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The Power of Movement in Plants, a non-fiction book by Charles Darwin |
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Chapter 2. General Considerations On The Movements And Growth Of Seedling Plants |
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_ CHAPTER II. GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF SEEDLING PLANTS Generality of the circumnutating movement--Radicles, their circumnutation of service--Manner in which they penetrate the ground--Manner in which hypocotyls and other organs break through the ground by being arched-- Singular manner of germination in Megarrhiza, etc.--Abortion of cotyledons- -Circumnutation of hypocotyls and epicotyls whilst still buried and arched- -Their power of straightening themselves--Bursting of the seed-coats-- Inherited effect of the arching process in hypogean hypocotyls-- Circumnutation of hypocotyls and epicotyls when erect--Circumnutation of cotyledons--Pulvini or joints of cotyledons, duration of their activity, rudimentary in Oxalis corniculata, their development--Sensitiveness of cotyledons to light and consequent disturbance of their periodic movements- -Sensitiveness of cotyledons to contact. THE circumnutating movements of the several parts or organs of a considerable number of seedling plants have been described in the last chapter. A list is here appended of the Families, Cohorts, Sub-classes, etc., to which they belong, arranged and numbered according to the classification adopted by Hooker.* Any one who will consider this list will see that the young plants selected for observation, fairly represent the whole vegetable series excepting the lowest cryptogams, and the movements of some of the latter when mature will hereafter be described. As all the seedlings which were observed, including Conifers, Cycads and Ferns, which belong to the most ancient * As given in the 'General System of Botany,' by Le Maout and Decaisne, 1873. types amongst plants, were continually circumnutating, we may infer that this kind of movement is common to every seedling species.
Radicles.--In all the germinating seeds observed by us, the first change is the protrusion of the radicle, which immediately bends downwards and endeavours to penetrate the ground. In order to effect this, it is almost necessary that the seed should be pressed down so as to offer some resistance, unless indeed the soil is extremely loose; for otherwise the seed is lifted up, instead of the radicle penetrating the surface. But seeds often get covered by earth thrown up by burrowing quadrupeds or scratching birds, by the castings of earth-worms, by heaps of excrement, the decaying branches of trees, etc., and will thus be pressed down; and they must often fall into cracks when the ground is dry, or into holes. Even with seeds lying on the bare surface, the first developed root-hairs, by becoming attached to stones or other objects on the surface, are able to hold down the upper part of the radicle, whilst the tip penetrates the ground. Sachs has shown* how well and closely root-hairs adapt themselves by growth to the most irregular particles in the soil, and become firmly attached to them. This attachment seems to be effected by the softening or liquefaction of the outer surface of the wall of the hair and its subsequent consolidation, as will be on some future occasion more fully described. This intimate union plays an important part, according to Sachs, in the absorption of water and of the inorganic matter dissolved in it. The mechanical aid afforded by the root-hairs in penetrating the ground is probably only a secondary service. The tip of the radicle, as soon as it protrudes from the seed-coats, begins to circumnutate, and the whole * 'Physiologie Végétale,' 1868, pp. 199, 205. growing part continues to do so, probably for as long as growth continues. This movement of the radicle has been described in Brassica, Aesculus, Phaseolus, Vicia, Cucurbita, Quercus and Zea. The probability of its occurrence was inferred by Sachs,* from radicles placed vertically upwards being acted on by geotropism (which we likewise found to be the case), for if they had remained absolutely perpendicular, the attraction of gravity could not have caused them to bend to any one side. Circumnutation was observed in the above specified cases, either by means of extremely fine filaments of glass affixed to the radicles in the manner previously described, or by their being allowed to grow downwards over inclined smoked glass-plates, on which they left their tracks. In the latter cases the serpentine course (see Figs. 19, 21, 27, 41) showed unequivocally that the apex had continually moved from side to side. This lateral movement was small in extent, being in the case of Phaseolus at most about 1 mm. from a medial line to both sides. But there was also movement in a vertical plane at right angles to the inclined glass-plates. This was shown by the tracks often being alternately a little broader and narrower, due to the radicles having alternately pressed with greater and less force on the plates. Occasionally little bridges of soot were left across the tracks, showing that the apex had at these spots been lifted up. This latter fact was especially apt to occur * 'Ueber das Wachsthum der Wurzeln: Arbeiten des bot. Instituts in Würzburg,' Heft iii. 1873, p. 460. This memoir, besides its intrinsic and great interest, deserves to be studied as a model of careful investigation, and we shall have occasion to refer to it repeatedly. Dr. Frank had previously remarked ('Beiträge zur Pflanzenphysiologie, 1868, p. 81) on the fact of radicles placed vertically upwards being acted on by geotropism, and he explained it by the supposition that their growth was not equal on all sides. when the radicle instead of travelling straight down the glass made a semicircular bend; but Fig. 52 shows that this may occur when the track is rectilinear. The apex by thus rising, was in one instance able to surmount a bristle cemented across an inclined glass-plate; but slips of wood only 1/40 of an inch in thickness always caused the radicles to bend rectangularly to one side, so that the apex did not rise to this small height in opposition to geotropism. In those cases in which radicles with attached filaments were placed so as to stand up almost vertically, they curved downwards through the action of geotropism, circumnutating at the same time, and their courses were consequently zigzag. Sometimes, however, they made great circular sweeps, the lines being likewise zigzag. Radicles closely surrounded by earth, even when this is thoroughly soaked and softened, may perhaps be quite prevented from circumnutating. Yet we should remember that the circumnutating sheath-like cotyledons of Phalaris, the hypocotyls of Solanum, and the epicotyls of Asparagus formed round themselves little circular cracks or furrows in a superficial layer of damp argillaceous sand. They were also able, as well as the hypocotyls of Brassica, to form straight furrows in damp sand, whilst circumnutating and bending towards a lateral light. In a future chapter it will be shown that the rocking or circumnutating movement of the flower-heads of Trifolium subterraneum aids them in burying themselves. It is therefore probable that the circumnutation of the tip of the radicle aids it slightly in penetrating the ground; and it may be observed in several of the previously given diagrams, that the movement is more strongly pronounced in radicles when they first protrude from the seed than at a rather later period; but whether this is an accidental or an adaptive coincidence we do not pretend to decide. Nevertheless, when young radicles of Phaseolus multiflorus were fixed vertically close over damp sand, in the expectation that as soon as they reached it they would form circular furrows, this did not occur,--a fact which may be accounted for, as we believe, by the furrow being filled up as soon as formed by the rapid increase of thickness in the apex of the radicle. Whether or not a radicle, when surrounded by softened earth, is aided in forming a passage for itself by circumnutating, this movement can hardly fail to be of high importance, by guiding the radicle along a line of least resistance, as will be seen in the next chapter when we treat of the sensibility of the tip to contact. If, however, a radicle in its downward growth breaks obliquely into any crevice, or a hole left by a decayed root, or one made by the larva of an insect, and more especially by worms, the circumnutating movement of the tip will materially aid it in following such open passage; and we have observed that roots commonly run down the old burrows of worms.* When a radicle is placed in a horizontal or inclined position, the terminal growing part, as is well known, bends down towards the centre of the earth; and Sachs** has shown that whilst thus bending, the growth of the lower surface is greatly retarded, whilst that * See, also, Prof. Hensen's statements ('Zeitschrift für Wissen, Zool.,' B. xxviii. p. 354, 1877) to the same effect. He goes so far as to believe that roots are able to penetrate the ground to a great depth only by means of the burrows made by worms. of the upper surface continues at the normal rate, or may be even somewhat increased. He has further shown by attaching a thread, running over a pulley, to a horizontal radicle of large size, namely that of the common bean, that it was able to pull up a weight of only one gramme, or 15.4 grains. We may therefore conclude that geotropism does not give a radicle force sufficient to penetrate the ground, but merely tells it (if such an expression may be used) which course to pursue. Before we knew of Sachs' more precise observations we covered a flat surface of damp sand with the thinnest tin-foil which we could procure (.02 to .03 mm., or .00012 to .00079 of an inch in thickness), and placed a radicle close above, in such a position that it grew almost perpendicularly downwards. When the apex came into contact with the polished level surface it turned at right angles and glided over it without leaving any impression; yet the tin-foil was so flexible, that a little stick of soft wood, pointed to the same degree as the end of the radicle and gently loaded with a weight of only a quarter of an ounce (120 grains) plainly indented the tin-foil. Radicles are able to penetrate the ground by the force due to their longitudinal and transverse growth; the seeds themselves being held down by the weight of the superincumbent soil. In the case of the bean the apex, protected by the root-cap, is sharp, and the growing part, from 8 to 10 mm. in length, is much more rigid, as Sachs has proved, than the part immediately above, which has ceased to increase in length. We endeavoured to ascertain the downward pressure of the growing part, by placing germinating beans between two small metal plates, the upper one of which was loaded with a known weight; and the radicle was then allowed to grow into a narrow hole in wood, 2 or 3 tenths of an inch in depth, and closed at the bottom. The wood was so cut that the short space of radicle between the mouth of the hole and the bean could not bend laterally on three sides; but it was impossible to protect the fourth side, close to the bean. Consequently, as long as the radicle continued to increase in length and remained straight, the weighted bean would be lifted up after the tip had reached the bottom of the shallow hole. Beans thus arranged, surrounded by damp sand, lifted up a quarter of a pound in 24 h. after the tip of the radicle had entered the hole. With a greater weight the radicles themselves always became bent on the one unguarded side; but this probably would not have occurred if they had been closely surrounded on all sides by compact earth. There was, however, a possible, but not probable, source of error in these trials, for it was not ascertained whether the beans themselves go on swelling for several days after they have germinated, and after having been treated in the manner in which ours had been; namely, being first left for 24 h. in water, then allowed to germinate in very damp air, afterwards placed over the hole and almost surrounded by damp sand in a closed box. Fig. 55. Outline of piece of stick (reduced to one-half natural size) with a hole through which the radicle of a bean grew. Thickness of stick at narrow end .08 inch, at broad end .16; depth of hole .1 inch. We succeeded better in ascertaining the force exerted transversely by these radicles. Two were so placed as to penetrate small holes made in little sticks, one of which was cut into the shape here exactly copied (Fig. 55). The short end of the stick beyond the hole was purposely split, but not the opposite end. As the wood was highly elastic, the split or fissure closed immediately after being made. After six days the stick and bean were dug out of the damp sand, and the radicle was found to be much enlarged above and beneath the hole. The fissure which was at first quite closed, was now open to a width of 4 mm.; as soon as the radicle was extracted, it immediately closed to a width of 2 mm. The stick was then suspended horizontally by a fine wire passing through the hole lately filled by the radicle, and a little saucer was suspended beneath to receive the weights; and it required 8 lbs. 8 ozs. to open the fissure to the width of 4 mm.-- that is, the width before the root was extracted. But the part of the radicle (only .1 of an inch in length) which was embedded in the hole, probably exerted a greater transverse strain even than 8 lbs. 8 ozs., for it had split the solid wood for a length of rather more than a quarter of an inch (exactly .275 inch), and this fissure is shown in Fig. 55. A second stick was tried in the same manner with almost exactly the same result. Fig. 56. Wooden pincers, kept closed by a spiral brass spring, with a hole (.14 inch in diameter and .6 inch in depth) bored through the narrow closed part, through which a radicle of a bean was allowed to grow. Temp. 50o - 60o F. We then followed a better plan. Holes were bored near the narrow end of two wooden clips or pincers (Fig. 56), kept closed by brass spiral springs. Two radicles in damp sand were allowed to grow through these holes. The pincers rested on glass-plates to lessen the friction from the sand. The holes were a little larger (viz..14 inch) and considerably deeper (viz..6 inch) than in the trials with the sticks; so that a greater length of a rather thicker radicle exerted a transverse strain. After 13 days they were taken up. The distance of two dots (see the figure) on the longer ends of the pincers was now carefully measured; the radicles were then extracted from the holes, and the pincers of course closed. They were then suspended horizontally in the same manner as were the bits of sticks, and a weight of 1500 grams (or 3 pounds 4 ounces) was necessary with one of the pincers to open them to the same extent as had been effected by the transverse growth of the radicle. As soon as this radicle had slightly opened the pincers, it had grown into a flattened form and had escaped a little beyond the hole; its diameter in one direction being 4.2 mm., and at rightangles 3.5 mm. If this escape and flattening could have been prevented, the radicle would probably have exerted a greater strain than the 3 pounds 4 ounces. With the other pincers the radicle escaped still further out of the hole; and the weight required to open them to the same extent as had been effected by the radicle, was only 600 grams. With these facts before us, there seems little difficulty in understanding how a radicle penetrates the ground. The apex is pointed and is protected by the root-cap; the terminal growing part is rigid, and increases in length with a force equal, as far as our observations can be trusted, to the pressure of at least a quarter of a pound, probably with a much greater force when prevented from bending to any side by the surrounding earth. Whilst thus increasing in length it increases in thickness, pushing away the damp earth on all sides, with a force of above 8 pounds in one case, of 3 pounds in another case. It was impossible to decide whether the actual apex exerts, relatively to its diameter, the same transverse strain as the parts a little higher up; but there seems no reason to doubt that this would be the case. The growing part therefore does not act like a nail when hammered into a board, but more like a wedge of wood, which whilst slowly driven into a crevice continually expands at the same time by the absorption of water; and a wedge thus acting will split even a mass of rock. Manner in which Hypocotyls, Epicotyls, etc., rise up and break through the ground.--After the radicle has penetrated the ground and fixed the seed, the hypocotyls of all the dicotyledonous seedlings observed by us, which lift their cotyledons above the surface, break through the ground in the form of an arch. When the cotyledons are hypogean, that is, remain buried in the soil, the hypocotyl is hardly developed, and the epicotyl or plumule rises in like manner as an arch through the ground. In all, or at least in most of such cases, the downwardly bent apex remains for a time enclosed within the seed-coats. With Corylus avellena the cotyledons are hypogean, and the epicotyl is arched; but in the particular case described in the last chapter its apex had been injured, and it grew laterally through the soil like a root; and in consequence of this it had emitted two secondary shoots, which likewise broke through the ground as arches. Cyclamen does not produce any distinct stem, and only a single cotyledon appears at first;* its petiole * This is the conclusion arrived at by Dr. H. Gressner ('Bot. Zeitung,' 1874, p. 837), who maintains that what has been considered by other botanists as the first true leaf is really the second cotyledon, which is greatly delayed in its development. breaks through the ground as an arch (Fig. 57). Abronia has only a single fully developed cotyledon, but in this case it is the hypocotyl which first emerges and is arched. Abronia umbellata, however, presents this peculiarity, that the enfolded blade of the one developed cotyledon (with the enclosed endosperm) whilst still beneath the surface has its apex upturned and parallel to the descending leg of the arched hypocotyl; but it is dragged out of the ground by the continued growth of the hypocotyl, with the apex pointing downward. With Cycas pectinata the cotyledons are hypogean, and a true leaf first breaks through the ground with its petiole forming an arch. Fig. 57. Cyclamen Persicum: seedling, figure enlarged: c, blade of cotyledon, not yet expanded, with arched petiole beginning to straighten itself; h, hypocotyl developed into a corm; r, secondary radicles. In the genus Acanthus the cotyledons are likewise hypogean. In A. mollis, a single leaf first breaks through the ground with its petiole arched, and with the opposite leaf much less developed, short, straight, of a yellowish colour, and with the petiole at first not half as thick as that of the other. The undeveloped leaf is protected by standing beneath its arched fellow; and it is an instructive fact that it is not arched, as it has not to force for itself a passage through the ground. In the accompanying sketch (Fig. 58) the petiole of the first leaf has already partially straightened itself, and the blade is beginning to unfold. The small second leaf ultimately grows to an equal size with the first, but this process is effected at very different rates in different individuals: in one instance the second leaf did not appear fully above the ground until six weeks after the first leaf. As the leaves in the whole family of the Acanthaceae stand either opposite one another or in whorls, and as these are of equal size, the great inequality between the first two leaves is a singular fact. We can see how this inequality of development and the arching of the petiole could have been gradually acquired, if they were beneficial to the seedlings by favouring their emergence; for with A. candelabrum, spinosus, and latifolius there was a great variability in the inequality between the two first leaves and in the arching of their petioles. In one seedling of A. candelabrum the first leaf was arched and nine times as long as the second, which latter consisted of a mere little, yellowish-white, straight, hairy style. In other seedlings the difference in length between the two leaves was as 3 to 2, or as 4 to 3, or as only .76 to .62 inch. In these latter cases the first and taller leaf was not properly arched. Lastly, in another seedling there was not the least difference in size between the two first leaves, and both of them had their petioles straight; their laminae were enfolded and pressed against each other, forming a lance or wedge, by which means they had broken through the ground. Therefore in different individuals of this same species of Acanthus the first pair of leaves breaks through the ground by two widely different methods; and if either had proved decidedly advantageous or disadvantageous, one of them no doubt would soon have prevailed. Asa Gray has described* the peculiar manner of germination of three widely different plants, in which the hypocotyl is hardly at all developed. These were therefore observed by us in relation to our present subject. Delphinium nudicaule.--The elongated petioles of the two cotyledons are confluent (as are sometimes their blades at the base), and they break through the ground as an arch. They thus resemble in a most deceptive manner a hypocotyl. At first they are solid, but after a time become tubular; and the basal part beneath the ground is enlarged into a hollow chamber, within which the young leaves are developed without any prominent plumule. Externally root-hairs are formed on the confluent petioles, either a little above, or on a level with, the plumule. The first leaf at an early period of its growth and whilst within the chamber is quite straight, but the petiole soon becomes arched; and the swelling of this part (and probably of the blade) splits open one side of the chamber, and the leaf then emerges. The slit was found in one case to be 3.2 mm. in length, and it is seated on the line of confluence of the two petioles. The leaf when it first escapes from the chamber is buried beneath the ground, and now an upper part of the petiole near the blade becomes arched in the usual manner. The second leaf comes out of the slit either straight or somewhat arched, but afterwards the upper part of the petiole,--certainly in some, and we believe in all cases,--arches itself whilst forcing a passage through the soil. * 'Botanical Text-Book,' 1879, p. 22. Megarrhiza Californica.--The cotyledons of this Gourd never free themselves from the seed-coats and are hypogean. Their petioles are completely confluent, forming a tube which terminates downwards in a little solid point, consisting of a minute radicle and hypocotyl, with the likewise minute plumule enclosed within the base of the tube. This structure was well exhibited in an abnormal specimen, in which one of the two cotyledons failed to produce a petiole, whilst the other produced one consisting of an open semicylinder ending in a sharp point, formed of the parts just described. As soon as the confluent petioles protrude from the seed they bend down, as they are strongly geotropic, and penetrate the ground. The seed itself retains its original position, either on the surface or buried at some depth, as the case may be. If, however, the point of the confluent petioles meets with some obstacle in the soil, as appears to have occurred with the seedlings described and figured by Asa Gray,* the cotyledons are lifted up above the ground. The petioles are clothed with root-hairs like those on a true radicle, and they likewise resemble radicles in becoming brown when immersed in a solution of permanganate of potassium. Our seeds were subjected to a high temperature, and in the course of three or four days the petioles penetrated the soil perpendicularly to a depth of from 2 to 2 ½ inches; and not until then did the true radicle begin to grow. In one specimen which was closely observed, the petioles in 7 days after their first protrusion attained a length of 2 ½ inches, and the radicle by this time had also become well developed. The plumule, still enclosed within the tube, was now * 'American Journal of Science,' vol. xiv. 1877, p. 21. .3 inch in length, and was quite straight; but from having increased in thickness it had just begun to split open the lower part of the petioles on one side, along the line of their confluence. By the following morning the upper part of the plumule had arched itself into a right angle, and the convex side or elbow had thus been forced out through the slit. Here then the arching of the plumule plays the same part as in the case of the petioles of the Delphinium. As the plumule continued to grow, the tip became more arched, and in the course of six days it emerged through the 2 ½ inches of superincumbent soil, still retaining its arched form. After reaching the surface it straightened itself in the usual manner. In the accompanying figure (Fig. 58, A) we have a sketch of a seedling in this advanced state of development; the surface of the ground being represented by the line G...........G. Fig. 58, A. Megarrhiza Californica: sketch of seedling, copied from Asa Gray, reduced to one-half scale: c, cotyledons within seed-coats; p, the two confluent petioles; h and r, hypocotyl and radicle; p1, plumule; G..........G, surface of soil. The germination of the seeds in their native Californian home proceeds in a rather different manner, as we infer from an interesting letter from Mr. Rattan, sent to us by Prof. Asa Gray. The petioles protrude from the seeds soon after the autumnal rains, and penetrate the ground, generally in a vertical direction, to a depth of from 4 to even 6 inches. they were found in this state by Mr. Rattan during the Christmas vacation, with the plumules still enclosed within the tubes; and he remarks that if the plumules had been at once developed and had reached the surface (as occurred with our seeds which were exposed to a high temperature), they would surely have been killed by the frost. As it is, they lie dormant at some depth beneath the surface, and are thus protected from the cold; and the root-hairs on the petioles would supply them with sufficient moisture. We shall hereafter see that many seedlings are protected from frost, but by a widely different process, namely, by being drawn beneath the surface by the contraction of their radicles. We may, however, believe that the extraordinary manner of germination of Megarrhiza has another and secondary advantage. The radicle begins in a few weeks to enlarge into a little tuber, which then abounds with starch and is only slightly bitter. It would therefore be very liable to be devoured by animals, were it not protected by being buried whilst young and tender, at a depth of some inches beneath the surface. Ultimately it grows to a huge size. Ipomoea leptophylla.--In most of the species of this genus the hypocotyl is well developed, and breaks through the ground as an arch. But the seeds of the present species in germinating behave like those of Megarrhiza, excepting that the elongated petioles of the cotyledons are not confluent. After they have protruded from the seed, they are united at their lower ends with the undeveloped hypocotyl and undeveloped radicle, which together form a point only about .1 inch in length. They are at first highly geotropic, and penetrate the ground to a depth of rather above half an inch. The radicle then begins to grow. On four occasions after the petioles had grown for a short distance vertically downwards, they were placed in a horizontal position in damp air in the dark, and in the course of 4 hours they again became curved vertically downwards, having passed through 90o in this time. But their sensitiveness to geotropism lasts for only 2 or 3 days; and the terminal part alone, for a length of between .2 and .4 inch, is thus sensitive. Although the petioles of our specimens did not penetrate the ground to a greater depth than about ½ inch, yet they continued for some time to grow rapidly, and finally attained the great length of about 3 inches. The upper part is apogeotropic, and therefore grows vertically upwards, excepting a short portion close to the blades, which at an early period bends downwards and becomes arched, and thus breaks through the ground. Afterwards this portion straightens itself, and the cotyledons then free themselves from the seed-coats. Thus we here have in different parts of the same organ widely different kinds of movement and of sensitiveness; for the basal part is geotropic, the upper part apogeotropic, and a portion near the blades temporarily and spontaneously arches itself. The plumule is not developed for some little time; and as it rises between the bases of the parallel and closely approximate petioles of the cotyledons, which in breaking through the ground have formed an almost open passage, it does not require to be arched and is consequently always straight. Whether the plumule remains buried and dormant for a time in its native country, and is thus protected from the cold of winter, we do not know. The radicle, like that of the Megarrhiza, grows into a tuber-like mass, which ultimately attains a great size. So it is with Ipomoea pandurata, the germination of which, as Asa Gray informs us, resembles that of I. leptophylla. The following case is interesting in connection with the root-like nature of the petioles. The radicle of a seedling was cut off, as it was completely decayed, and the two now separated cotyledons were planted. They emitted roots from their bases, and continued green and healthy for two months. The blades of both then withered, and on removing the earth the bases of the petioles (instead of the radicle) were found enlarged into little tubers. Whether these would have had the power of producing two independent plants in the following summer, we do not know. In Quercus virens, according to Dr. Engelmann,* both the cotyledons and their petioles are confluent. The latter grow to a length "of an inch or even more;" and, if we understand rightly, penetrate the ground, so that they must be geotropic. The nutriment within the cotyledons is then quickly transferred to the hypocotyl or radicle, which thus becomes developed into a fusiform tuber. The fact of tubers being formed by the foregoing three widely distinct plants, makes us believe that their protection from animals at an early age and whilst tender, is one at least of the advantages gained by the remarkable elongation of the petioles of the cotyledons, together with their power of penetrating the ground like roots under the guidance of geotropism. The following cases may be here given, as they bear on our present subject, though not relating to seedlings. The flower-stem of the parasitic Lathraea squamaria, which is destitute of true leaves, breaks through the ground as an arch;** so does the flower- * 'Transact. St. Louis Acad. Science,' vol. iv. p. 190. stem of the parasitic and leafless Monotropa hypopitys. With Helleborus niger, the flower-stems, which rise up independently of the leaves, likewise break through the ground as arches. This is also the case with the greatly elongated flower-stems, as well as with the petioles of Epimedium pinnatum. So it is with the petioles of Ranunculus ficaria, when they have to break through the ground, but when they arise from the summit of the bulb above ground, they are from the first quite straight; and this is a fact which deserves notice. The rachis of the bracken fern (Pteris aquilina), and of some, probably many, other ferns, likewise rises above ground under the form of an arch. No doubt other analogous instances could be found by careful search. In all ordinary cases of bulbs, rhizomes, root-stocks, etc., buried beneath the ground, the surface is broken by a cone formed by the young imbricated leaves, the combined growth of which gives them force sufficient for the purpose. With germinating monocotyledonous seeds, of which, however, we did not observe a large number, the plumules, for instance, those of Asparagus and Canna, are straight whilst breaking through the ground. With the Gramineae, the sheath-like cotyledons are likewise straight; they, however, terminate in a sharp crest, which is white and somewhat indurated; and this structure obviously facilitates their emergence from the soil: the first true leaves escape from the sheath through a slit beneath the chisel-like apex and at right angles to it. In the case of the onion (Allium cepa) we again meet with an arch; the leaf-like cotyledon being abruptly bowed, when it breaks through the ground, with the apex still enclosed within the seed-coats. The crown of the arch, as previously described, is developed into a white conical protuberance, which we may safely believe to be a special adaptation for this office. The fact of so many organs of different kinds--hypocotyls and epicotyls, the petioles of some cotyledons and of some first leaves, the cotyledons of the onion, the rachis of some ferns, and some flower-stems--being all arched whilst they break through the ground, shows how just are Dr. Haberlandt's* remarks on the importance of the arch to seedling plants. He attributes its chief importance to the upper, young, and more tender parts of the hypocotyl * 'Die Schutzeinrichtungen in der Entwickelung der Keimpflanze,' 1877. We have learned much from this interesting essay, though our observations lead us to differ on some points from the author. or epicotyl, being thus saved from abrasion and pressure whilst breaking through the ground. But we think that some importance may be attributed to the increased force gained by the hypocotyl, epicotyl, or other organ by being at first arched; for both legs of the arch increase in length, and both have points of resistance as long as the tip remains enclosed within the seed-coats; and thus the crown of the arch is pushed up through the earth with twice as much force as that which a straight hypocotyl, etc., could exert. As soon, however, as the upper end has freed itself, all the work has to be done by the basal leg. In the case of the epicotyl of the common bean, the basal leg (the apex having freed itself from the seed-coats) grew upwards with a force sufficient to lift a thin plate of zinc, loaded with 12 ounces. Two more ounces were added, and the 14 ounces were lifted up to a very little height, and then the epicotyl yielded and bent to one side. With respect to the primary cause of the arching process, we long thought in the case of many seedlings that this might be attributed to the manner in which the hypocotyl or epicotyl was packed and curved within the seed-coats; and that the arched shape thus acquired was merely retained until the parts in question reached the surface of the ground. But it is doubtful whether this is the whole of the truth in any case. For instance, with the common bean, the epicotyl or plumule is bowed into an arch whilst breaking through the seed-coats, as shown in Fig. 59 (p. 92). The plumule first protrudes as a solid knob (e in A), which after twenty-four hours' growth is seen (e in B) to be the crown of an arch. Nevertheless, with several beans which germinated in damp air, and had otherwise been treated in an unnatural manner, little plumules were developed in the axils of the petioles of both cotyledons, and these were as perfectly arched as the normal plumule; yet they had not been subjected to any confinement or pressure, for the seed-coats were completely ruptured, and they grew in the open air. This proves that the plumule has an innate or spontaneous tendency to arch itself. In some other cases the hypocotyl or epicotyl protrudes from the seed at first only slightly bowed; but the bowing afterwards increases independently of any constraint. The arch is thus made narrow, with the two legs, which are sometimes much elongated, parallel and close together, and thus it becomes well fitted for breaking through the ground. With many kinds of plants, the radicle, whilst still enclosed within the seed and likewise after its first protrusion, lies in a straight line with the future hypocotyl and with the longitudinal axis of the cotyledons. This is the case with Cucurbita ovifera: nevertheless, in whatever position the seeds were buried, the hypocotyl always came up arched in one particular direction. Seeds were planted in friable peat at a depth of about an inch in a vertical position, with the end from which the radicle protrudes downwards. Therefore all the parts occupied the same relative positions which they would ultimately hold after the seedlings had risen clear above the surface. Notwithstanding this fact, the hypocotyl arched itself; and as the arch grew upwards through the peat, the buried seeds were turned either upside down, or were laid horizontally, being afterwards dragged above the ground. Ultimately the hypocotyl straightened itself in the usual manner; and now after all these movements the several parts occupied the same position relatively to one another and to the centre of the earth, which they had done when the seeds were first buried. But it may be argued in this and other such cases that, as the hypocotyl grows up through the soil, the seed will almost certainly be tilted to one side; and then from the resistance which it must offer during its further elevation, the upper part of the hypocotyl will be doubled down and thus become arched. This view seems the more probable, because with Ranunculus ficaria only the petioles of the leaves which forced a passage through the earth were arched; and not those which arose from the summits of the bulbs above the ground. Nevertheless, this explanation does not apply to the Cucurbita, for when germinating seeds were suspended in damp air in various positions by pins passing through the cotyledons, fixed to the inside of the lids of jars, in which case the hypocotyls were not subjected to any friction or constraint, yet the upper part became spontaneously arched. This fact, moreover, proves that it is not the weight of the cotyledons which causes the arching. Seeds of Helianthus annuus and of two species of Ipomoea (those of 'I. bona nox' being for the genus large and heavy) were pinned in the same manner, and the hypocotyls became spontaneously arched; the radicles, which had been vertically dependent, assumed in consequence a horizontal position. In the case of Ipomoea leptophylla it is the petioles of the cotyledons which become arched whilst rising through the ground; and this occurred spontaneously when the seeds were fixed to the lids of jars. It may, however, be suggested with some degree of probability that the arching was aboriginally caused by mechanical compulsion, owing to the confinement of the parts in question within the seed-coats, or to friction whilst they were being dragged upwards. But if this is so, we must admit from the cases just given, that a tendency in the upper part of the several specified organs to bend downwards and thus to become arched, has now become with many plants firmly inherited. The arching, to whatever cause it may be due, is the result of modified circumnutation, through increased growth along the convex side of the part; such growth being only temporary, for the part always straightens itself subsequently by increased growth along the concave side, as will hereafter be described. It is a curious fact that the hypocotyls of some plants, which are but little developed and which never raise their cotyledons above the ground, nevertheless inherit a slight tendency to arch themselves, although this movement is not of the least use to them. We refer to a movement observed by Sachs in the hypocotyls of the bean and some other Leguminosae, and which is shown in the accompanying figure (Fig. 59), copied from his Essay.* The hypocotyl and radicle at first grow perpendicularly downwards, as at A, and then bend, often in the course of 24 hours, into the position shown at B. As we shall hereafter often have to recur to this movement, we will, for brevity sake, call it "Sachs' curvature." At first sight it might be thought that the altered position of the radicle in B was wholly due to the outgrowth of the epicotyl (e), the petiole (p) serving as a hinge; and it is probable that this is partly the cause; but the hypocotyl and upper part of the radicle themselves become slightly curved. The above movement in the bean was repeatedly seen by us; but our observations were made chiefly on Phaseolus multiflorus, the cotyledons of which are likewise * 'Arbeiten des bot. Instit. Würzburg,' vol. i. 1873, p. 403. hypogean. Some seedlings with well-developed radicles were first immersed in a solution of permanganate of potassium; and, judging from the changes of colour (though these were not very clearly defined), the hypocotyl is about .3 inch in length. Straight, thin, black lines of this length were now drawn from the bases of the short petioles along the hypocotyls Fig. 59. Vicia faba: germinating seeds, suspended in damp air: A, with radicle growing perpendicularly downwards; B, the same bean after 24 hours and after the radicle has curved itself; r. radicle; h, short hypocotyl; e, epicotyl appearing as a knob in A and as an arch in B; p, petiole of the cotyledon, the latter enclosed within the seed-coats. of 23 germinating seeds, which were pinned to the lids of jars, generally with the hilum downwards, and with their radicles pointing to the centre of the earth. After an interval of from 24 to 48 hours the black lines on the hypocotyls of 16 out of the 23 seedlings became distinctly curved, but in very various degrees (namely, with radii between 20 and 80 mm. on Sachs' cyclometer) in the same relative direction as shown at B in Fig. 59. As geotropism will obviously tend to check this curvature, seven seeds were allowed to germinate with proper precautions for their growth in a klinostat,* by which means geotropism was eliminated. The position of the hypocotyls was observed during four successive days, and they continued to bend towards the hilum and lower surface of the seed. On the fourth day they were deflected by an average angle of 63o from a line perpendicular to the lower surface, and were therefore considerably more curved than the hypocotyl and radicle in the bean at B (Fig. 59), though in the same relative direction. It will, we presume, be admitted that all leguminous plants with hypogean cotyledons are descended from forms which once raised their cotyledons above the ground in the ordinary manner; and in doing so, it is certain that their hypocotyls would have been abruptly arched, as in the case of every other dicotyledonous plant. This is especially clear in the case of Phaseolus, for out of five species, the seedlings of which we observed, namely, P. multiflorus, caracalla, vulgaris, Hernandesii and Roxburghii (inhabitants of the Old and New Worlds), the three last-named species have well-developed hypocotyls which break through the ground as arches. Now, if we imagine a seedling of the common bean or of P. multiflorus, to behave as its progenitors once did, the hypocotyl (h, Fig. 59), in whatever position the seed may have been buried, would become so much arched that the upper part would be doubled down parallel to the lower part; and * An instrument devised by Sachs, consisting essentially of a slowly revolving horizontal axis, on which the plant under observation is supported: see 'Würzburg Arbeiten,' 1879, p. 209. this is exactly the kind of curvature which actually occurs in these two plants, though to a much less degree. Therefore we can hardly doubt that their short hypocotyls have retained by inheritance a tendency to curve themselves in the same manner as they did at a former period, when this movement was highly important to them for breaking through the ground, though now rendered useless by the cotyledons being hypogean. Rudimentary structures are in most cases highly variable, and we might expect that rudimentary or obsolete actions would be equally so; and Sachs' curvature varies extremely in amount, and sometimes altogether fails. This is the sole instance known to us of the inheritance, though in a feeble degree, of movements which have become superfluous from changes which the species has undergone. Rudimentary Cotyledons.--A few remarks on this subject may be here interpolated. It is well known that some dicotyledonous plants produce only a single cotyledon; for instance, certain species of Ranunculus, Corydalis, Chaerophyllum; and we will here endeavour to show that the loss of one or both cotyledons is apparently due to a store of nutriment being laid up in some other part, as in the hypocotyl or one of the two cotyledons, or one of the secondary radicles. Fig. 60. Citrus aurantium: two young seedlings: c, larger cotyledon; c', smaller cotyledon; h, thickened hypocotyl; r, radicle. In A the epicotyl is still arched, in B it has become erect. With the orange (Citrus aurantium) the cotyledons are hypogean, and one is larger than the other, as may be seen in A (Fig. 60). In B the inequality is rather greater, and the stem has grown between the points of insertion of the two petioles, so that they do not stand opposite to one another; in another case the separation amounted to one-fifth of an inch. The smaller cotyledon of one seedling was extremely thin, and not half the length of the larger one, so that it was clearly becoming rudimentary,* In all these seedlings the hypocotyl was enlarged or swollen. Fig. 61. Abronia umbellata: seedling twice natural size: c cotyledon; c', rudimentary cotyledon; h, enlarged hypocotyl, with a heel or projection (h') at the lower end; r, radicle. With Abronia umbellata one of the cotyledons is quite rudimentary, as may be seen (c') in Fig. 61. In this specimen it consisted of a little green flap, 1/84th inch in length, destitute of a petiole and covered with glands like those on the fully developed cotyledon (c). At first it stood opposite to the larger cotyledon; but as the petiole of the latter increased in length and grew in the same line with the hypocotyl (h), the rudiment appeared in older seedlings as if seated some way down the hypocotyl. With Abronia arenaria there is a similar rudiment, which in one * In Pachira aquatica, as described by Mr. R. I. Lynch ('Journal Linn. Soc. Bot.' vol. xvii. 1878, p. 147), one of the hypogean cotyledons is of immense size; the other is small and soon falls off; the pair do not always stand opposite. In another and very different water-plant, 'Trapa natans', one of the cotyledons, filled with farinaceous matter, is much larger than the other, which is scarcely visible, as is stated by Aug. de Candolle, 'Physiologie Veg.' tom. ii. p. 834, 1832. specimen was only 1/100th and in another 1/60th inch in length; it ultimately appeared as if seated halfway down the hypocotyl. In both these species the hypocotyl is so much enlarged, especially at a very early age, that it might almost be called a corm. The lower end forms a heel or projection, the use of which will hereafter be described. In Cyclamen Persicum the hypocotyl, even whilst still within the seed, is enlarged into a regular corm,* and only a single cotyledon is at first developed (see former Fig. 57). With Ranunculus ficaria two cotyledons are never produced, and here one of the secondary radicles is developed at an early age into a so-called bulb.** Again, certain species of Chaerophyllum and Corydalis produce only a single cotyledon;*** in the former the hypocotyl, and in the latter the radicle is enlarged, according to Irmisch, into a bulb. In the several foregoing cases one of the cotyledons is delayed in its development, or reduced in size, or rendered rudimentary, or quite aborted; but in other cases both cotyledons are represented by mere rudiments. With Opuntia basilaris this is not the case, for both cotyledons are thick and large, and the hypocotyl shows at first no signs of enlargement; but afterwards, when the cotyledons have withered and disarticulated themselves, it becomes thickened, and from its tapering form, together with its smooth, tough, brown skin, appears, when ultimately drawn down to some depth into the soil, like a root. On the other * Dr. H. Gressner, 'Bot. Zeitung,' 1874, p. 824. hand, with several other Cacteae, the hypocotyl is from the first much enlarged, and both cotyledons are almost or quite rudimentary. Thus with Cereus Landbeckii two little triangular projections, representing the cotyledons, are narrower than the hypocotyl, which is pear-shaped, with the point downwards. In Rhipsalis cassytha the cotyledons are represented by mere points on the enlarged hypocotyl. In Echinocactus viridescens the hypocotyl is globular, with two little prominences on its summit. In Pilocereus Houlletii the hypocotyl, much swollen in the upper part, is merely notched on the summit; and each side of the notch evidently represents a cotyledon. Stapelia sarpedon, a member of the very distinct family of the Asclepiadeae, is fleshy like a cactus; and here again the upper part of the flattened hypocotyl is much thickened and bears two minute cotyledons, which, measured internally, were only .15 inch in length, and in breadth not equal to one-fourth of the diameter of the hypocotyl in its narrow axis; yet these minute cotyledons are probably not quite useless, for when the hypocotyl breaks through the ground in the form of an arch, they are closed or pressed against one another, and thus protect the plumule. They afterwards open. From the several cases now given, which refer to widely distinct plants, we may infer that there is some close connection between the reduced size of one or both cotyledons and the formation, by the enlargement of the hypocotyl or of the radicle, of a so-called bulb. But it may be asked, did the cotyledons first tend to abort, or did a bulb first begin to be formed? As all dicotyledons naturally produce two well-developed cotyledons, whilst the thickness of the hypocotyl and of the radicle differs much in different plants, it seems probable that these latter organs first became from some cause thickened--in several instances apparently in correlation with the fleshy nature of the mature plant--so as to contain a store of nutriment sufficient for the seedling, and then that one or both cotyledons, from being superfluous, decreased in size. It is not surprising that one cotyledon alone should sometimes have been thus affected, for with certain plants, for instance the cabbage, the cotyledons are at first of unequal size, owing apparently to the manner in which they are packed within the seed. It does not, however, follow from the above connection, that whenever a bulb is formed at an early age, one or both cotyledons will necessarily become superfluous, and consequently more or less rudimentary. Finally, these cases offer a good illustration of the principle of compensation or balancement of growth, or, as Goethe expresses it, "in order to spend on one side, Nature is forced to economise on the other side." Circumnutation and other movements of Hypocotyls and Epicotyls, whilst still arched and buried beneath the ground, and whilst breaking through it.--According to the position in which a seed may chance to have been buried, the arched hypocotyl or epicotyl will begin to protrude in a horizontal, a more or less inclined, or in a vertical plane. Except when already standing vertically upwards, both legs of the arch are acted on from the earliest period by apogeotropism. Consequently they both bend upwards until the arch becomes vertical. During the whole of this process, even before the arch has broken through the ground, it is continually trying to circumnutate to a slight extent; as it likewise does if it happens at first to stand vertically up,--all which cases have been observed and described, more or less fully, in the last chapter. After the arch has grown to some height upwards the basal part ceases to circumnutate, whilst the upper part continues to do so. That an arched hypocotyl or epicotyl, with the two legs fixed in the ground, should be able to circumnutate, seemed to us, until we had read Prof. Wiesner's observations, an inexplicable fact. He has shown* in the case of certain seedlings, whose tips are bent downwards (or which nutate), that whilst the posterior side of the upper or dependent portion grows quickest, the anterior and opposite side of the basal portion of the same internode grows quickest; these two portions being separated by an indifferent zone, where the growth is equal on all sides. There may be even more than one indifferent zone in the same internode; and the opposite sides of the parts above and below each such zone grow quickest. This peculiar manner of growth is called by Wiesner "undulatory nutation." Circumnutation depends on one side of an organ growing quickest (probably preceded by increased turgescence), and then another side, generally almost the opposite one, growing quickest. Now if we look at an arch like this [upside down U] and suppose the whole of one side--we will say the whole convex side of both legs--to increase in length, this would not cause the arch to bend to either side. But if the outer side or surface of the left leg were to increase in length the arch would be pushed over to the right, and this would be aided by the inner side of the right leg increasing in length. If afterwards the process were reversed, the arch would be pushed over to the opposite or left side, and so on alternately,--that is, it would circumnutate. As an arched hypocotyl, * 'Die undulirende Nutation der Internodien,' Akad. der Wissench. (Vienna), Jan. 17th, 1878. Also published separately, see p. 32. with the two legs fixed in the ground, certainly circumnutates, and as it consists of a single internode, we may conclude that it grows in the manner described by Wiesner. It may be added, that the crown of the arch does not grow, or grows very slowly, for it does not increase much in breadth, whilst the arch itself increases greatly in height. The circumnutating movements of arched hypocotyls and epicotyls can hardly fail to aid them in breaking through the ground, if this be damp and soft; though no doubt their emergence depends mainly on the force exerted by their longitudinal growth. Although the arch circumnutates only to a slight extent and probably with little force, yet it is able to move the soil near the surface, though it may not be able to do so at a moderate depth. A pot with seeds of Solanum palinacanthum, the tall arched hypocotyls of which had emerged and were growing rather slowly, was covered with fine argillaceous sand kept damp, and this at first closely surrounded the bases of the arches; but soon a narrow open crack was formed round each of them, which could be accounted for only by their having pushed away the sand on all sides; for no such cracks surrounded some little sticks and pins which had been driven into the sand. It has already been stated that the cotyledons of Phalaris and Avena, the plumules of Asparagus and the hypocotyls of Brassica, were likewise able to displace the same kind of sand, either whilst simply circumnutating or whilst bending towards a lateral light. As long as an arched hypocotyl or epicotyl remains buried beneath the ground, the two legs cannot separate from one another, except to a slight extent from the yielding of the soil; but as soon as the arch rises above the ground, or at an earlier period if the pressure of the surrounding earth be artificially removed, the arch immediately begins to straighten itself. This no doubt is due to growth along the whole inner surface of both legs of the arch; such growth being checked or prevented, as long as the two legs of the arch are firmly pressed together. When the earth is removed all round an arch and the two legs are tied together at their bases, the growth on the under side of the crown causes it after a time to become much flatter and broader than naturally occurs. The straightening process consists of a modified form of circumnutation, for the lines described during this process (as with the hypocotyl of Brassica, and the epicotyls of Vicia and Corylus) were often plainly zigzag and sometimes looped. After hypocotyls or epicotyls have emerged from the ground, they quickly become perfectly straight. No trace is left of their former abrupt curvature, excepting in the case of Allium cepa, in which the cotyledon rarely becomes quite straight, owing to the protuberance developed on the crown of the arch. The increased growth along the inner surface of the arch which renders it straight, apparently begins in the basal leg or that which is united to the radicle; for this leg, as we often observed, is first bowed backwards from the other leg. This movement facilitates the withdrawal of the tip of the epicotyl or of the cotyledons, as the case may be, from within the seed-coats and from the ground. But the cotyledons often emerge from the ground still tightly enclosed within the seed-coats, which apparently serve to protect them. The seed-coats are afterwards ruptured and cast off by the swelling of the closely conjoined cotyledons, and not by any movement or their separation from one another. Nevertheless, in some few cases, especially with the Cucurbitaceae, the seed-coats are ruptured by a curious contrivance, described by M. Flahault.* A heel or peg is developed on one side of the summit of the radicle or base of the hypocotyl; and this holds down the lower half of the seed-coats (the radicle being fixed into the ground) whilst the continued growth of the arched hypocotyl forced upwards the upper half, and tears asunder the seed-coats at one end, and the cotyledons are then easily withdrawn. Fig. 62. Cucurbita ovifera: germinating seed, showing the heel or peg projecting on one side from summit of radicle and holding down lower tip of seed-coats, which have been partially ruptured by the growth of the arched hypocotyl. The accompanying figure (Fig. 62) will render this description intelligible. Forty-one seeds of Cucurbita ovifera were laid on friable peat and were covered by a layer about an inch in thickness, not much pressed down, so that the cotyledons in being dragged up were subjected to very little friction, yet forty of them came up naked, the seed-coats being left buried in the peat. This was certainly due to the action of the peg, for when it was prevented from acting, the cotyledons, as we shall presently see, were lifted up still enclosed in their seed-coats. They were, however, cast off in the course of two or three days by the swelling of the cotyledons. Until this occurs light is excluded, and the cotyledons cannot decompose carbonic acid; but no one probably would have thought that the advantage thus gained by a little earlier casting * 'Bull. Soc. Bot. de France,' tom. xxiv. 1877, p. 201. off of the seed-coats would be sufficient to account for the development of the peg. Yet according to M. Flahault, seedlings which have been prevented from casting their seed-coats whilst beneath the ground, are inferior to those which have emerged with their cotyledons naked and ready to act. The peg is developed with extraordinary rapidity; for it could only just be distinguished in two seedlings, having radicles .35 inch in length, but after an interval of only 24 hours was well developed in both. It is formed, according to Flahault, by the enlargement of the layers of the cortical parenchyma at the base of the hypocotyl. If, however, we judge by the effects of a solution of permanganate of potassium, it is developed on the exact line of junction between the hypocotyl and radicle; for the flat lower surface, as well as the edges, were coloured brown like the radicle; whilst the upper slightly inclined surface was left uncoloured like the hypocotyl, excepting indeed in one out of 33 immersed seedlings in which a large part of the upper surface was coloured brown. Secondary roots sometimes spring from the lower surface of the peg, which thus seems in all respects to partake of the nature of the radicle. The peg is always developed on the side which becomes concave by the arching of the hypocotyl; and it would be of no service if it were formed on any other side. It is also always developed with the flat lower side, which, as just stated, forms a part of the radicle, at right angles to it, and in a horizontal plane. This fact was clearly shown by burying some of the thin flat seeds in the same position as in Fig. 62, excepting that they were not laid on their flat broad sides, but with one edge downwards. Nine seeds were thus planted, and the peg was developed in the same position, relatively to the radicle, as in the figure; consequently it did not rest on the flat tip of the lower half of the seed-coats, but was inserted like a wedge between the two tips. As the arched hypocotyl grew upwards it tended to draw up the whole seed, and the peg necessarily rubbed against both tips, but did not hold either down. The result was, that the cotyledons of five out of the nine seeds thus placed were raised above the ground still enclosed within their seed-coats. Four seeds were buried with the end from which the radicle protrudes pointing vertically downwards, and owing to the peg being always developed in the same position, its apex alone came into contact with, and rubbed against the tip on one side; the result was, that the cotyledons of all four emerged still within their seed-coats. These cases show us how the peg acts in co-ordination with the position which the flat, thin, broad seeds would almost always occupy when naturally sown. When the tip of the lower half of the seed-coats was cut off, Flahault found (as we did likewise) that the peg could not act, since it had nothing to press on, and the cotyledons were raised above the ground with their seed-coats not cast off. Lastly, nature shows us the use of the peg; for in the one Cucurbitaceous genus known to us, in which the cotyledons are hypogean and do not cast their seed-coats, namely, Megarrhiza, there is no vestige of a peg. This structure seems to be present in most of the other genera in the family, judging from Flahault's statements' we found it well-developed and properly acting in Trichosanthes anguina, in which we hardly expected to find it, as the cotyledons are somewhat thick and fleshy. Few cases can be advanced of a structure better adapted for a special purpose than the present one. With Mimosa pudica the radicle protrudes from a small hole in the sharp edge of the seed; and on its summit, where united with the hypocotyl, a transverse ridge is developed at an early age, which clearly aids in splitting the tough seed-coats; but it does not aid in casting them off, as this is subsequently effected by the swelling of the cotyledons after they have been raised above the ground. The ridge or heel therefore acts rather differently from that of Cucurbita. Its lower surface and the edges were coloured brown by the permanganate of potassium, but not the upper surface. It is a singular fact that after the ridge has done its work and has escaped from the seed-coats, it is developed into a frill all round the summit of the radicle.* At the base of the enlarged hypocotyl of Abronia umbellata, where it blends into the radicle, there is a projection or heel which varies in shape, but its outline is too angular in our former figure (Fig. 61). The radicle first protrudes from a small hole at one end of the tough, leathery, winged fruit. At this period the upper part of the radicle is packed within the fruit parallel to the hypocotyl, and the single cotyledon is doubled back parallel to the latter. The swelling of these three parts, and especially the rapid development of the thick heel between the hypocotyl and radicle at the point where they are doubled, ruptures the tough fruit at the upper end and allows the arched hypocotyl to emerge; and this seems to be the function of the heel. A seed was cut out of the fruit and * Our attention was called to this case by a brief statement by Nobbe in his 'Handbuch der Samenkunde,' 1876, p. 215, where a figure is also given of a seedling of Martynia with a heel or ridge at the junction of the radicle and hypocotyl. This seed possesses a very hard and tough coat, and would be likely to require aid in bursting and freeing the cotyledons. allowed to germinate in damp air, and now a thin flat disc was developed all round the base of the hypocotyl and grew to an extraordinary breadth, like the frill described under Mimosa, but somewhat broader. Flahault says that with Mirabilis, a member of the same family with Abronia, a heel or collar is developed all round the base of the hypocotyl, but more on one side than on the other; and that it frees the cotyledons from their seed-coats. We observed only old seeds, and these were ruptured by the absorption of moisture, independently of any aid from the heel and before the protrusion of the radicle; but it does not follow from our experience that fresh and tough fruits would behave in a like manner. In concluding this section of the present chapter it may be convenient to summarise, under the form of an illustration, the usual movements of the hypocotyls and epicotyls of seedlings, whilst breaking through the ground and immediately afterwards. We may suppose a man to be thrown down on his hands and knees, and at the same time to one side, by a load of hay falling on him. He would first endeavour to get his arched back upright, wriggling at the same time in all directions to free himself a little from the surrounding pressure; and this may represent the combined effects of apogeotropism and circumnutation, when a seed is so buried that the arched hypocotyl or epicotyl protrudes at first in a horizontal or inclined plane. The man, still wriggling, would then raise his arched back as high as he could; and this may represent the growth and continued circumnutation of an arched hypocotyl or epicotyl, before it has reached the surface of the ground. As soon as the man felt himself at all free, he would raise the upper part of his body, whilst still on his knees and still wriggling; and this may represent the bowing backwards of the basal leg of the arch, which in most cases aids in the withdrawal of the cotyledons from the buried and ruptured seed-coats, and the subsequent straightening of the whole hypocotyl or epicotyl--circumnutation still continuing. Circumnutation of Hypocotyls and Epicotyls, when erect.--The hypocotyls, epicotyls, and first shoots of the many seedlings observed by us, after they had become straight and erect, circumnutated continuously. The diversified figures described by them, often during two successive days, have been shown in the woodcuts in the last chapter. It should be recollected that the dots were joined by straight lines, so that the figures are angular; but if the observations had been made every few minutes the lines would have been more or less curvilinear, and irregular ellipses or ovals, or perhaps occasionally circles, would have been formed. The direction of the longer axes of the ellipses made during the same day or on successive days generally changed completely, so as to stand at right angles to one another. The number of irregular ellipses or circles made within a given time differs much with different species. Thus with Brassica oleracea, Cerinthe major, and Cucurbita ovifera about four such figures were completed in 12 h.; whereas with Solanum palinacanthum and Opuntia basilaris, scarcely more than one. The figures likewise differ greatly in size; thus they were very small and in some degree doubtful in Stapelia, and large in Brassica, etc. The ellipses described by Lathyrus nissolia and Brassica were narrow, whilst those made by the Oak were broad. The figures are often complicated by small loops and zigzag lines. As most seedling plants before the development of true leaves are of low, sometimes very low stature, the extreme amount of movement from side to side of their circumnutating stems was small; that of the hypocotyl of Githago segetum was about .2 of an inch, and that of Cucurbita ovifera about .28. A very young shoot of Lathyrus nissolia moved about .14, that of an American oak .2, that of the common nut only .04, and a rather tall shoot of the Asparagus .11 of an inch. The extreme amount of movement of the sheath-like cotyledon of Phalaris Canariensis was .3 of an inch; but it did not move very quickly, the tip crossing on one occasion five divisions of the micrometer, that is, 1/100th of an inch, in 22 m. 5 s. A seedling Nolana prostrata travelled the same distance in 10 m. 38 s. Seedling cabbages circumnutate much more quickly, for the tip of a cotyledon crossed 1/100th of an inch on the micrometer in 3 m. 20 s.; and this rapid movement, accompanied by incessant oscillations, was a wonderful spectacle when beheld under the microscope. The absence of light, for at least a day, does not interfere in the least with the circumnutation of the hypocotyls, epicotyls, or young shoots of the various dicotyledonous seedlings observed by us; nor with that of the young shoots of some monocotyledons. The circumnutation was indeed much plainer in darkness than in light, for if the light was at all lateral the stem bent towards it in a more or less zigzag course. Finally, the hypocotyls of many seedlings are drawn during the winter into the ground, or even beneath it so that they disappear. This remarkable process, which apparently serves for their protection, has been fully described by De Vries.* He shows that * 'Bot. Zeitung,' 1879, p. 649. See also Winkler in 'Verhandl. des Bot. Vereins der P. Brandenburg,' Jahrg. xvi. p. 16, as quoted by Haberlandt, 'Schutzeinrichungen der Keimpflanze,' 1877, p. 52. it is effected by the contraction of the parenchyma-cells of the root. But the hypocotyl itself in some cases contracts greatly, and although at first smooth becomes covered with zigzag ridges, as we observed with Githago segetum. How much of the drawing down and burying of the hypocotyl of Opuntia basilaris was due to the contraction of this part and how much to that of the radicle, we did not observe. Circumnutation of Cotyledons.--With all the dicotyledonous seedlings described in the last chapter, the cotyledons were in constant movement, chiefly in a vertical plane, and commonly once up and once down in the course of the 24 hours. But there were many exceptions to such simplicity of movement; thus the cotyledons of Ipomoea caerulea moved 13 times either upwards or downwards in the course of 16 h.. 18 m. Those of Oxalis rosea moved in the same manner 7 times in the course of 24 h.; and those of Cassia tora described 5 irregular ellipses in 9 h. The cotyledons of some individuals of Mimosa pudica and of Lotus Jacobaeus moved only once up and down in 24 h., whilst those of others performed within the same period an additional small oscillation. Thus with different species, and with different individuals of the same species, there were many gradations from a single diurnal movement to oscillations as complex as those of the Ipomoea and Cassia. The opposite cotyledons on the same seedling move to a certain extent independently of one another. This was conspicuous with those of Oxalis sensitiva, in which one cotyledon might be seen during the daytime rising up until it stood vertically, whilst the opposite one was sinking down. Although the movements of cotyledons were generally in nearly the same vertical plane, yet their upward and downward courses never exactly coincided; so that ellipses, more or less narrow, were described, and the cotyledons may safely be said to have circumnutated. Nor could this fact be accounted for by the mere increase in length of the cotyledons through growth, for this by itself would not induce any lateral movement. That there was lateral movement in some instances, as with the cotyledons of the cabbage, was evident; for these, besides moving up and down, changed their course from right to left 12 times in 14 h. 15 m. With Solanum lycopersicum the cotyledons, after falling in the forenoon, zigzagged from side to side between 12 and 4 P.M., and then commenced rising. The cotyledons of Lupinus luteus are so thick (about .08 of an inch) and fleshy,* that they seemed little likely to move, and were therefore observed with especial interest; they certainly moved largely up and down, and as the line traced was zigzag there was some lateral movement. The nine cotyledons of a seedling Pinus pinaster plainly circumnutated; and the figures described approached more nearly to irregular circles than to irregular ovals or ellipses. The sheath-like cotyledons of the Gramineae circumnutate, that is, move to all sides, as plainly as do the hypocotyls or epicotyls of any dicotyledonous plants. Lastly, the very young fronds of a Fern and of a Selaginella circumnutated. In a large majority of the cases which were carefully observed, the cotyledons sink a little downwards in the forenoon, and rise a little in the afternoon or evening. They thus stand rather more highly inclined during the night than during the mid-day, at which * The cotyledons, though bright green, resemble to a certain extent hypogean ones; see the interesting discussion by Haberlandt ('Die Schutzeinrichtungen,' etc., 1877, p. 95), on the gradations in the Leguminosae between subaërial and subterranean cotyledons. time they are expanded almost horizontally. The circumnutating movement is thus at least partially periodic, no doubt in connection, as we shall hereafter see, with the daily alternations of light and darkness. The cotyledons of several plants move up so much at night as to stand nearly or quite vertically; and in this latter case they come into close contact with one another. On the other hand, the cotyledons of a few plants sink almost or quite vertically down at night; and in this latter case they clasp the upper part of the hypocotyl. In the same genus Oxalis the cotyledons of certain species stand vertically up, and those of other species vertically down, at night. In all such cases the cotyledons may be said to sleep, for they act in the same manner as do the leaves of many sleeping plants. This is a movement for a special purpose, and will therefore be considered in a future chapter devoted to this subject. In order to gain some rude notion of the proportional number of cases in which the cotyledons of dicotyledonous plants (hypogean ones being of course excluded) changed their position in a conspicuous manner at night, one or more species in several genera were cursorily observed, besides those described in the last chapter. Altogether 153 genera, included in as many families as could be procured, were thus observed by us. The cotyledons were looked at in the middle of the day and again at night; and those were noted as sleeping which stood either vertically or at an angle of at least 60o above or beneath the horizon. Of such genera there were 26; and in 21 of them the cotyledons of some of the species rose, and in only 6 sank at night; and some of these latter cases are rather doubtful from causes to be explained in the chapter on the sleep of cotyledons. When cotyledons which at noon were nearly horizontal, stood at night at more than 20o and less than 60o above the horizon, they were recorded as "plainly raised;" and of such genera there were 38. We did not meet with any distinct instances of cotyledons periodically sinking only a few degrees at night, although no doubt such occur. We have now accounted for 64 genera out of the 153, and there remain 89 in which the cotyledons did not change their position at night by as much as 20o--that is, in a conspicuous manner which could easily be detected by the unaided eye and by memory; but it must not be inferred from this statement that these cotyledons did not move at all, for in several cases a rise of a few degrees was recorded, when they were carefully observed. The number 89 might have been a little increased, for the cotyledons remained almost horizontal at night in some species in a few genera, for instance, Trifolium and Geranium, which are included amongst the sleepers, such genera might therefore have been added to the 89. Again, one species of Oxalis generally raised its cotyledons at night more than 20o and less than 60o above the horizon; so that this genus might have been included under two heads. But as several species in the same genus were not often observed, such double entries have been avoided. In a future chapter it will be shown that the leaves of many plants which do not sleep, rise a few degrees in the evening and during the early part of the night; and it will be convenient to defer until then the consideration of the periodicity of the movements of cotyledons. On the Pulvini or Joints of Cotyledons.--With several of the seedlings described in this and the last chapter, the summit of the petiole is developed into a pulvinus, cushion, or joint (as this organ has been variously called), like that with which many leaves are provided. It consists of a mass of small cells usually of a pale colour from the absence of chlorophyll, and with its outline more or less convex, as shown in the annexed figure. In the case of Oxalis sensitiva two-thirds of the petiole, and in that of Mimosa pudica, apparently the whole of the short sub-petioles of the leaflets have been converted into pulvini. With pulvinated leaves (i.e. those provided with a pulvinus) their periodical movements depend, according to Pfeffer,* on the cells of the pulvinus alternately expanding more quickly on one side than on the other; whereas the similar movements of leaves not provided with pulvini, depend on their growth being alternately more rapid on one side than on the other.** As long as a leaf provided with a pulvinus is young and continues to grow, its movement depends on both these causes combined;*** and if the view now held by many botanists be sound, namely, that growth is always preceded by the expansion of the growing cells, then the difference between the movements induced by the aid of pulvini and Fig. 63. Oxalis rosea: longitudinal section of a pulvinus on the summit of the petiole of a cotyledon, drawn with the camera lucida, magnified 75 times: p, p, petiole; f, fibro-vascular bundle: b, b, commencement of blade of cotyledon. without such aid, is reduced to the expansion of the cells not being followed by growth in the first case, and being so followed in the second case. Dots were made with Indian ink along the midrib of both pulvinated cotyledons of a rather old seedling of Oxalis Valdiviana; their distances were repeatedly measured with an eye-piece micrometer during 8 3/4 days, and they did not exhibit the least trace of increase. It is therefore almost certain that the pulvinus itself was not then growing. Nevertheless, during this whole time and for ten days afterwards, these cotyledons rose vertically every night. In the case of some seedlings raised from seeds purchased under the name of Oxalis floribunda, the cotyledons continued for a long time to move vertically down at night, and the movement apparently depended exclusively on the pulvini, for their petioles were of nearly the same length in young, and in old seedlings which had produced true leaves. With some species of Cassia, on the other hand, it was obvious without any measurement that the pulvinated cotyledons continued to increase greatly in length during some weeks; so that here the expansion of the cells of the pulvini and the growth of the petiole were probably combined in causing their prolonged periodic movements. It was equally evident that the cotyledons of many plants, not provided with pulvini, increased rapidly in length; and their periodic movements no doubt were exclusively due to growth. In accordance with the view that the periodic movements of all cotyledons depend primarily on the expansion of the cells, whether or not followed by growth, we can understand the fact that there is but little difference in the kind or form of movement in the two sets of cases. This may be seen by comparing the diagrams given in the last chapter. Thus the movements of the cotyledons of Brassica oleracea and of Ipomoea caerulea, which are not provided with pulvini, are as complex as those of Oxalis and Cassia which are thus provided. The pulvinated cotyledons of some individuals of Mimosa pudica and Lotus Jacobaeus made only a single oscillation, whilst those of other individuals moved twice up and down in the course of 24 hours; so it was occasionally with the cotyledons of Cucurbita ovifera, which are destitute of a pulvinus. The movements of pulvinated cotyledons are generally larger in extent than those without a pulvinus; nevertheless some of the latter moved through an angle of 90o. There is, however, one important difference in the two sets of cases; the nocturnal movements of cotyledons without pulvini, for instance, those in the Cruciferae, Cucurbitaceae, Githago, and Beta, never last even for a week, to any conspicuous degree. Pulvinated cotyledons, on the other hand, continue to rise at night for a much longer period, even for more than a month, as we shall now show. But the period no doubt depends largely on the temperature to which the seedlings are exposed and their consequent rate of development.
Trifolium is a natural genus, and the leaves of all * Pfeffer, 'Die Period. Bewegungen,' 1875, p. 157. the species seen by us are pulvinated; so it is with the cotyledons of T. subterraneum and strictum, which stand vertically at night; whereas those of T. resupinatum exhibit not a trace of a pulvinus, nor of any nocturnal movement. This was ascertained by measuring the distance between the tips of the cotyledons of four seedlings at mid-day and at night. In this species, however, as in the others, the first-formed leaf, which is simple or not trifoliate, rises up and sleeps like the terminal leaflet on a mature plant. In another natural genus, Oxalis, the cotyledons of O. Valdiviana, rosea, floribunda, articulata, and sensitiva are pulvinated, and all move at night into an upward or downward vertical position. In these several species the pulvinus is seated close to the blade of the cotyledon, as is the usual rule with most plants. Oxalis corniculata (var. Atro-purpurea) differs in several respects; the cotyledons rise at night to a very variable amount, rarely more than 45o; and in one lot of seedlings (purchased under the name of O. tropaeoloides, but certainly belonging to the above variety) they rose only from 5o to 15o above the horizon. The pulvinus is developed imperfectly and to an extremely variable degree, so that apparently it is tending towards abortion. No such case has hitherto, we believe, been described. It is coloured green from its cells containing chlorophyll; and it is seated nearly in the middle of the petiole, instead of at the upper end as in all the other species. The nocturnal movement is effected partly by its aid, and partly by the growth of the upper part of the petiole as in the case of plants destitute of a pulvinus. From these several reasons and from our having partially traced the development of the pulvinus from an early age, the case seems worth describing in some detail. [When the cotyledons of O. corniculata were dissected out of a seed from which they would soon have naturally emerged, no trace of a pulvinus could be detected; and all the cells forming the short petiole, 7 in number in a longitudinal row, were of nearly equal size. In seedlings one or two days old, the pulvinus was so indistinct that we thought at first that it did not exist; but in the middle of the petiole an ill-defined transverse zone of cells could be seen, which were much shorter than those both above and below, although of the same breadth with them. They presented the appearance of having been just formed by the transverse division of longer cells; and there can be little doubt that this had occurred, for the cells in the petiole which had Fig. 64. Oxalis corniculata: A and B the almost rudimentary pulvini of the cotyledons of two rather old seedlings, viewed as transparent objects. Magnified 50 times. been dissected out of the seed averaged in length 7 divisions of the micrometer (each division equalling .003 mm.), and were a little longer than those forming a well-developed pulvinus, which varied between 4 and 6 of these same divisions. After a few additional days the ill-defined zone of cells becomes distinct, and although it does not extend across the whole width of the petiole, and although the cells are of a green colour from containing chlorophyll, yet they certainly constitute a pulvinus, which as we shall presently see, acts as one. These small cells were arranged in longitudinal rows, and varied from 4 to 7 in number; and the cells themselves varied in length in different parts of the same pulvinus and in different individuals. In the accompanying figures, A and B (Fig. 64), we have views of the epidermis* in the middle part of the petioles of two seedlings, in which the pulvinus was for this species well developed. They offer a striking contrast with the pulvinus of O. rosea (see former Fig. 63), or of O. Valdiviana. With the seedlings, falsely called O. tropaeoloides, the cotyledons of which rise very little at night, the small cells were still fewer in number and in parts formed a single transverse row, and in other parts short longitudinal rows of only two or three. Nevertheless they sufficed to attract the eye, when the whole petiole was viewed as a transparent object beneath the microscope. In these seedlings there could hardly be a doubt that the pulvinus was becoming rudimentary and tending to disappear; and this accounts for its great variability in structure and function. In the following Table some measurements of the cells in fairly well-developed pulvini of O. corniculata are given:--
* Longitudinal sections show that the forms of the epidermic cells may be taken as a fair representation of those constituting the pulvinus. We here see that the cells of the pulvinus increase but little in length with advancing age, in comparison with those of the petiole both above and below it; but they continue to grow in width, and keep equal in this respect with the other cells of the petiole. The rate of growth, however, varies in all parts of the cotyledons, as may be observed in the measurements of the 8-days' old seedling. The cotyledons of seedlings only a day old rise at night considerably, sometimes as much as afterwards; but there was much variation in this respect. As the pulvinus is so indistinct at first, the movement probably does not then depend on the expansion of its cells, but on periodically unequal growth in the petiole. By the comparison of seedlings of different known ages, it was evident that the chief seat of growth of the petiole was in the upper part between the pulvinus and the blade; and this agrees with the fact (shown in the measurements above given) that the cells grow to a greater length in the upper than in the lower part. With a seedling 11 days old, the nocturnal rise was found to depend largely on the action of the pulvinus, for the petiole at night was curved upwards at this point; and during the day, whilst the petiole was horizontal, the lower surface of the pulvinus was wrinkled with the upper surface tense. Although the cotyledons at an advanced age do not rise at night to a higher inclination than whilst young, yet they have to pass through a larger angle (in one instance amounting to 63o) to gain their nocturnal position, as they are generally deflected beneath the horizon during the day. Even with the 11-days' old seedling the movement did not depend exclusively on the pulvinus, for the blade where joined to the petiole was curved upwards, and this must be attributed to unequal growth. Therefore the periodic movements of the cotyledons of 'O. corniculata' depend on two distinct but conjoint actions, namely, the expansion of the cells of the pulvinus and on the growth of the upper part of the petiole, including the base of the blade.
Turning now to very young seedlings, the cotyledons of which do not rise at night and are only from 2 to 2 ½ mm. in length, their petioles do not exhibit any defined zone of small cells, destitute of chlorophyll and differing in shape exteriorly from the lower ones. Nevertheless, the cells at the place where a pulvinus will afterwards be developed are smaller (being on an average .015 mm. in length) than those in the lower parts of the same petiole, which gradually become larger in proceeding downwards, the largest being .030 mm. in length. At this early age the cells of the blade are about .027 mm. in length. We thus see that the pulvinus is formed by the cells in the uppermost part of the petiole, continuing for only a short time to increase in length, then being arrested in their growth, accompanied by the loss of their chlorophyll grains; whilst the cells in the lower part of the petiole continue for a long time to increase in length, those of the epidermis becoming more conical. The singular fact of the cotyledons of this plant not sleeping at first is therefore due to the pulvinus not being developed at an early age.] We learn from these two cases of Lotus and Oxalis, that the development of a pulvinus follows from the growth of the cells over a small defined space of the petiole being almost arrested at an early age. With Lotus Jacobaeus the cells at first increase a little in length; in Oxalis corniculata they decrease a little, owing to self-division. A mass of such small cells forming a pulvinus, might therefore be either acquired or lost without any special difficulty, by different species in the same natural genus: and we know that with seedlings of Trifolium, Lotus, and Oxalis some of the species have a well-developed pulvinus, and others have none, or one in a rudimentary condition. As the movements caused by the alternate turgescence of the cells in the two halves of a pulvinus, must be largely determined by the extensibility and subsequent contraction of their walls, we can perhaps understand why a large number of small cells will be more efficient than a small number of large cells occupying the same space. As a pulvinus is formed by the arrestment of the growth of its cells, movements dependent on their action may be long-continued without any increase in length of the part thus provided; and such long-continued movements seem to be one chief end gained by the development of a pulvinus. Long-continued movement would be impossible in any part, without an inordinate increase in its length, if the turgescence of the cells was always followed by growth. Disturbance of the Periodic Movements of Cotyledons by Light.--The hypocotyls and cotyledons of most seedling plants are, as is well known, extremely heliotropic; but cotyledons, besides being heliotropic, are affected paratonically (to use Sachs' expression) by light; that is, their daily periodic movements are greatly and quickly disturbed by changes in its intensity or by its absence. It is not that they cease to circumnutate in darkness, for in all the many cases observed by us they continued to do so; but the normal order of their movements in relation to the alternations of day and night is much disturbed or quite annulled. This holds good with species the cotyledons of which rise or sink so much at night that they may be said to sleep, as well as with others which rise only a little. But different species are affected in very different degrees by changes in the light. [For instance, the cotyledons of Beta vulgaris, Solanum lycopersicum, Cerinthe major, and Lupinus luteus, when placed in darkness, moved down during the afternoon and early night, instead of rising as they would have done if they had been exposed to the light. All the individuals of the Solanum did not behave in the same manner, for the cotyledons of one circumnutated about the same spot between 2.30 and 10 P.M. The cotyledons of a seedling of Oxalis corniculata, which was feebly illuminated from above, moved downwards during the first morning in the normal manner, but on the second morning it moved upwards. The cotyledons of Lotus Jacoboeus were not affected by 4 h. of complete darkness, but when placed under a double skylight and thus feebly illuminated, they quite lost their periodical movements on the third morning. On the other hand, the cotyledons of Cucurbita ovifera moved in the normal manner during a whole day in darkness. Seedlings of Githago segetum were feebly illuminated from above in the morning before their cotyledons had expanded, and they remained closed for the next 40 h. Other seedlings were placed in the dark after their cotyledons had opened in the morning and these did not begin to close until about 4 h. had elapsed. The cotyledons of Oxalis rosea sank vertically downwards after being left for 1 h. 20 m. in darkness; but those of some other species of Oxalis were not affected by several hours of darkness. The cotyledons of several species of Cassia are eminently susceptible to changes in the degree of light to which they are exposed: thus seedlings of an unnamed S. Brazilian species (a large and beautiful tree) were brought out of the hot-house and placed on a table in the middle of a room with two north-east and one north-west window, so that they were fairly well illuminated, though of course less so than in the hot-house, the day being moderately bright; and after 36 m. the cotyledons which had been horizontal rose up vertically and closed together as when asleep; after thus remaining on the table for 1 h. 13 m. they began to open. The cotyledons of young seedlings of another Brazilian species and of C. neglecta, treated in the same manner, behaved similarly, excepting that they did not rise up quite so much: they again became horizontal after about an hour. Here is a more interesting case: seedlings of Cassia tora in two pots, which had stood for some time on the table in the room just described, had their cotyledons horizontal. One pot was now exposed for 2 h. to dull sunshine, and the cotyledons remained horizontal; it was then brought back to the table, and after 50 m. the cotyledons had risen 68o above the horizon. The other pot was placed during the same 2 h. behind a screen in the room, where the light was very obscure, and the cotyledons rose 63o above the horizon; the pot was then replaced on the table, and after 50 m. the cotyledons had fallen 33o. These two pots with seedlings of the same age stood close together, and were exposed to exactly the same amount of light, yet the cotyledons in the one pot were rising, whilst those in the other pot were at the same time sinking. This fact illustrates in a striking manner that their movements are not governed by the actual amount, but by a change in the intensity or degree of the light. A similar experiment was tried with two sets of seedlings, both exposed to a dull light, but different in degree, and the result was the same. The movements of the cotyledons of this Cassia are, however, determined (as in many other cases) largely by habit or inheritance, independently of light; for seedlings which had been moderately illuminated during the day, were kept all night and on the following morning in complete darkness; yet the cotyledons were partially open in the morning and remained open in the dark for about 6 h. The cotyledons in another pot, similarly treated on another occasion, were open at 7 A.M. and remained open in the dark for 4 h. 30 m., after which time they began to close. Yet these same seedlings, when brought in the middle of the day from a moderately bright into only a moderately dull light raised, as we have seen, their cotyledons high above the horizon. Sensitiveness of Cotyledons to contact.--This subject does not possess much interest, as it is not known that sensitiveness of this kind is of any service to seedling plants. We have observed cases in only four genera, though we have vainly observed the cotyledons of many others. The genus cassia seems to be pre-eminent in this respect: thus, the cotyledons of C. tora, when extended horizontally, were both lightly tapped with a very thin twig for 3 m. and in the course of a few minutes they formed together an angle of 90o, so that each had risen 45o. A single cotyledon of another seedling was tapped in a like manner for 1 m., and it rose 27o in 9 m.; and after eight additional minutes it had risen 10o more; the opposite cotyledon, which was not tapped, hardly moved at all. The cotyledons in all these cases became horizontal again in less than half an hour. The pulvinus is the most sensitive part, for on slightly pricking three cotyledons with a pin in this part, they rose up vertically; but the blade was found also to be sensitive, care having been taken that the pulvinus was not touched. Drops of water placed quietly on these cotyledons produced no effect, but an extremely fine stream of water, ejected from a syringe, caused them to move upwards. When a pot of seedlings was rapidly hit with a stick and thus jarred, the cotyledons rose slightly. When a minute drop of nitric acid was placed on both pulvini of a seedling, the cotyledons rose so quickly that they could easily be seen to move, and almost immediately afterwards they began to fall; but the pulvini had been killed and became brown. The cotyledons of an unnamed species of Cassia (a large tree from S. Brazil) rose 31o in the course of 26 m. after the pulvini and the blades had both been rubbed during 1 m. with a twig; but when the blade alone was similarly rubbed the cotyledons rose only 8o. The remarkably long and narrow cotyledons, of a third unnamed species from S. Brazil, did not move when their blades were rubbed on six occasions with a pointed stick for 30 s. or for 1 m.; but when the pulvinus was rubbed and slightly pricked with a pin, the cotyledons rose in the course of a few minutes through an angle of 60o. Several cotyledons of C. neglecta (likewise from S. Brazil) rose in from 5 m. to 15 m. to various angles between 16o and 34o, after being rubbed during 1 m. with a twig. Their sensitiveness is retained to a somewhat advanced age, for the cotyledons of a little plant of C. neglecta, 34 days old and bearing three true leaves, rose when lightly pinched between the finger and thumb. Some seedlings were exposed for 30 m. to a wind (temp. 50o F.) sufficiently strong to keep the cotyledons vibrating, but this to our surprise did not cause any movement. The cotyledons of four seedlings of the Indian C. glauca were either rubbed with a thin twig for 2 m. or were lightly pinched: one rose 34o; a second only 6o; a third 13o; and a fourth 17o. A cotyledon of C. florida similarly treated rose 9o; one of C. corymbosa rose 7 1/2o, and one of the very distinct C. mimosoides only 6o. Those of C. pubescens did not appear to be in the least sensitive; nor were those of C. nodosa, but these latter are rather thick and fleshy, and do not rise at night or go to sleep. Smithia sensitiva.--This plant belongs to a distinct sub-order of the Leguminosae from Cassia. Both cotyledons of an oldish seedling, with the first true leaf partially unfolded, were rubbed for 1 m. with a fine twig, and in 5 m. each rose 32o; they remained in this position for 15 m., but when looked at again 40 m. after the rubbing, each had fallen 14o. Both cotyledons of another and younger seedling were lightly rubbed in the same manner for 1 m., and after an interval of 32 m. each had risen 30o. They were hardly at all sensitive to a fine jet of water. The cotyledons of S. Pfundii, an African water plant, are thick and fleshy; they are not sensitive and do not go to sleep. Mimosa pudica and albida.--The blades of several cotyledons of both these plants were rubbed or slightly scratched with a needle during 1 m. or 2 m.; but they did not move in the least. When, however, the pulvini of six cotyledons of M. pudica were thus scratched, two of them were slightly raised. In these two cases perhaps the pulvinus was accidentally pricked, for on pricking the pulvinus of another cotyledon it rose a little. It thus appears that the cotyledons of Mimosa are less sensitive than those of the previously mentioned plants.* Oxalis sensitiva.--The blades and pulvini of two cotyledons, standing horizontally, were rubbed or rather tickled for 30 s. with a fine split bristle, and in 10 m. each had risen 48o; when looked at again in 35 m. after being rubbed they had risen 4o more; after 30 additional minutes they were again horizontal. On hitting a pot rapidly with a stick for 1 m., the cotyledons of two seedlings were considerably raised in the course of 11 m. A pot was carried a little distance on a tray and thus jolted; and the cotyledons of four seedlings were all raised in 10 m.; after 17 m. one had risen 56o, a second 45o, a third almost 90o, and a fourth 90o. After an additional interval of 40 m. three of them had re-expanded to a considerable extent. These observations were made before we were aware at what an extraordinarily rapid rate the cotyledons circumnutate, and are therefore liable to error. Nevertheless it is extremely improbable that the cotyledons in the eight cases given, should all have been rising at the time when they were irritated. The cotyledons of Oxalis Valdiviana and rosea were rubbed and did not exhibit any sensitiveness.] Finally, there seems to exist some relation between * The sole notice which we have met with on the sensitiveness of cotyledons, relates to Mimosa; for Aug. P. De Candolle says ('Phys. Vég.,' 1832, tom. ii. p. 865), "les cotyledons du M. pudica tendent à se raprocher par leurs faces supérieures lorsqu'on les irrite." the habit of cotyledons rising vertically at night or going to sleep, and their sensitiveness, especially that of their pulvini, to a touch; for all the above-named plants sleep at night. On the other hand, there are many plants the cotyledons of which sleep, and are not in the least sensitive. As the cotyledons of several species of Cassia are easily affected both by slightly diminished light and by contact, we thought that these two kinds of sensitiveness might be connected; but this is not necessarily the case, for the cotyledons of Oxalis sensitiva did not rise when kept on one occasion for 1 ½ h., and on a second occasion for nearly 4 h., in a dark closet. Some other cotyledons, as those of Githago segetum, are much affected by a feeble light, but do not move when scratched by a needle. That with the same plant there is some relation between the sensitiveness of its cotyledons and leaves seems highly probable, for the above described Smithia and Oxalis have been called sensitiva, owing to their leaves being sensitive; and though the leaves of the several species of Cassia are not sensitive to a touch, yet if a branch be shaken or syringed with water, they partially assume their nocturnal dependent position. But the relation between the sensitiveness to contact of the cotyledons and of the leaves of the same plant is not very close, as may be inferred from the cotyledons of Mimosa pudica being only slightly sensitive, whilst the leaves are well known to be so in the highest degree. Again, the leaves of Neptunia oleracea are very sensitive to a touch, whilst the cotyledons do not appear to be so in any degree. _ |
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