Picturesque Experiments

________________________________________________
Title:     Picturesque Experiments
Author: Francis Darwin [More Titles by Darwin]

To those who have never made experiments on plants it may seem that 'picturesque' is an odd term to apply to laboratory methods. But to an experimentalist the adjective does not seem overstrained. There is not merely the pleasure of seeing a prediction verified—that may be experienced in more everyday matters. There is a peculiar delight in the discovery of a method of revealing some detail in the natural history of living things. I remember vividly the pleasure which I felt when I first tried the experiment on Sorghum, described in the essay on the Movements of Plants in this volume. {210} I hoped that the seedlings would curve in the elaborate manner shown in Fig. 4. But I had so little expectation of success that I did not explain the object of the trial to my laboratory assistant, and it came as a shock of delight when he told me that the seedlings had "curled up like corkscrews." I do not think that it is an exaggeration to say, that this result is a picturesque illustration of the distribution of gravitational sensitiveness in plants. The instances in the present essay are not concerned with the movements of plants, and are so far less interesting, but I think the reader will not refuse them the same adjective.

We all know that in plants—from the smallest weed to the giant trees of America—there flows a stream of water from the root to the topmost leaf. Nevertheless, it is an experience to have ocular proof of this life-giving current. A branch of laurel is so arranged that it has to suck up the water it needs through a coarse thermometer tube, dipping into a beaker. The laurel does not wither, and we know therefore that it is continuously supplied with water. If the beaker is removed we shall see the absorption, for the thermometer tube does not remain full of water; a minute column of air is seen at its lower end which rapidly increases in size, and finally when the tube is emptied of its water-content, bubbles of air escape one after another into the larger tube, which contains the cut end of the branch. This, the simplest possible experiment, is nevertheless a vivid ocular proof of the laurel's power of absorbing water. It can be shown that the sucking power of the branch depends on its leaves, for if these are removed the rate of the current is very greatly diminished. It can also be proved that it depends on some quality of the leaf surface, for if a new specimen is taken, and if the lower sides of its leaves are rubbed with vaseline, the rate of absorption will be seen to diminish very greatly. Greasing the upper surface of the leaves does not produce this result, and when we examine the two surfaces it is found that the lower one is riddled with innumerable microscopic holes (stomata), while the upper side of the leaf has no such apertures. The stomata in fact are the arbiters of what shall pass in or out of the body of the leaf; they are the gate-keepers who regulate both export and import. They are known by actual inspection (with a microscope) to close at night: the result of this is that the evaporation of the leaves is much slower at night, and this is true when allowance has been made for the fact that evaporation is also checked at night by the dampness of the air.

[Picture: Fig. 7. The Porometer]

The microscopical inspection of stomata is not a completely satisfactory method of discovering to what degree they are open. It has, however, been my good fortune to resuscitate and simplify a method of studying the stomatal condition. The method was many years ago tried in a hopelessly cumbersome form by a German, but never came into use. My apparatus is described in the Proceedings of the Royal Society, {212} and is known as the Porometer. Its essential part is shown in Fig. 7. It consists of a funnel-shaped tube, having a broad flange, which is cemented on to the stomata-bearing surface of a leaf. The leaf is represented by the obliquely shaded object and is enormously magnified. To the upper orifice of the funnel is fixed a rubber tube, and by means of it steady suction can be supplied. The result is that a current of air is drawn through the stomata into the leaf, and then out of the leaf into the cavity of the porometer. The rate of this current is an index of the degree to which the stomata are open. With this apparatus a number of interesting points can be determined.

[Picture: Fig. 8. Curve of Porometer readings in light and darkness (black)]

Fig. 8 shows the effect of alternate periods of light and darkness. The fall of the curve represents partial closure, and is seen to occur in the periods of darkness (black), and to rise when the plant is re-illumined. These changes are necessarily accompanied by rise and fall in the evaporation of the leaf, but into the question of the accuracy of this correlation I shall not enter.

There are other methods of demonstrating the movements of the stomata. Stahl had the happy inspiration of making use of the colour-changes of cobalt chloride. A piece of filter paper soaked in a 5 p.c. solution of this salt is blue when dried, and turns pink in damp air. A dry piece of this material, applied with proper precaution to the stomata-bearing surface of a leaf, rapidly changes to pink if the stomata are open. When, however, the same trial is made on the upper surface of a leaf, where stomata do not occur, no such change occurs. If two leaves are treated at the same time, one in the normal position and the other upside down, it is delightful to watch the appearance of a pink picture of that leaf whose stomatic surface is in contact with the paper, while no such change takes place over that which exposes no stomata to the tell-tale material. Another method was discovered by the accident of finding in an old house in Wales a Chinese figure of a man, cut out of a thin shaving of horn, which writhed and twisted when placed on the hand. It was clearly very sensitive to moisture, and it seemed possible that horn-shavings might be used to test the condition of the stomata. The first difficulty was to obtain a supply of this material. Having discovered from the P.O. Directory that there were two horn-pressers in London I proceeded to visit one of them somewhere in Hoxton. He turned out to be of a highly suspicious disposition, but his wife had more discernment, and persuaded him that I was a harmless customer, with no designs on trade secrets, and I finally obtained what I wanted. A delicate strip of horn was fixed to a little block of cork and placed on a leaf, and to my delight showed the stomata to be open by violently curving upwards. It was only necessary to fix a graduated arc to the cork, and to fasten a delicate hair on to the horn so as to serve as index. The instrument is not of course accurately quantitative, but it does at least show whether the stomata are nearly shut, moderately open, or widely so. Rough as it is I found it good enough for determining a number of interesting facts in the physiology of stomata. {215a}

I now pass on to a different subject, the all-important process on which the life of green plants depends, an act therefore by which our own existence and that of all other animals is conditioned. I mean the process known as assimilation. This is the truly miraculous feat of using as a source of food the carbonic acid gas (CO2) which exists in minute quantities in the atmosphere. The plant is in fact a carbon-catching machine, and the machine is driven by the energy of the sun, and can therefore only work in light. The eminent Russian botanist, Timiriazeff, in a lecture on this subject {215b} before the Royal Society, made a witty use of Gulliver's Travels—a book not commonly quoted as an authority in scientific matters. He pointed out that the philosophers of Lagado, who were extracting sun-beams from cucumbers, were not doing anything absurd. On the contrary, since the cucumbers had been built with the help of sunshine, it was a reasonable expectation that energy corresponding to the sunshine should be obtainable. This indeed is what we do when we drive a steam engine by burning coal which ages ago was built by vegetable machinery driven by sunlight.

It is possible to show the existence of this process by very simple experiments. The most direct, but the least interesting, experiment is to take two similar plants, and expose plant A to an atmosphere containing CO2 while B is in air freed from that gas. Both specimens are placed in bright light, and after a sufficient interval of time their leaves are tested for the presence of starch. This is a simple matter; the green colouring matter is washed out of them by means of alcohol, and they are then placed in a dilute solution of iodine, which has the property of staining starch purple. It is always pleasant to see the leaf that had been supplied with CO2 turn blue, while the starved leaf remains a hungry yellow.

Some of the prettiest methods of demonstrating this process depend, not on the manufacture of starch in the leaf, but on the fact that an assimilating plant sets free oxygen, by breaking up the molecule CO2, building the carbon (C) into its own tissues, and letting the oxygen (O) go free. A beautiful method was discovered on these lines by Engelmann, which I was never tired of seeing year after year in my Cambridge class. Defibrinated bullock's blood is freed from air by means of an air pump and charged with CO2. In the course of this process it acquires the dingy tint of venous blood. A single leaf of the American weed (Elodea) is mounted on a glass slide in a drop of this blood and covered by an ordinary cover slip. Then comes the dramatic moment: the preparation is exposed to sunshine, and in 3 or 4 minutes a delicate scarlet border begins to appear round the leaf and grows rapidly, making a curious sunset effect in contrast with dingy purple of the venous blood. The meaning is very clear; the Elodea leaf in sunshine took the carbon from the CO2, and the oxygen thus set free gave the venous blood the scarlet hue characteristic of the arterial condition. Professor Farmer has designed a striking method based on another well-known experiment of Engelmann's. A drop of water containing the products of decay, and therefore swarming with bacteria, supplies the test. A drop of this fluid is placed on a glass slip, one or two delicate leaves of a green water plant (Elodea) are added, and a square of thin glass is placed on it. Round the edges of the cover-slip the preparation must be sealed with a preparation of wax, which melts at a low temperature, and when cold serves to prevent the preparation drying; it also isolates it from the surrounding atmosphere. After making sure under the microscope that the bacteria are in active movement, the glass slip is placed in the dark for some 3 or 4 hours. It is then examined, and the bacteria will be found to have ceased to move because they and the leaves between them have consumed the oxygen dissolved in the water, and bacterial activity being dependent on oxygen naturally came to an end. The preparation is placed under the microscope and illumined with bright incandescent gas, and after a short time the bacteria begin to stir and are soon once more whirling in their insensate dance. The reason is obvious—the green leaves under the influence of light were able to seize the carbon from the CO2, and the O thus set free put the bacteria in motion. The bacterial dance is therefore evidence of the act of assimilation carried on by the Elodea leaf.

Yet another method is worth mention, viz., that of Boussingault. The plant is placed in an inverted glass vessel resting in a dish of water, and is filled with hydrogen mixed with a percentage of CO2. Inside the vessel a fragment of phosphorus is suspended, and as a small amount of oxygen is sure to be mixed with the hydrogen the phosphorus will be oxygenated and white fumes will fill the vessel. The observer must wait until these clouds have subsided, which may need a couple of hours. This must take place in the dark, and as soon as the atmosphere is clear, the whole preparation is placed in bright light, when obvious clouds will again appear—a proof that oxygen has been set free by the assimilation of the green plants. With this example I must bring my short series of experiments to a close, with the hope that my readers may not deny that they are picturesque.

NOTES:

{210} See p. 50.

{212} A new method of estimating the aperture of stomata. B., Vol. 84, 1911.

{215a} Phil. Trans., B. vol 190, 1898.

{215b} See above, p. 136.

[The end]
Francis Darwin's essay: Picturesque Experiments

________________________________________________



GO TO TOP OF SCREEN