Rustic Sounds, and Other Studies in Literature and Natural History

by Sir Francis Darwin

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.

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.