Page:Popular Science Monthly Volume 6.djvu/73

Rh connected with an aspirator, which causes the air in contact with the plant to pass gently over the baryta solution. For instance, take a freshly-opened bud of the chestnut, and presently, or at least after a very little while, there is seen to form, in the daylight, a deposit of carbonate of baryta, and this increases very rapidly. Of course, care must be taken to deprive the air of its carbonic acid before it is admitted into the bell-glass.

A very simple experiment will make it plain that, in the course of this first period, the nascent leaves absorb to an appreciable extent the oxygen of the air both day and night. We have only to place the plant in a small bell-glass containing common air, the mouth of the vessel being stopped by means of a solution of caustic potash in a saucer. Soon we observe the solution rising in the bell-glass, and standing still at a certain point, which it never goes beyond. (Care must be taken not to allow the alkaline liquid to touch the petioles of the leaves.) If we now examine the elastic fluid which remains unabsorbed, we find that it contains nothing but nitrogen. In this operation the oxygen is inhaled by the leaves, which transform that gas into carbonic acid; this they expire in variable proportions according to their age, and it is absorbed by the caustic-potash solution.

But this power of absorbing oxygen and of exhaling carbonic acid in the daytime, while very evident at the instant of the opening out of the buds, becomes sensibly less pronounced, according as the leaves grow, and, as a general rule, this phenomenon ceases to be presented after these organs have attained their normal development. Hence, it is certain that plants, in their earlier stages, respire after the manner of animals, absorbing oxygen and exhaling carbonic acid. These physiological facts were demonstrated by M. Corenwinder, in a memoir published in 1866 by the Société des Sciences of Lille.

It is not alone young plants just produced from the seed or from the bud in the spring that offer these characters: all foliaceous organs, while young, tender, and injected with nitrogenized materials, and just beginning to derive their nourishment from the carbon of the atmosphere, sensibly exhale carbonic acid in the daytime. If we observe the young branches which, during summer, grow on trees of persistent foliage, the Laurocerasus, for example, we find that, in these new growths, the phenomenon of respiration predominates: they exhale sensibly carbonic acid in the daytime.

But, if we place under a closed bell-glass an entire branch bearing leaves of the current and of the preceding year, collecting the air that has been in contact with them in a receiver holding baryta-water, and provided with an aspirator, we find that the result varies according to the relative quantities of new and old leaves. If the latter are in excess, they absorb the carbonic acid exhaled by the former, and the