Page:Popular Science Monthly Volume 6.djvu/74

64 baryta-water remains clear, but it grows turbid when the new leaves predominate.

If the experiment be made at the period when all the leaves of the current year have attained their adult age, the branch of Laurocerasus gives out no carbonic acid while exposed to light, provided the light is not very feeble.

The point at which plants cease perceptibly to give out carbonic acid in the daytime varies widely according to the species. Corenwinder has found some which exhibit this property for a long time, while others lose it very early. In the first category we may class a perennial plant common in our gardens, Diclitra spectabilis, and in the second the young leaves of the beet.

The cause of this peculiarity cannot at present be assigned; certain it is, however, that it largely depends on external circumstances, heat, for instance, which quickens all the chemical actions of oxygen, or the intensity of the light which promotes the assimilation of the carbon. But the special nature of the plant also plays a part. Hence we must not jump at conclusions after one of these experiments, if we would avoid setting up artificial laws with many exceptions.

It was at first difficult to account a priori for the fact of this property of nascent plants constantly exhaling carbonic acid, being at the outset very patent, and then diminishing in intensity as they grow, and finally disappearing. But experiments of another kind, described eight years ago by M. Corenwinder, put him on the track of this phenomenon, and gave him a plausible explanation of it.

Adopting the same processes which enabled Bonnet, Ingenhousz, and Sennebier, to lay the foundations of plant physiology, he placed buds and young stems bearing new leaves in bell-glasses filled with spring-water containing bicarbonate of lime or in water charged with carbonic acid, and then exposed them to the sun. As was to be expected, the leaves were soon covered with bubbles, and gave off oxygen; and this is the case even with leaves whose evolution is not yet complete. Hence it is plain that, from the earliest period of their life, plants decompose the carbonic acid of the atmosphere and assimilate its carbon.

Thus the foregoing experiments prove two facts which seem to be contradictory, and which, nevertheless, are simultaneous: Inhalation of oxygen, accompanied with emission of carbonic acid;  Absorption of carbonic acid, leading to a discharge of oxygen. Hence, in young plants, there is simultaneity of the two modes of respiration commonly attributed to older plants; but, in the latter, these two modes have different conditions or different organs. This was the starting-point, and it had to be made clear by means of accurate research.

As we now see, the plant begins, in the early stages of its life, to respire as the animal does, absorbing oxygen, and exhaling carbonic