Page:Popular Science Monthly Volume 6.djvu/80

70 the air in the tube. The acid was absorbed, as soon as formed, by the alkaline solution, and the latter rose in the tube until there was no gas left except nitrogen.

The bud was kept in the nitrogen till May 2d; it then began to give signs of decay. During all this time it never gained in size, and retained its original form. The conclusion is, that bud development cannot go on in an atmosphere deprived of oxygen.

We know, further, from the observations of Th. de Saussure, that germination is impossible when the embryo, in process of growth, does not find, in the atmosphere in which it lives, the amount of oxygen needed for its life. Hence Corenwinder's experiment gives us a fresh instance of the resemblance between germination and the evolution of leaf-axes.

Th. de Saussure also examined several plants placed in an atmosphere of nitrogen. According to their behavior under these circumstances, he divides them into two categories, viz., those which vegetate in such an atmosphere only for a few days, and those which live and even flourish there for a certain length of time. Plants of the latter class are chiefly those which inhabit marshy situations, such as Lythrum salicaria, Epilobium hirsutum, Polygonum amphibium, etc. He has expressed the opinion that plants possessed of this latter property consume less oxygen, vegetating in atmospheric air without much light.

If, in M. Corenwinder's experiments, plants wither rapidly, the reason is, that in the morning he drew off the carbonic acid formed during the night by the agency of the oxygen contained in the cells. When this is not done, the leaves may decompose the acid in the daytime, give out oxygen, and so live for a long time, the oxygen being inhaled and exhaled over and over again.

Finally, if the leaves be kept in absolute darkness, the reducing action is null, and then the act of respiration, which, of necessity, is never completely suspended, alone appears, and the plant disengages only carbonic acid. This function, however, is curiously affected by temperature, so that, at 32° Fahr., leaves usually exhale but little carbonic acid.

These early observations would of themselves suffice to show the existence in plants, at every stage of growth, of a respiratory action, like that of animals, viz., an absorption of oxygen.

What in the books is called the diurnal respiration of plants, is in reality an assimilation of carbon; in other words, it is the act whereby the leaf-organs decompose the carbonic acid of the air, and give out its oxygen. This act depends essentially on the influence of light. It is at the maximum intensity when the plant is under the direct action of the sun's rays, and gradually diminishes in importance in proportion as the light grows feebler; for instance, when the sky is overcast with clouds, and when the weather is thick and rainy. This was demonstrated in a memoir by Corenwinder, published in 1858.