Page:Popular Science Monthly Volume 6.djvu/78

58 the plant, the humidity of the soil, and the heat to which it is exposed. It was proved by De Saussure that the quantity of ash is much less in nascent leaves than in those which have attained the term of their existence.

4. De Saussure also proved that in the ash of buds and nascent leaves there is more phosphoric acid than is found at any later stage. Since his time this fact has been confirmed by Garreau, of Lille, and by Corenwinder. The present series of experiments furnishes a new demonstration of this important phenomenon.

In the tables we have given the reader will observe that the proportion of phosphoric acid, which at the outset was considerable, especially in the maple-leaves, rapidly grows less, and when the process of vegetation is at an end it is very small indeed. Thus, when coming from the bud, they contain (in the dry state) about .028 phosphoric acid; but at last they contain only about .001. It was long ago proved by M. Corenwinder that the phosphorus contained in plants is an essentially variable quantity. It almost entirely disappears from the tissue of annual plants at the end of their growth, being condensed in the seeds, and ultimately serving to perpetuate the species. In perennial plants, the phosphorus does not go into the seeds merely—it is also diffused through the trunk and the branches; further, it hibernates in the buds, which contain the essential elements of the seed, and which perform the same functions as the latter in the evolution of leaves.

Having now made the experiments tell their story, and described the comparative evolution of the various elements of the leaves during their annual life, let us next see whether these variations in chemical constitution may be coupled together under a theory which shall explain the modifications undergone by the gaseous exhalations of plants at the various stages of their life.

When we study closely the figures relating, for instance, to the maple, we find that, in the first stage of growth the nitrogenized matters are very considerable. Probably they have an organization of their own, and exist independently of the vegetal cells; at all events, they discharge functions which may be called animal—they respire, and in this early stage respiration is the dominant function. The carbonic acid resulting from this operation is at first only in part retained in the plant by the reducing action of the chlorophyll. The young plant, when exposed to the light and placed in atmospheric air, exhales an excess of carbonic acid.

In the second period, the relative proportion of nitrogenized matters grows less, while, on the other hand, the carbonaceous matters increase. The plant now exhales only a small amount of carbonic acid, the latter being almost entirely retained by the chlorophyll, which decomposes it, and fixes its carbon.