THE chlorophyll cells and the leaves of the plant may be regarded as little laboratories elaborating vegetable matter; they work upon the carbonic acid, which the enormous quantity of water they contain enables them to extract from the atmosphere, reduce it, and form with the residue from its decomposition, after the elimination of oxygen, sugars and cellulose, straw-gum, vasculose, and all the ternary matters composed of carbon, oxygen, and hydrogen; these cells likewise reduce the nitrates which are brought to them at the same time with phosphoric acid, potash, and silica, by the water which constantly traverses the plant, entering it at the root and being exhaled from the leaves.
If rain is frequent and the soil well moistened, the cells will continue their work for a long time; they will elaborate much vegetable matter, and the plant will grow. But the course is not the same if rain is scarce and the soil is parsimonious in providing for the enormous expenditure of water which the wheat makes. I have found that a leaf of wheat exhales, under one hour of insolation, a weight of water equal to its own. When the earth, insufficiently watered by rain, becomes incapable of supplying this prodigious consumption, desiccation of the organs is produced, and it is always the oldest leaves which dry up and perish first. A May rarely passes without one seeing the little leaves fixed at the base of the stem soft, flabby, and withered. If we submit them to analysis, we find that they have let escape some nitrogenized matter, phosphoric acid, and potash, which they contained while they were living, green, and turgescent. It is well to lay stress upon this death of the leaves, and on the departure of the materials they contain; when the leaf dies, one of the small agglomerations of working cells is closed, the quantity of matter elaborated is then less than if it had continued its task, and as the closure of these little laboratories is determined by their desiccation, we conclude that the quantity of vegetable matter formed during dry years is limited, and that the stems are shortened and there is little straw.
At the moment when desiccation begins the nitrogenized matter which forms the protoplasm, the living part of the cell, is metamorphosed, and takes an itinerant property that permits it to pass through the membranes and migrate toward the new leaves, carrying with it its usual accompaniment of phosphoric acid and potash. This transportation of some of the elaborated material from the lower leaves toward the upper leaves goes on through