Page:Popular Science Monthly Volume 21.djvu/305

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of the cells from the epidermis of the leaf no contents were apparent; in fact, the cells are tabular, very thin in proportion to their width, and any contents they may possess are so nearly homogeneous as to be transparent and invisible. But let us make a thin transverse section of the same leaf. Here (Fig. 6) we find cells different in shape from any we have yet seen, and evidently possessing different contents. The cells from the inside of the leaf are here seen filled with tiny green PSM V21 D305 Leaf cross section showing chlorophyl in cells.jpgFig. 6.—Cross-Section of a Leaf, showing the Cells containing Chlorophyl. bodies, sometimes closely packed together,sometimes scattered more sparsely. These little green bodies are the chlorophyl-granules, affording to our vegetation all the lovely tints which render charming a landscape in spring. Children of the light are these little green grains, lovers and worshipers of the sun. At all events, they appear by uncounted millions in the bright light of the open sky, become fewer and fewer in proportion as the light received by any plant is diminished, and finally disappear entirely when the plant is left in total darkness. Every one will recall the appearance of potato-stalks where growth has started in some dark corner of the cellar. Cells taken from such growth afford not a sign of chlorophyl. Botanists tell us that the petals of flowers are only altered leaves. In petal-cells, then, instead of chlorophyl-grains, we find in some cases granules of yellow, sometimes of orange. Sometimes the cell contains no such granules, but rather some colored fluid, red, blue, or purple, and then our flowers are tinted accordingly; sometimes the cells of a petal contain air only, and then the flower is white. But these tiny green grains in the leaf-cells do vastly more than simply lend their color to the foliage; they are readjusters and organizers, and perform, in those diminutive laboratories we have been calling cells, feats which the chemist strives in vain to rival. They take possession of molecules of carbon dioxide and of water, compel the binding chemical forces to relax their hold, combine again, to serve the purposes of the plant, the atoms of carbon, of hydrogen, using such part of the oxygen as may be necessary, and setting the remainder free in the open atmosphere—all this in the sunlight. The chlorophyl bodies thus work while it is day, have charge of nearly all the income of the plant, and provide in themselves for the temporary storage, of its daily accumulations, mostly in the form of starch. When the night comes, these same little factors give up at once their labors and their stores, other cells of the plant begin to work, change and transfer and change again, until all the wondrous series of vegetable products with which we are familiar (the sugars, the oils, the alkaloids, crystals of various forms and kinds) are formed and properly deposited. We might go on now to examine cells containing many of these sub-