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Page:Popular Science Monthly Volume 46.djvu/411

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THE uniform co-operation of living cells in the vegetable organism appears less problematical to us when we know that these elements are connected by fine threads of living substance. These protoplasmic threads penetrate the cell walls; they immediately transmit the stimulus from cell to cell, and conduct it to a distance; and the continuity of the living substance in the whole organism is thus preserved. It formerly appeared otherwise, when the single living cell bodies were supposed to be completely separated by their walls, and these cell walls were thought to bring about the transmission. The physiological arrangements of plants have now become very much like those of animals, and nearly approach them in perfection. Very striking among the life expressions of organisms are certain processes which mutually influence and condition one another, and which we call manifestations of correlation. A particular condition in the organism invokes another, so brought about that a general balance in the functions is preserved, and is restored if disturbed. A red-beech tree growing in the open, where it is immediately exposed to the effects of the light, has small but relatively thick leaves. Red beeches, as undergrowth in the shadow of the woods, are distinguished by considerably larger but thinner leaves. The cause of this variation lies chiefly in the difference in the conditions of transpiration. The growing leaves in the isolated tree give out more vapor to the atmosphere than do those in the shade. The increased evaporation affects the structure of the leaf surfaces, and they are compressed, chiefly because less air space is found between their cells, and partly, also, because the cells turn perpendicularly to the surface instead of increasing in breadth. AU this increases the thickness of the leaf at the cost of its superficial diameter. This condition is immediately useful to the plant, because a thin and comparatively broad leaf surface would transpire too rapidly in an open situation, involving the tree in danger of drying out. In the shade, on the other hand, a large surface is necessary to give as much vapor out to the atmosphere as the life processes of the plant require; for evaporation promotes the accession of food-salts from the soil. These salts, dissolved in water, reach the plant, and are retained by it, while the water is evaporated. More rays of light fall upon a large leaf than upon a small one. In this view too, then, an enlargement of the leaf surface is more advantageous and useful in a feebly lighted situation. The intense light which falls upon a fully exposed beech may, in its greater intensity, perform as