Page:Popular Science Monthly Volume 33.djvu/699

Rh this one end. Leaf-formation will be scanty; assimilation will be suspended; and the whole organism will reach out for the sunlight, as thousands of generations had done before its own life. That fungi of the mushroom type—needing no light, for they make no chlorophyl—reach upward too—and it is undeniably true that they elongate more rapidly in darkness—is to be considered as evidence of descent from an alga stock, and this is rendered probable by morphological as well as by this interesting physiological consideration. Heredity may come into play here as well as it does in the case of the moss antherozoids, which are attracted by the archegonium, or in the case of the fish-mold zoöspores, which swim toward decaying fish or putrid extracts of meat.

From all this, the meaning of heliotropism in the natural order of things becomes apparent. The phenomena which have been studied fit into the evolution theory as if made for the theory and not the theory for them. Plants which must have light to live are impelled toward this light by their own conditions of structure. The reaching upward is sometimes almost instinctive—almost conscious, one might fancy. Knight observed a vine-leaf try first one way and then another to reach the position of best illumination—a transverse one, which is now considered to be a result of the palisade structure, and not of a peculiar kind of irritability. Dutrochet noticed the tendril of a pea trying to avoid the light, and it finally seemed to send an impulse down to the petiole, and this bent backward. The question of resistance is probably, however, the only one which needs to be considered as modifying plant-action in such instances.

Climbing and twining plants, as Darwin observed, have lost their heliotropism because they would be pulled away from their supports if they always followed the sun. For the same reason tendrils, aerial roots, the suckers of Parthenocissus quinquefolia—the Virginia creeper—are, considered as separate organs, apheliotropic rather than the reverse. Carnivorous plants, which at least partially depend for sustenance upon a peculiar position of leaves and stem, and which have less need of light for assimilation, have also lost their powers of* response to the heliotropic stimulus. It must not be supposed, however, for a moment, that heliotropic irritability is not present. It may be there, and well developed too; but inhibited by heredity, by growth, by environment. Just as the compass-plant when grown in darkness allows its leaves to adopt the horizontal position, so does the Venus's-flytrap when growing normally, and it is probable that, if generation after generation of compass-plants could be grown in the darkness, the erect position of the leaves would permanently disappear. Just so the Venus's-flytrap has lost the power of responding to its heliotropic tendency, through its carnivorous habits. Vines, indeed, thinks