case a modified curvature. This is the one way which. a plant has of reacting to the external world. Motile organisms, as, for instance, the microscopic swarm-spores of Hæmatococcus or Botrydium—a couple of fresh-water algæ—may seem to stretch the strict interpretation; but their movements may be considered under the law if one remembers that they are free, solitary cells, and must act accordingly.
Twining plants are, perhaps, the most interesting examples of an-heliotropic irritability, for their habit of growth—by no means leaving the instinctive circumnutation of the tip out of account—is a manifestation of insensibility to light. The morning-glory is a perfect example. Regardless of the sun, it twines regularly along its support, never for a moment being deflected or turned aside through conditions of unequal illumination. Of the same thing the wistaria is an equally instructive illustration. It may be safely presumed that, originally, twining plants were not twining plants at all, but were creeping in their habits; and from this it seems probable that heliotropism was once present, but is now lost. In its first appearance the habit of twining must have been accidental, and just how heliotropic tendencies were overcome by the newly developed trait is difficult to explain. It is interesting to notice, however, that the shoot of the morning-glory, when it first peeps from the ground, is distinctly heliotropic, and this must be considered an embryonic feature significant precisely as the branchial development of the fœtal mammal is significant—that is, there is indicated by it a line of descent.
A distinction must first be made between the periodic movements of leaves and stems and the true heliotropic movements. As pointed out by Dr. Julius Sachs, the first—such as the well known phenomenon of sleep—are dependent upon the intensity of illumination, while the second are almost entirely due to the direction from which the rays chance to be falling upon the plant. It will be indispensable to a clear comprehension of what true heliotropism is to speak somewhat generally of light-action in vegetable physiology.
There is certainly no more important agent in the whole system of Nature than light. It is only in light that green plants can form their chlorophyl, and, since this chlorophyl is absolutely essential in the assimilating processes, its importance can be conceived. Even chlorophylless plants—the fungi and slime-molds—and all animals are indirectly dependent, as is well known, upon the possibility of chlorophyl-formation. The whole organized world, then, depends upon light as one of its essentials, for undulations of the ether are necessary to the well-being of all protoplasm. But not only does protoplasm react to these undulations, in very many instances, by the elaboration from itself of the
