Page:Popular Science Monthly Volume 9.djvu/235

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HEAT and light are physical influences to which even the lowest units of living matter respond, whether their mode of life and nutrition is most akin to that of plants or to that of animals. These influences act on such organisms, either by stimulating, retarding, or otherwise modifying the chemical changes naturally occurring in their interior, and upon the existence of which their life depends. Where the vital processes of the organism are stimulated by these physical agencies, their incidence may, in many instances, become the cause of so-called "spontaneous movements." The term, however, as applied to movements, is a bad one—since all the movements of an organism are alike dependent upon a series of antecedent states of contractile and other tissues. There is some sort of foundation, it is true, for the popular mode of expression. A movement is not said to be "spontaneous" if it follows immediately upon some external impression as a cause; the term is generally applied where the cause of the movement is not distinctly recognizable. In some instances the undetected or unconsidered external cause may be the incidence of a diffused physical agent such as heat, which, by stimulating the vital processes, seems to give rise to spontaneous movements. In other cases so-called "spontaneous" movements are to be referred to internal states or changes, whose origin is even less distinctly traceable, to impressions, it may be, which emanate from some of the internal organs, and thence are transmitted to ganglia in direct relation with some of the organs of locomotion.

Heat mostly acts on organisms upon all sides alike, so that, though it may stimulate their life-processes generally, and, in some instances, give rise to movements, these movements are not determined in one, more than in another, direction. Thus, while heat stimulates the "to-and-fro" or the gyratory movements of bacteria, and also renders more striking and rapid those changes of form which all amœboid organisms are apt to display, the movements evoked are random, and apparently devoid of all purpose.

It is not altogether similar, however, with the influence of light. This agent almost always, and necessarily, falls more on one side than on another; and consequently it often suffices to induce movements to be made in definite directions, by the lower forms of life, just as it causes definite and responsive movements to be executed by certain parts of higher plants, which come fully under its influence. In each case the movement, or altered position, is due to some nutritive change; that is, to some alteration, whatever its nature, in the activity of the life-processes taking place in the part impressed by the