Page:Popular Science Monthly Volume 18.djvu/866

846 of special research in some by-path or other subordinated to the main course of the biological system associated with his name; and it has been an unfailing source of interest to see the central ideas of the evolution and the continuity of life developed in detail through a series of special treatises, each wellnigh exhaustive of the materials available for its subject. It is in the department of plant-life that he has of late years devoted himself to working out the laws which govern the whole realm of vital phenomena. That these laws in their origin and ultimate operation are common to plant and animal alike has long formed a characteristic principle or axiom of his philosophy. In the experimental study needed for the elaboration of the vital processes and the making good the resulting generalizations, the kingdom of plant-life offers decided advantages beyond that of animals, if it were only that observations of this class are free from all possible taint of inhumanity. Mr. Darwin has in the quietude of his hothouse, and with a boundless variety of forms for selection, experimented upon the vital organism of plants, seconded by the untiring energy and patience of his son. Night and day seem to have come alike to the aid of this enthusiastic pair of naturalists. The electric light has served them on the failure of the sun's beams, and has in truth opened up of itself a wholly new field for observation as regards the agency of light upon the phenomena of life. To the vista of knowledge revealed by these experiments upon the elementary processes of life in movement, growth, nutrition, respiration, sensation, etc., imagination can set no bounds. It is impossible, Mr. Darwin remarks at the close of his record of these interesting experiments, not to be struck with the resemblance between the foregoing movements of plants and many of the actions performed unconsciously by the lower animals. This analogy has been made the subject of much interesting investigation by Sachs, Frank, and other leading biologists on the Continent, and we may expect that the highly original and elaborate experiments recorded in the volume before us will give fresh stimulus to this most important course of investigation, laying as they do a new and more solid basis for the comparative study of plant and animal life. Plants, of course, possess neither nerves nor a central nervous system, and there is consequently lacking in them that which gives its most distinctive character to animal life as a whole. Yet that sensitive impressions are present in plants, with the power of movement in obedience to the stimulus thereby imparted to the organism, may be held to be conclusively shown by facts such as those produced by Mr. Darwin. Most striking of all, he urges, as a point of resemblance, is the localization of their sensitiveness, and the transmission of an influence from the excited part to another, which consequently moves. May it not be inferred that in animals the nervous structures serve merely for the more perfect transmission of impressions and for the more complete intercommunication of parts? From the earliest sign of germination in plants—namely, the protrusion of the radicle from the seed-coats under the soil—there is manifest a sensitiveness to external influences, with a movement in response to the conditions of light or pressure, etc., which is not sharply to be distinguished from the rudimentary intelligence in animals. In the sensitive point or tip of the radicle, which we might compare with the antennæ in insects, there is to be seen an organic power equivalent, in a lesser degree, to the action of the brain in the lower animals:

We believe that there is no structure in plants more wonderful, as far as its functions are concerned, than the tip of the radicle. If the tip be lightly pressed or burned or cut, it transmits an influence to the upper adjoining part, causing it to bend away from the affected side; and, what is more surprising, the tip can distinguish between a slightly harder and softer object, by which it is simultaneously pressed on opposite sides. If, however, the radicle is pressed by a similar object a little above the tip, the pressed part does not transmit any influence to the more distant parts, but bends abruptly toward the object. If the tip perceives the air to be moister on one side than on the other, it likewise transmits an influence to the upper adjoining part, which bends toward the source of moisture. When the tip is excited by light (though in the case of radicles this was ascertained in only a single instance) the adjoining part bends from the light; but when excited by gravitation the same part bends toward the center of gravity. In almost every case we can clearly perceive the final purpose or advantage of the several movements. Two, or perhaps more, of the exciting causes often act simultaneously on the tip, and one conquers the other, no doubt in accordance with its importance for the