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Page:Popular Science Monthly Volume 18.djvu/867

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life of the plant. The course pursued by the radicle in penetrating the ground must be determined by the tip; hence it has acquired such diverse kinds of sensitiveness. It is hardly an exaggeration to say that the tip of the radicle thus endowed, and having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain being seated within the anterior end of the body, receiving impressions from the sense organs, and directing the several movements.

In this suggestive passage, with which our authors bring their present course of investigations to a close, we see opened up a far-reaching prospect for the biological progress of the future. For the present it must suffice to have made good so much as our authors have been able to report from their patient study of the simpler and more easily observable vital phenomena.

A great part of Mr. Darwin's work is taken up with the details of experiments for measuring the quantity and direction of motion in plants, both under natural and artificial conditions. Direct observations have been made in numerous cases under the microscope, and in others use has been made of delicate apparatus of various kinds. Minute bits of card or tissue paper have been attached to the radicles, filaments, or terminals of stems, and tiny particles of metal or beads of shellac have been employed as weights to test the power of rigidity or of sensitiveness in the fibers of plants. Pins stuck in the soil around the stem have served to mark the conduct of the plant when impeded in its growth or its spontaneous habits of movement. The movements of the tenderest filaments or leaflets have been made to trace themselves in lines upon smoked glass. A series of diagrams has in this way been worked out, and set before the eye in numerous woodcuts, generally magnified two or three fold, showing the general law of circumnutation indefinitely modified by special conditions. The differences of movement in seedling and mature plants, in monocotyledons and dicotyledons, with the indications of certain movements having been acquired for a special purpose, are pursued through widely contrasted classes of plants. The circumnutating powers of young leaves are described in thirty-three genera belonging to twenty-five families, widely distributed among ordinary and gymnospermous dicotyledons, and among monocotyledons, together with many cryptogams. Here the seat of movement is generally seen to lie in the petiole, but sometimes both in the petiole and the blade, or in the blade alone. The movement is chiefly in a vertical plane; yet, as the ascending and descending lines never coincide, there is always some lateral movement, resulting in irregular ellipses, so that the motion becomes really one of circumnutation. It is interesting to mark the periodicity. of leaf-movement, a gentle rise being observed in the evening and the early part of the night, with a sinking toward morning. In Dionæa and certain graminiæ, a strange jerking and oscillatory movement is to be seen under the microscope, curiously contrasted with the immobility of the tentacles of Drosera rotundifolia, which are yet sensitive enough to curl inward in twenty-three seconds so as to absorb a bit of raw meat. The distinction of epinastic and hyponastic growth—according as the growth takes place more rapidly in the upper or lower surface of an organ, causing it to bend downward or upward respectively—introduced by De Vries, has been illustrated in the case of a number of plants. To Frank is due the introduction of the useful terms of "heliotropism," for the tendency to turn to the light, with its correlative "apheliotropism," the opposite tendency, occasionally to be observed, "geotropism," for the bending toward the earth, and "apogeotropism," expressing motion in opposition to gravity or from the center of the earth. For the measurement of movements, sometimes excessively minute, various expedients were adopted. Dots were made from time to time upon sheets of glass placed vertically and horizontally near the plant, these dots being then copied on tracing-paper and joined by ruled lines, arrows being added to show the direction of the movement. The plants were exposed to varied conditions of light, sometimes being wholly protected, the light at other times being admitted from above or from either side. In addition to the sun's rays, the electric light was made the subject of experiment, with results comparable with those of Dr. Siemens. A valuable chapter is given to the sensitiveness of plants to light, with its transmitted effects. That growth in general is checked by light, which acts upon