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Page:Popular Science Monthly Volume 18.djvu/516

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THE POPULAR SCIENCE MONTHLY.

the apex of the radicle, and if it could have remained there all the time, the movement exhibited would have been much greater, for at the close of our observations the tip, instead of standing upward, had become bowed downward, through geotropism [gravitation], so as almost to touch the zinc plate. As far as we could roughly ascertain by measurements made with compasses on other seeds, the tip alone, for a length of only 2/100 to 3/100 of an inch, is acted on by geotropism. But the tracing shows that the basal part of the radicle continued to circumnutate irregularly during the whole time. The actual extreme amount of movement of the bead at the end of the filament was nearly ·05 inch, but to what extent the movement of the radicle was magnified by the filament, which was nearly three fourths of an inch in length, it was impossible to estimate. . . .

"Another seed was treated and observed in the same manner, but the radicle in this case protruded! inch, and was not fastened so as to project quite vertically upward. The filament was fixed close to its base. The tracing (Fig. 2, reduced one half) shows the movement

PSM V18 D516 Brassica oleracea traced for 46 hours.jpg
Fig. 2.—Brassica Oleracea: circumnutating and geotropic movement of radicle, traced on horizontal glass during forty-six hours.

from 9 a. m., January 31st, to 7 a. m., February 2d; but it continued to move during the whole of the day in the same general direction and in a similar zigzag manner." The chapter contains fifty-four diagrams, giving the movements of all the parts of the seedlings of all sorts of plants.

Mr. Darwin thinks that these movements of the radicle are useful at least in enabling it to take the line of least resistance, if they do not directly aid it in forming a passage for itself; and he adds: "If, however, a radicle in its downward growth breaks obliquely into any crevice, or a hole left by a decayed root, or one made by the larva of an insect, and more especially by worms, the circumnutating movement of the tip will materially aid its descent; and we have observed that roots commonly run down the old burrows of worms." He says, further, that the force due to longitudinal and transverse growth materially assists the radicle in penetrating the ground. He experimented upon these points, and we give two of these experiments which relate to the force exerted transversely by the radicles of beans. We may say that these radicles have a sharp apex protected by a root-cap, and their growing part is more rigid than the part just above.