Modified circumnutation: Movements excited by gravitation.
Means of observation — Apogeotropism—Cytisus—Verbena—Beta—Gradual
conversion of the movement of circumnutation into apogeotropism
in Rubus, Lilium, Phalaris, Avena, and Brassica—Apogeotropism
retarded by heliotropism—Effected by the
aid of joints or pulvini—Movements of flower-peduncles
of Oxalis—General remarks on apogeotropism—Geotropism—
Movements of radicles—Burying of seed-capsules—Use
of process—Trifolium subterraneum—Arac
Our object in the present chapter is to show that geotropism, apogeotropism, and diageotropism are modified forms of circumnutation. Extremely fine filaments of glass, bearing two minute triangles of paper, were fixed to the summits of young stems, frequently to the hypocotyls of seedlings, to flower-peduncles, radicles, etc., and the movements of the parts were then traced in the manner already described on vertical and horizontal glass-plates. It should be remembered that as the stems or other parts become more and more oblique with respect to the glasses, the figures traced on them necessarily become more and more magnified. The plants were protected from light, excepting whilst each observation was being made, and then the light, which was always a dim one, was allowed to enter so as to interfere as little as possible with the movement in progress; and we did not detect any evidence of such interference.
When observing the gradations between circumnu-[page 494] tation and heliotropism, we had the great advantage of being able to lessen the light; but with geotropism analogous experiments were of course impossible. We could, however, observe the movements of stems placed at first only a little from the perpendicular, in which case geotropism did not act with nearly so much power, as when the stems were horizontal and at right