52 shows that this may occur when the track is
rectilinear. The apex by thus rising, was in one instance able to surmount
a bristle cemented across an inclined glass-plate; but slips of wood only
1/40 of an inch in thickness always caused the radicles to bend
rectangularly to one side, so that the apex did not rise to this small
height in opposition to geotropism.
In those cases in which radicles with attached filaments were placed so as
to stand up almost vertically, they curved downwards through the action of
geotropism, circumnutating at the same time, and their courses were
consequently zigzag. Sometimes, however, they made great circular sweeps,
the lines being likewise zigzag.
Radicles closely surrounded by earth, even when this is thoroughly soaked
and softened, may perhaps be quite prevented from circumnutating. Yet we
should remember that the circumnutating sheath-like cotyledons of Phalaris,
the hypocotyls of Solanum, and the epicotyls of Asparagus formed round
themselves little circular cracks or furrows in a superficial layer of damp
argillaceous sand.
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