The Power of Movement in Plants | Page 7

A.M. Jan. 31st to 7 A.M. Feb. 2nd; but it continued
to move during the whole of the [page 12] 2nd in the same general
direction, and in a similar zigzag manner. From the radicle not being
quite perpendicular when the filament was affixed geotropism came
into play at once; but the irregular zigzag course shows that there was
growth (probably preceded by turgescence), sometimes on one and
sometimes on another side. Occasionally the bead remained stationary
for about an hour, and then probably growth occurred on the side
opposite to that which caused the geotropic curvature. In the case
previously described the basal part of the very short radicle from being
turned vertically upwards, was at first very little affected by geotropism.
Filaments were affixed in two other instances to rather longer radicles
protruding obliquely from seeds which had been turned upside down;
and in these cases the lines traced on the horizontal glasses were only
slightly zigzag, and the movement was always in the same general
direction, through the action of geotropism. All these observations are
liable to several causes of error, but we believe, from what will
hereafter be shown with respect to the movements of the radicles of
other plants, that they may be largely trusted.
Hypocotyl.--The hypocotyl protrudes through the seed-coats as a
rectangular projection, which grows rapidly into an arch like the letter
U turned upside down; the cotyledons being still enclosed within the
seed. In whatever position the seed may be embedded in the earth or
otherwise fixed, both legs of the arch bend upwards through

apogeotropism, and thus rise vertically above the ground. As soon as
this has taken place, or even earlier, the inner or concave surface of the
arch grows more quickly than the upper or convex surface; and this
tends to separate the two legs and aids in drawing the cotyledons out of
the buried seed-coats. By the growth of the whole arch the cotyledons
are ultimately dragged from beneath the ground, even from a
considerable depth; and now the hypocotyl quickly straightens itself by
the increased growth of the concave side.
Even whilst the arched or doubled hypocotyl is still beneath the ground,
it circumnutates as much as the pressure of the surrounding soil will
permit; but this was difficult to observe, because as soon as the arch is
freed from lateral pressure the two legs begin to separate, even at a very
early age, before the arch would naturally have reached the surface.
Seeds were allowed to germinate on the surface of damp earth, and
after they had fixed themselves by their radicles, and after the, as yet,
only [page 13] slightly arched hypocotyl had become nearly vertical, a
glass filament was affixed on two occasions near to the base of the
basal leg (i.e. the one in connection with the radicle), and its
movements were traced in darkness on a horizontal glass. The result
was that long lines were formed running in nearly the plane of the
vertical arch, due to the early separation of the two legs now freed from
pressure; but as the lines were zigzag, showing lateral movement, the
arch must have been circumnutating, whilst it was straightening itself
by growth along its inner or concave surface.
A somewhat different method of observation was next followed: Fig. 3.
Brassica oleracea: circumnutating movement of buried and arched
hypocotyl (dimly illuminated from above), traced on horizontal glass
during 45 hours. Movement of bead of filament magnified about 25
times, and here reduced to one-half of original scale.
as soon as the earth with seeds in a pot began to crack, the surface was
removed in parts to the depth of .2 inch; and a filament was fixed to the
basal leg of a buried and arched hypocotyl, just above the summit of
the radicle. The cotyledons were still almost completely enclosed
within the much-cracked seed-coats; and these were again covered up

with damp adhesive soil pressed pretty firmly down. The movement of
the filament was traced (Fig. 3) from 11 A.M. Feb. 5th till 8 A.M. Feb.
7th. By this latter period the cotyledons had been dragged from beneath
the pressed-down earth, but the upper part of the hypocotyl still formed
nearly a right angle with the lower part. The tracing shows that the
arched hypocotyl tends at this early [page 14] age to circumnutate
irregularly. On the first day the greater movement (from right to left in
the figure) was not in the plane of the vertical and arched hypocotyl,
but at right angles to it, or in the plane of the two cotyledons, which
were still in close contact. The basal leg of the arch at the time when
the filament was affixed to it, was already bowed considerably
backwards, or from the cotyledons; had