Botany; Geotropism; Plants -- Irritability and movements
The needle-like leaves of Pinus pinaster form a bundle whilst young;
afterwards they slowly diverge, so that those on the upright shoots
become horizontal. The movements of one such
young leaf was traced during 4½ days, and the tracing here given (Fig.
121) shows that it descended at first in a nearly straight line, but
afterwards zigzagged, making one or two little loops. The diverging and
descending movements of a rather older leaf were also traced (see
former Fig. 113, p. 251): it descended during the first day and night
in a somewhat zigzag line; it then circumnutated round a small space
and again descended. By this time the leaf had nearly assumed its final
position, and now plainly circumnutated. As in the case of the
Carnation, the leaves, whilst very young, do not seem to be much
affected by geotropism or heliotropism, for those on a young plant laid
horizontally, and those on another plant left upright, both kept in the
dark, continued to diverge in the usual manner without bending to
either side.
Fig. 121. Pinus pinaster: epinastic downward movement of a young leaf,
produced by a young plant in a pot, traced on a vertical glass under a
skylight, from 6.45 A.M. June 2nd to 10.40 P.M. 6th.
With Cobœa scandens, the young leaves, as they successively diverge
from the leading shoot which is bent to one side, rise up so as to
project vertically, and they retain this position for some time whilst
the tendril is revolving. The diverging and ascending movements of the
petiole of one such a leaf, were traced on a vertical glass under a
skylight; and the course pursued was in most parts nearly straight, but
there were two
well-marked zigzags (one of them forming an angle of 112°), and this
indicates circumnutation.
The still closed lobes of a young leaf of Dionaea projected at right
angles to the petiole, and were in the act of slowly rising. A glass
filament was attached to the under side of the midrib, and its
movements were traced on a vertical glass. It circumnutated once in the
evening, and on the next day rose, as already described (see Fig. 106,
p. 240), by a number of acutely zigzag lines, closely approaching in
character to ellipses. This movement no doubt was due to epinasty,
aided by apogeotropism, for the closed lobes of a very young leaf on a
plant which had been placed horizontally, moved into nearly the same
line with the petiole, as if the plant had stood upright; but at the
same time the lobes curved laterally upwards, and thus occupied an
unnatural position, obliquely to the plane of the foliaceous petiole.
Public-domain text, read in full here on John Shaqi.
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