Botany; Geotropism; Plants -- Irritability and movements
Trifolium subterraneum.—The blades of the cotyledons on the first day
after germination (Nov. 21st) were not fully expanded, being inclined
at about 35° above the horizon; at night they rose to about 75°. Two
days afterwards the blades at noon were horizontal, with the petioles
highly inclined upwards; and it is remarkable that the nocturnal
movement is almost wholly confined to the blades, being effected by the
pulvinus at their bases; whilst the petioles retain day and night
nearly the same inclination. On this night (Nov. 23rd), and for some
few succeeding nights, the blades rose from a horizontal into a
vertical position, and then became bowed inwards at about an average
angle of 10°; so that they had passed through an angle of 100°. Their
tips now almost touched one another, their bases being slightly
divergent. The two blades thus formed a highly inclined roof over the
axis of the seedling. This movement is the same as that of the terminal
leaflet of the tripartite leaves of many species of Trifolium. After an
interval of 8 days (Nov. 29th) the blades were horizontal during the
day, and vertical at night, and now they were no longer bowed inwards.
They continued to move in the same manner for the following two months,
by which time they had increased greatly in size, their petioles being
no less than .8 of an inch in length, and two true leaves had by this
time been developed.
Trifolium strictum.—On the first day after germination the cotyledons,
which are provided with a pulvinus, stood at noon horizontally, and at
night rose to only about 45° above the horizon. Four days afterwards
the seedlings were again observed at night, and now the blades stood
vertically and were in contact, excepting the tips, which were much
deflexed, so that they faced the zenith. At this age the petioles are
curved
upwards, and at night, when the bases of the blades are in contact, the
two petioles together form a vertical ring surrounding the plumule. The
cotyledons continued to act in nearly the same manner for 8 or 10 days
from the period of germination; but the petioles had by this time
become straight and had increased much in length. After from 12 to 14
days the first simple true leaf was formed, and during the ensuing
fortnight a remarkable movement was repeatedly observed. At I. (Fig.
125) we have a sketch, made in the middle of the day, of a seedling
about a fortnight old. The two cotyledons, of which Rc is the right and
Lc the left one, stand directly opposite one another, and the first
true leaf (F) projects at right angles to them. At night (see II. and
III.) the right cotyledon (Rc) is greatly raised, but is not otherwise
changed in position. The left cotyledon (Lc) is likewise raised, but it
is also twisted so that its blade, instead of exactly facing the
opposite one, now stands at nearly right angles to it. This nocturnal
twisting movement is effected not by means of the pulvinus, but by the
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