Botany; Geotropism; Plants -- Irritability and movements
In order to observe the movements of the epicotyl at a somewhat more
advanced age, a filament was fixed near the base of one which was no
longer arched, for its upper half now formed a right angle with the
lower half. This bean had germinated on bare damp sand, and the
epicotyl began to straighten itself much sooner than would have
occurred if it had been properly planted. The course pursued during 50
h. (from 9 A.M. Dec. 26th, to 11 A.M. 28th) is here shown (Fig. 22);
and we see that the epicotyl circumnutated during the whole time. Its
basal part grew so much during the 50 h. that the filament at the end
of our observations was attached at the height of .4 inch above the
upper surface of the bean, instead of close to it. If the bean had been
properly planted, this part of the epicotyl would still have been
beneath the soil.
Fig. 22. Vicia faba: circumnutation of young epicotyl, traced in
darkness during 50 hours on a horizontal glass. Movement of bead of
filament magnified 20 times, here reduced to one-half of original
scale.
Late in the evening of the 28th, some hours after the above
observations were completed, the epicotyl had grown much straighter,
for the upper part now formed a widely open angle with the lower part.
A filament was fixed to the upright basal part, higher up than before,
close beneath the lowest scale-like process or homologue of a leaf; and
its movement was traced
during 38 h. (Fig. 23). We here again have plain evidence of continued
circumnutation. Had the bean been properly planted, the part of the
epicotyl to which the filament was attached, the movement of which is
here shown, would probably have just risen above the surface of the
ground.
Fig. 23. Vicia faba: circumnutation of the same epicotyl as in Fig. 22,
a little more advanced in age, traced under similar conditions as
before, from 8.40 A.M. Dec. 28th, to 10.50 A.M. 30th. Movement of bead
here magnified 20 times.
Lathyrus nissolia (Leguminosae).—This plant was selected for
observation from being an abnormal form with grass-like leaves. The
cotyledons are hypogean, and the epicotyl breaks through the ground in
an arched form. The movements of a stem, 1.2 inch in height, consisting
of three internodes, the lower one almost wholly subterranean, and the
upper one bearing a short,
narrow leaf, is shown during 24 h., in Fig. 24. No glass filament was
employed, but a mark was placed beneath the apex of the leaf. The
actual length of the longer of the two ellipses described by the stem
was about .14 of an inch. On the previous day the chief line of
movement was nearly at right angles to that shown in the present
figure, and it was more simple.
Fig. 24. Lathyrus nissolia: circumnutation of stem of young seedling,
traced in darkness on a horizontal glass, from 6.45 A.M. Nov. 22nd, to
7 A.M. 23rd. Movement of end of leaf magnified about 12 times, here
reduced to one-half of original scale.
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