Botany; Geotropism; Plants -- Irritability and movements
[10] The passage of the flower-stem of the Lathraea through the ground
cannot fail to be greatly facilitated by the extraordinary quantity of
water secreted at this period of the year by the subterranean
scale-like leaves; not that there is any reason to suppose that the
secretion is a special adaptation for this purpose: it probably
follows from the great quantity of sap absorbed in the early spring by
the parasitic roots. After a long period without any rain, the earth
had become light-coloured and very dry, but it was dark-coloured and
damp, even in parts quite wet, for a distance of at least six inches
all round each flower-stem. The water is secreted by glands (described
by Cohn, ‘Bericht. Bot. Sect. der Schlesischen Gesell.,’ 1876, p. 113)
which line the longitudinal channels running through each scale-like
leaf. A large plant was dug up, washed so as to remove the earth, left
for some time to drain, and then placed in the evening on a dry
glass-plate, covered with a bell-glass, and by next morning it had
secreted a large pool of water. The plate was wiped dry, and in the
course of the succeeding 7 or 8 hours another little pool was
secreted, and after 16 additional hours several large drops. A smaller
plant was washed and placed in a large jar, which was left inclined
for an hour, by which time no more water drained off. The jar was then
placed upright and closed: after 23 hours two drachms of water were
collected from the bottom, and a little more after 25 additional
hours. The flower-stems were now cut off, for they do not secrete, and
the subterranean part of the plant was found to weigh 106.8 grams
(1611 grains), and the water secreted during the 48 hours weighed 11.9
grams (183 grains),—that is, one-ninth of the whole weight of the
plant, excluding the flower-stems. We should remember that plants in a
state of nature would probably secrete in 48 hours much more than the
above large amount, for their roots would continue all the time
absorbing sap from the plant on which they were parasitic.
With germinating monocotyledonous seeds, of which, however, we did not
observe a large number, the plumules, for instance, those of Asparagus
and Canna, are straight whilst breaking through the ground. With the
Gramineæ, the sheath-like cotyledons are likewise straight; they,
however, terminate in a sharp crest, which is white and somewhat
indurated; and this structure obviously facilitates their emergence
from the soil: the first true leaves escape from the sheath through a
slit beneath the chisel-like apex and at right angles to it. In the
case of the onion (Allium cepa) we again meet with an arch; the
leaf-like cotyledon being abruptly bowed, when it breaks through the
ground, with the apex still enclosed within the seed-coats. The crown
of the arch, as previously described, is developed into a white conical
protuberance, which we may safely believe to be a special adaptation
for this office.
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