on their backs, but without any meat. Two of these leaves, after a day,
had a few tentacles reflexed. The drops had now increased considerably
in size, from having imbibed moisture, so as to trickle down the backs
of the tentacles and footstalks. On the second day, one leaf had its
blade much reflexed; on the third day the tentacles of two were much
reflexed, as well as the blades of all four to a greater or less
degree. The upper side of one leaf, instead of being, as at first,
slightly concave, now presented a strong convexity upwards. Even on the
fifth day the leaves did not appear dead. Now, as sugar does not in the
least excite Drosera, we may safely attribute the reflexion of the
blades and tentacles of the above leaves to exosmose from the cells
which were in contact with the syrup, and their consequent contraction.
When drops of syrup are placed on the leaves of plants with their roots
still in damp earth, no inflection ensues, for the roots, no doubt,
pump up water as quickly as it is lost by exosmose. But if cut-off
leaves are immersed in syrup, or in any dense fluid, the tentacles are
greatly, though irregularly, inflected, some of them assuming the shape
of corkscrews; and the leaves soon become flaccid. If they are now
immersed in a fluid of low specific gravity, the tentacles re-expand.
From these [page 233] facts we may conclude that drops of syrup placed
on the backs of leaves do not act by exciting a motor impulse which is
transmitted to the tentacles; but that they cause reflexion by inducing
exosmose. Dr. Nitschke used the secretion for sticking insects to the
backs of the leaves; and I suppose that he used a large quantity, which
from being dense probably caused exosmose. Perhaps he experimented on
cut-off leaves, or on plants with their roots not supplied with enough
water.
As far, therefore, as our present knowledge serves, we may conclude
that the glands, together with the immediately underlying cells of the
tentacles, are the exclusive seats of that irritability or
sensitiveness with which the leaves are endowed. The degree to which a
gland is excited can be measured only by the number of the surrounding
tentacles which are inflected, and by the amount and rate of their
movement. Equally vigorous leaves, exposed to the same temperature (and
this is an important condition), are excited in different degrees under
the following circumstances. A minute quantity of a weak solution
produces no effect; add more, or give a rather stronger solution, and
the tentacles bend. Touch a gland once or twice, and no movement
follows; touch it three or four times, and the tentacle becomes
213