blades in their ordinary state may be roughly touched without their
closing; yet some effect must be thus caused and transmitted across the
whole leaf, for if the glands have recently absorbed animal matter,
even a delicate touch causes them to close instantly. On the whole we
may conclude that the acquirement of a high degree of sensitiveness and
of the power of movement by certain genera of the Droseraceae presents
no greater difficulty than that presented by the similar but feebler
powers of a multitude of other plants.
* See the abstract of his memoir on the contractile tissues of plants,
in the Annals and Mag. of Nat. Hist. 3rd series, vol. xi. p. 188.)
[page 365]
The specialised nature of the sensitiveness possessed by Drosera and
Dionaea, and by certain other plants, well deserves attention. A gland
of Drosera may be forcibly hit once, twice, or even thrice, without any
effect being produced, whilst the continued pressure of an extremely
minute particle excites movement. On the other hand, a particle many
times heavier may be gently laid on one of the filaments of Dionaea
with no effect; but if touched only once by the slow movement of a
delicate hair, the lobes close; and this difference in the nature of
the sensitiveness of these two plants stands in manifest adaptation to
their manner of capturing insects. So does the fact, that when the
central glands of Drosera absorb nitrogenous matter, they transmit a
motor impulse to the exterior tentacles much more quickly than when
they are mechanically irritated; whilst with Dionaea the absorption of
nitrogenous matter causes the lobes to press together with extreme
slowness, whilst a touch excites rapid movement. Somewhat analogous
cases may be observed, as I have shown in another work, with the
tendrils of various plants; some being most excited by contact with
fine fibres, others by contact with bristles, others with a flat or a
creviced surface. The sensitive organs of Drosera and Dionaea are also
specialised, so as not to be uselessly affected by the weight or impact
of drops of rain, or by blasts of air. This may be accounted for by
supposing that these plants and their progenitors have grown accustomed
to the repeated action of rain and wind, so that no molecular change is
thus induced; whilst they have been rendered more sensitive by means of
natural selection to the rarer impact or pressure of solid bodies.
Although the absorption by the glands of Drosera of various fluids
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