294 CONCLUDING REMARKS [Cuar. XV. 
from those of ordinary plants; it is therefore probable that 
all leaves are to a slight degree irritable. Even if an insect 
alights on a leaf, a slight molecular change is probably trans- 
mitted to some distance across its tissue, with the sole 
difference that no perceptible effect is produced. We have 
some evidence in favour of this belief, for we know that a 
single touch on the glands of Drosera does not excite inflec- 
tion ; yet it must produce some effect, for if the glands have 
been immersed in a solution of camphor, inflection follows 
within a shorter time than would have followed from the 
effects of camphor alone. So again with Dionza, the blades 
in their ordinary state may be roughly touched without 
their closing ; yet some effect must be thus caused and trans- 
mitted 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 Drose- 
races presents no greater difficulty than that presented by 
the similar but feebler powers of a multitude of other plants. 
The specialised nature of the sensitiveness possessed by 
Drosera and Dionza, 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 
Dionza with no effect ; but if touched only once by the slow 
movement of a delicate hair, the lobes close; and this differ- 
ence 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 Dionea 
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 Dionxa 
are also specialised, so as not to be uselessly affected by the 
