364 CONCLUDING REMARKS Cuar. XV. 
number of the most widely distinct orders,—in the 
leaves of the many plants which go to sleep at night, 
or move when shaken,—and in the irritable stamens 
and pistils of not a few species. We may therefore 
infer that the power of movement can be by some 
means readily acquired. Such movements imply iri- 
tability or sensitiveness, but, as Cohn has remarked,* 
the tissues of the plants thus endoyed do not differ 
in any uniform manner 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 transmitted 
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 inflection; 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 Dionwa, the 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 Droseracee presents no greater difficulty 
than that presented by tne similar but feebler powers 
of a multitude of other plants. 
* See the abstract of his me- Mag. of Nat. Hist.’ 3rd series, 
moir on the contractile tissues vol. xi. p. 188. 
of plants, in the ‘Annals and 
