petioles and flower-peduncles, &c., modified for prehension) belonging to a large [page 364] 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 irritability or sensitiveness, but, as Cohn has remarked,* the tissues of the plants thus endowed 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 Dionaea, 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 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