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Show 364 CONCLUDING REMARKS CnAP. XV. number of the most widely distinct orders,-in the leaves of the many plants which go to sleep at nio·ht d . h . b ' or move when shaken,-an 1n t e uritable 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 ini" tability 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 molec.ular change is probably transmitted to some distance. across its tissue, with the sole difference that no p rceptible effect is produced. We have some evidence in favour of this belief, for we know that a single touch on the glands of Droscra 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 Dionma, the blades in their ordinary state may be roughly touched without their closing; yet some effect must b thus caused and transmitted across the whole leaf, for if tho 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 move1nent by certain g nera of the Droseracero presents no greater difficulty than that presented by the similar but feebler powers of a multi tude of other plants. * See the abstract of his memoir on the contractile tissues of plants, in the ' Annals and 1\Iag. of Nat. lEst.' 3rd series, vol. xi. p. 188. CHAP. xv. ON THE DROSERACEM. 365 The specialised nature of the sensitiveness possessed by Drosera and Diona:a, 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 Dionrea 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 Dionrea the absorption of nitrogeneous 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 Dionrea are also specialised, so as not to be useless! y affected by the weight or impact of drops of rain, or by bla~ts of air. This may be accounted for by supposing that these plants and their progenitors have g~own accustomed to the repeated action of rain and WI~d, 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. .A.lthough the absorption by the glands of Drosera of various fluids excites move- |