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Show 308 DION.lEA MUSCIPULA. CHAP. XIII. resist being opened, as by a thin wedge driven between them, with astonishing force, and are generally ruptured rather than yi~ld. If not. ruptured, they close again, as Dr. ~~nby In~orms me In a letter, "with quite a loud flap. But If the end of a leaf · h ld firmly between the thumb and finger, or by a IS e . l h clip, so that the_ lobe~ . cannot b~gin .to. c ose, t ey exert, whilst in this position, very httle fo~ce. I thought at first that the graclu~l pressing together of the lobes was caused exclusively by captured insects crawling over and repeatedly irritating the sensitive filaments; and this view seemed the more robable when I learnt from Dr. Burdon Sanderson ~hat whenever the filaments of a closed leaf are irritated, the normal electric current is disturbed. Nevertheless, such irritation is by no means necessary, for a dead insect, or a bit of meat, or of albumen, all act equally well; proving that in ~hese c~ses it is the absorption of animal matter which excites the lobes slowly to press close together. We have s~en that the absorption of an extremely small quantity of such matter also causes a fully expanded leaf to close slowly; and this movement is clearly analogous ~o the slow pressing together of the concave lobes. This latter action is of high functional importance to the plant, for the glands on both sides are thus brought into contact with a captured insect, and consequently secrete. The secretion with animal matter in solution is then drawn by capillary attraction over the whole surface of the leaf, ·causing all the glands to secrete and allowing them to absorb the diffused animal matter. The movement, excited by the absorption of such matter, though slow, suffices for its final pur~~se, whilst the movement excited by one of the sensitive nlamen ts being touched is rapid, and this is in dis- CHAP. XIII. Jlt'IANNER OF CAPTURING INSECTS. 309 pensable for the capturing of insects. These two movements, excited by two such widely different means, are thus both well adapted, like all the other functions of the plant, for the purposes which they subserve. There is another wide difference in the action of leaves which enclose objects, such as bits of wood, cork, balls of paper, or which have had their filaments merely touched, and those which enclose organic bodies yielding soluble nitrogenous matter. In the former case the leaves, as we have ·seen, open in under 24 hrs. and are then ready, even before being fully expanded, to shut again. But if they have closed over nitrogen-yielding bodies, they remain closely shut for many days; and after re-expanding are torpid, and never act again, or only after a considerable interval of time. In four instances, leaves after catching insects never reopened, but began to wither, remaining closed-in one case for fifteen days over a fly ; in a second, for twenty-four <lays, though the fly was small; in a third for twenty-four days over a woodlouse ; and in a fourth, for thirty-five days over a large Tipula. In two other cases leaves remained closed for at least nine days over flies, and for how many more I do not know. It should, however, be added that in two instances in which very small insects had been naturally caught the leaf opened as quickly as if nothing had been caught; and I suppose that this was due to such small insects not having been crus_,P.ed or not having excreted any animal matter, so that the glands were not excited. Small angular bits of albumen and gelatine were placed at both ends of three leaves, two of which remained closed for thirteen and the other for twelve days. Two other leaves remained closed over bits of |
