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Show 246 DROSERA ROTUNDIFOLIA. CHAP. X. i1npulse in passing transversely across nearly the whole width of the disc had departed somewhat from a true course. This accords with what we have already seen of the impulse travelling less readily in a transverse than in a longitudinal direction. In some other cases, the exterior tentacles did not see1n capable of such accurate movement as the shorter and more central ones. Nothing could be more striking than the appearance of the above four leaves, each with their tentacles point:ing truly to the two little masses of the phosphate on their discs. We might imagine that we were looking at a lowly organised animal seizing prey with its arms. In the case of Drosera the explanation of this accurate power of movement, no doubt, lies in the motor impulse radiating in all directions, and whichever side of a tentacle it :first strikes, that side contracts, and the tentacle consequently bends towards the point of excitement. The pedicels of the tentacles .are flattened, or elliptic in section. Near the bases of the short central tentacles, the flattened or broad face is formed of about :five longitudinal rows of cells; in the outer tentacles of the disc it consists of about six or seven rows; and in the extreme marginal tentacles of above a dozen rows. As the fl~ttened bases are thus fonned of only a few rows of cells, the precision of the movements of the tentacles is the more remarkable; for when the motor impulse strikes the base of a tentacle in a very oblique direction relatively to its broad face, scarcely more than one or two cells towards one end can be affected at first, and the contraction of these cells must draw the whole tentacle into the proper direction. It is, perhaps, owing to the exterior pedicels being much flattened that they do not bend quite so accurately to the point of excite1nent as the CHAP. X. CONDUCTING TISSUES. 247 more central ones. The properly directed movement of the tentacles is not an unique case in the vegetable kingdom, for the tendrils of many plants curve towards the side which is touched; but the case of Drosera is far more interesting, as here the tentacles are not directly excited, but receive an impulse from a distant point; nevertheless, they bend accurately towards this point. On the Nature of the Tissues through which the Motor Impulse is Transmitted.-It will be necessary first to describe briefly the course of the main fi bravascular bundles. These are shown in the accompanying sketch (fig. 11) of a srnall leaf. Little vessels from the neighbouring bundles enter all the many tentacles with which the surface is studded; but these are not here represented. The central trunk, which runs up the foots talk, bifurcates near the centre of the leaf, each branch bifurcating again and again according to the size of the leaf. This FIG. 11. ( Drosera ?'otundijolia.) Diagram showing the distribution of the vascular tissue in a small leaf. central trunk sends off, low down on each side, a delicate branch, which may be called the sublateral branch. There is also, on each side, a main lateral branch or bundle, which bifurcates in the same manner as the others. Bifurcation does not imply that any single vessel divides, but that a bundle |