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Show 250 DROSERA ROTUNDIFOLIA. CHAP. X. proves that the spiral vessels of the central trunk may be divided, and yet the motor impulse be transmitted from the distal to the basal end; and this led me at first to suppose that the motor force was sent through the closely surrounding fibrous tissue; and that if one half of this tissue was left undivided, it sufficed for complete transmission. But opposed to this conclusion is the fact that no vessels pass directly from one side of the leaf to the other, and yet, as we have seen, if a rather large bit of meat is placed on one side, the 1notor impulse is sent, though slowly and imperfectly, in a transverse direction across the whole breadth of the leaf. Nor can this latter fact be accounted for by supposing that the transmission is effected through the two inosculations, or through the circumferential zigzag line of union, for had this been the case, the exterior tentacles on the opposite side of the disc would have been affected before the more central ones, which never occurred. We have also seen that tho extre1ne marginal tentacles appear to have no power to transmit an impulse to the adjoining tentacles; yet the little bundle of vessels which enters each 1narginal tentacle sends off a minute branch to those on both sides, and this I have not observed in any other tentacles; so that the marginal ones are more closely connected together by spiral vessels than are the others, and yet have much less power of communicating a motor impulse to one another. But besides these several facts and arguments we have conclusive evidence that the motor impulse is not sent, at least exclusively, through the spiral vessels, or through the tissue immediately surrounding thmn. We know that if a bit of meat is placed on a gland (the immediately adjoining ones having been removed) on any part of the elise, all the short sur- CHAP. X. CONDUCTING TISSUES. 251 rounding tentacles bend almost simultaneously with great precision towards it. Now there are tontacl s on the disc, for instance near the extremities of the sublateral bundles (fig. 11), which are supplied with vessels that do not come into contact with the branches that enter the surrounding tentacles, except by a very long and extremely circuitous course. Nevertheless, if a bit of meat is placed on the gland of a tentacle of this kind, all the surrounding ones are inflected towards it with great precision. It is, of course, possible. that an impulse might be sent through a long and circuitous course, but it is obviously impossible that the direction of the movement could be thus communicated, so that all the surrounding tentacles should bend precisely to the point of excitement. The impulse no doubt is transmitted in straight radiating lines from the excited gland to the surrounding tentacles; it cannGt, therefore, be sent along the fibrovascular bundles. The effect of cutting the central vessels, in the above cases, in preventing the transmission of the motor impulse from the distal to the basal end of a leaf, may be attributed to a considerable space of the cellular tissue having been divided. We shall hereafter see, when we treat of Dionrea, that this same conclusion, namely that the motor impulse is not transmitted by the fibro-vascular bundles, is plainly confirmed ; and Professor Cohn has come to the sa1ne conclusion with respect to Aldrovanda-both members of the Droseracere. As the motor impulse is not transmitted along the vessels, there remains for its passage only the cellular tissue ; and the structure of this tissue explains to a certain extent how it travels so quickly down the long exterior tentacles, and much more slowly across the blade of the leaf. We shall also see why it crosses |
