OCR Text |
Show 252 DRO;SERA ROTUNDIFOLIA. CHAP. X. the blade more quickly in a longitudinal than in a transverse direction; though with time it can pass in any direction. We know that the same stimulus causes movement of the tentacles and aggregation of the protoplasm, and that both influences originate in and proceed from the glands within the same brief space of time. It seems therefore probable that the motor impulse consists of the first commencement of a molecular change in the protoplasm, which, when well developed, is plainly visible, and has been designated aggregation ; but to this subject I shall return. We further know that in the transmission of the aggregating process the chief delay is caused by the passage of the transverse cell-walls; for as the aggregation travels down the tentacles, the contents of each successive cell seem almost to flash in to a cloudy mass. We may therefore infer that the motor impulse is in like manner delayed chiefly by passing through the cell-walls. The greater celerity with which the impulse is transmitted down the long exterior tentacles than across the disc may be largely attributed to its being closely confined within the narrow pedicel, instead of radiating forth on all sides as on the disc. But besides this confinement, the exterior cells of the tentacles are fully twice as long as those of the disc ; so that only half the number of transverse partitions have to be traversed in a given length of a tentacle, compared with an equal space on the disc; and there would be in the same proportion less retardation of the impulse. Moreover, in sections of the exterior tentacles given by Dr. Warming,* the parenchymatous * '':idenskabelige Meddelelser de la Soc. d'Hist. nat. de Copen-hague, Nos. 10-12, 1872, woodcuts iv. and v. . CHAP. X. CONDUCTING TISSUES. 253 cells are shown to be still more elongated; and these would form the most direct line of communication from the gland to the bending place of the tentacle. If the impulse travels down the exterior cells, it would have to cross from between twenty to thirty transverse partitions; but rather fewer if down the inner parenchymatous tissue. In either case it is remarkable that t~~ impulse is able to pass through so many partitions down nearly the whole length of the pedicel, and _to act on the ben~ing place, in ten seconds. Why the 1m pulse, after ha v1ng passed so quickly down one of the extreme marginal tentacles (about 2~ of an inch in length), should never, as far as I have seen, affect the adjoining tentacles, I do not understand. It may be in part accounted for by much energy being expended in the rapidity of the transmission. Most of t:he cells of the disc, both the superficial ones and the larger cells which form the five or six underlying layers, are about four ti1nes as long as broad. They are arranged almost longitudinally, radiating from the footstalk. The motor impulse, therefore, when transmitted across the disc, has to cross nearly four times. as many cell-walls as when transmitted in a longitudinal direction, and would consequently be much delayed in the former case. The cells of the disc converge towards the bases of the tentacles, and are thus fitted to convey the motor impulse to them from all sides. On the whole, the arrangement and shape of the cells, both those of the disc and tentacles, throw much light on the rate and manner of diffusion of the motor impulse. But why the impulse proceeding from the glands of the exterior rows of tentacles tends to travel laterally and towards the centre of the leaf, but not centrifugally, is by no means clear. |