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 cannot, therefore, be sent along the fibro-vascular 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 Dionaea, that this same conclusion, namely that the motor impulse is not transmitted by the fibro-vascular bundles, is plainly confirmed; and Prof. Cohn has come to the same conclusion with respect to Aldrovanda--both members of the Droseraceae.
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 [page 252] 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 into 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
* 'Videnskabelige Meddelelser de la Soc. d'Hist. nat. de Copenhague,' Nos. 10-12, 1872, woodcuts iv. and v. [page 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 the impulse is able to pass through so many partitions down nearly the whole length of the pedicel, and to act on the bending place, in ten seconds. Why the impulse, after having passed so quickly down one of the extreme marginal tentacles (about 1/20 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 the cells of the disc, both the superficial ones and the larger cells which form the five or six underlying layers, are about four times as long as broad.