Chap. X.] CONDUCTING TISSUES. 205 



and much more slowly across the blade of the leaf. We shall 

 also see why it crosses the blade more quickly in a longi- 

 tudinal 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 pro- 

 toplasm, 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 trans- 

 mitted 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 thq same 

 proportion less retardation of the impulse. Moreover, in 

 sections of the exterior tentacles given by Dr. Warming,' 

 the parenchymatous cells are shown to be still more elon- 

 gated; and these would form the most direct line of com- 

 municating 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 trans- 

 verse partitions: but rather fewer if down the inner paren- 

 chymatous 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 hav- 



* ' Vldennkahellge Mwldelelser hnjjne/ Nos. 10-12, 1872, wood- 

 de la Soc. d'lllst. nat. de Copen- cuts ir. and t. 



