Chap, X.] SENSITIVENESS OP THE LEAVES. 187 



CHAPTER X. 



ON THE SENSITIVENESS OF THE LEAVES, AND ON THE LINES OP 

 TRANSMISSION OF THE MOTOR IMPULSE. 



Glands and summits pf the tentacles alone sensitive ^Transmission of the 

 motor impulse down the pedicels of the tentacles, and across the blade 

 of the leaf Aggregation of the protoplasm, a reflex action First dis- 

 charge of the motor impulse sudden Direction of the movements of 

 the tentacles Motor impulse transmitted through the cellular tissue 

 Mechanism of the movements Nature of the motor impulse Ee- 

 expansion of the tentacles. 



We have seen in the previous chapters that many widely 

 different stimulants, mechanical and chemical, excite the 

 movement of the tentacles, as well as of the blade of the leaf ; 

 and we must now consider, firstly, what are the points which 

 are irritable or sensitive, and secondly how the motor impulse 

 is transmitted from one point to another. The glands are 

 almost exclusively the seat of irritability, yet this irritability 

 must extend for a very short distance below them; for when 

 they were cut off with a sharp pair of scissors without being 

 themselves touched, the tentacles often became inflected. 

 These headless tentacles frequently re-expanded; and when 

 afterwards drops of the two most powerful known stimulants 

 were placed on the cut-off ends, no effect was produced. 

 Nevertheless these headless tentacles are capable of sub- 

 sequent inflection if excited by an impulse sent from the disc. 

 I succeeded on several occasions in crushing glands between 

 fine pincers, but this did not excite any movement ; nor did 

 raw meat and salts of ammonia, when placed on such crushed 

 glands. It is probable that they were killed so instantly 

 that they were not able to transmit any motor impulse; for 

 in six observed cases (in two of which, however, the gland 

 was quite pinched off) the protoplasm within the cells of the 

 tentacles did not become aggregated; whereas in some ad- 

 joining tentacles, which were inflected from having been 

 roughly touched by the pincers, it was well aggregated. In 



