CHAP. X. SENSITIVENESS OF THE LEAVES. 233 



facts we may conclude that drops of syrup placed on 

 the backs of leaves do not act by exciting a motor 

 impulse which is transmitted to the tentacles ; but 

 that they cause reflexion by inducing exosmose. 

 Dr. Nitschke used the secretion for sticking insects 

 to the backs of the leaves ; and I suppose that he 

 used a large quantity, which from being dense pro- 

 bably caused exosmose. Perhaps he experimented on 

 cut-off leaves, or on plants with their roots not supplied 

 with enough water. 



As far, therefore, as our present knowledge serves, 

 we may conclude that the glands, together with the 

 immediately underlying cells of the tentacles, are 

 the exclusive seats of that irritability or sensitiveness 

 with which the leaves are endowed. The degree to 

 which a gland is excited can be measured only by 

 the number of the surrounding tentacles which are in- 

 flected, and by the amount and rate of their move- 

 ment. Equally vigorous leaves, exposed to the same 

 temperature (and this is an important condition), 

 are excited in different degrees under the following 

 circumstances. A minute quantity of a weak solu- 

 tion produces no effect ; add more, or give a rather 

 stronger solution, and the tentacles bend. Touch 

 a gland once or twice, and no movement follows ; 

 touch it three or four times, and the tentacle becomes 

 inflected. But the nature of the substance which is 

 given is a very important element : if equal-sized par- 

 ticles of glass (which acts only mechanically), of 

 gelatine, and raw meat, are placed on the discs ot 

 several leaves, the meat causes far more rapid, ener- 

 getic, and widely extended movement than the two 

 former substances. The number of glands which are 

 excited also makes a great difference in the result : 

 place a bit of meat on one or two of the discal 



