190 DROSERA ROTUNDIPOLIA. [Chip. X. 



and the leaves soon become flaccid. If they are now im- 

 mersed in a fluid of low specific gravity, the tentacles re- 

 expand. From these 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 reflection by inducing exosmose. Dr. Nitsch- 

 ke 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 probably caused exosmose. Per- 

 haps 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 inflected, and by the amount and rate of their 

 movement. Equally vigorous leaves, exposed to the same 

 temperature (and this is an important condition), are ex- 

 cited in various degrees under the following circumstances. 

 A minute quantity of a weak solution produces no effect; 

 add more, or give a rather stronger solution, and the ten- 

 tacles bend. Touch a gland once or twice, and no movement 

 follows; touch it three or four times, and the tentacle be- 

 comes inflected. But the nature of the substance which is 

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

 of glass (which acts only mechanically), of gelatine, and 

 raw meat are placed on the discs of several leaves, the meat 

 causes far more rapid, energetic, and widely extended move- 

 ment 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 glands, 

 and only a few of the immediately surrounding short tenta- 

 cles are inflected; place it on several glands, and many more 

 are acted on; place it on thirty or forty, and all the ten- 

 tacles, including the extreme marginal ones, become closely 

 inflected. We thus see that the impulses proceeding from a 

 number of glands strengthen one another, spread farther, 

 and act on a larger number of tentacles, than the impulse 

 from any single gland. 



