IRRITABILITY. 



485 



the case of less highly irritable tendrils, the stimulus must be 

 more powerful in order to produce a perceptible curvature, 

 and the time which elapses between the moment of stimulation 

 and the commencement of the curvature is longer. 



Fig. 53 (after Darwin). Tendril of Vitis. 



If the contact be not too long-continued, the tendril will 

 straighten out the curvature which it has made. Darwin, in 

 endeavouring to ascertain how often the tendril could thus 

 straighten itself after stimulation, found that it did so no less 

 than twenty-one times in fifty-four hours. After the cessation 

 of the stimulation, the curvature of the tendril continues to 

 increase for a considerable time, it then ceases, and after a 

 few hours the tendril uncurls itself and is again ready to act. 



Now with regard to the period during which tendrils are 

 irritable, and to the distribution of the irritability in a tendril, 

 Darwin has clearly shewn that a tendril is not irritable during 

 the whole of its existence. Speaking generally, tendrils do 

 not possess irritability when they are either very young or 



