STIMULATION AND ITS EESULTS 385 



retarded or stopped. The cells on the other side of the 

 root not being affected, a curvature results from their con- 

 tinued growth. These two capacities for curvature are of 

 great assistance to a root during its growth downwards 

 into the soil. On coming into contact with a particle of 

 earth which is directly opposed to its progress, the tip 

 becomes first stimulated and the subsequent curvature 

 causes it to be deflected past the obstacle if it is not too 

 large. A little further elongation, followed by a geotropic 

 movement, brings the growing zone into contact with the 

 particle and the converse curvature follows, so that the root 

 grows round the obstacle and then resumes its normal direc- 

 tion downwards, under the stimulus of gravity. 



Perhaps the best instance of sensitiveness to slight 

 contact is afforded by the behaviour of twining organs, 

 tendrils, petioles, and climbing stems, the twining of these 

 organs round their supports being altogether due to it. 

 Very great differences of irritability are met with, tendrils 

 generally possessing it in a very high degree, but climbing 

 stems often exhibiting it very feebly ; indeed some observers 

 deny that they possess this form of sensitiveness. In the 

 most sensitive cases a very slight touch is sufficient to 

 bring about a perceptible curvature in a very short space 

 of time. Darwin found that the contact of a small loop of 

 thread, weighing not more than ^ grain, with one of the 

 tendrils of Passiflora gracilis, caused it to bend, while a 

 mere touch with a hard substance induced it to assume the 

 form of a helix in about two minutes. This is perhaps 

 the most sensitive tendril known ; with others a stronger 

 stimulus is needed, and the time taken for the response is 

 longer, the irritability varying considerably. Slight rub- 

 bing is more effective than mere contact. 



The behaviour of tendrils in twining is somewhat pecu- 

 liar. When young they are generally cir cum nutating, and 

 if in their movement they come into contact with any 

 foreign body, they begin to curve round it. If the contact 

 is not prolonged the tendril will curve for some time, but 



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