STIMULATION AND ITS RESULTS 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 54 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 circumnutating, 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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