394 VEGETABLE PHYSIOLOGY 



appears to be due to the contact injuriously affecting the 

 cells which are pressed upon, so that their growth is retarded 

 or stopped. The cells on the other side of the root not 

 being affected, a curvature results from their continued 

 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 direction 

 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 -gV 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 rubbing is more 

 effective than mere contact. 



The behaviour of tendrils in twining is somewhat peculiar. 

 When young they are continually circumnutating, and if 

 in their movement they come into contact with any foreign 



