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 
erowth. 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 zy; 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 
