34 GEOTROPISM. [CH. Ill 



position the seed may be placed, the radicle will bend 

 until it reaches the vertical, and will go on growing 

 downwards towards the centre of the earth. This mode 

 of growth is known as geotropism, and is but one out of a 

 number of special powers which the plant possesses of 

 directing its growth according to external circumstances. 

 It used to be believed that the radicle attained the 

 vertically downward position in virtue of plasticity, that 

 it bent over by its own weight as a piece of sealing-wax 

 bends if kept in a warm place. This is quite a mistaken 

 view : we now know that the curvature of the root is an 

 active process due to a rearrangement of longitudinal 

 growth. That is to say the curvature results from one 

 longitudinal half of the root growing more quickly than 

 the other half. We further know that this rearrangement 

 of growth is a response to a stimulus quite as certainly 

 as that the movements of animals are brought about by 

 stimulation. It is not of course suggested that a plant has 

 consciousness, nor do we claim consciousness for muscles 

 or nerves. But botanists do claim for plants an irrita- 

 bility or sensitiveness by means of which the plant's 

 movements are directed to suit its environment: they 

 believe that by this sensitiveness the growth of the 

 plant is directed in the same unconscious way that the 

 flight of a moth may be supposed to be directed towards 

 a lamp. I shall return to this point when the upward 

 growth of the stem into the air is discussed, but I think 

 it is worth noting that at the very outset of the life 

 of the plant, in its germinating state, it is endowed 

 with and guided by a very remarkable kind of sensitive- 



