26 BOTANY 



ture is not met with, as a slight change in the direction 

 of growth causes the root to grow past the obstructing 

 body, and then the downward direction is resumed. 



We can thus show that the young root has not 

 only an appreciation of direction, but it can in some 

 way recognise when it is in contact with some solid 

 obstacle and that it can modify its 

 growth with a view to getting past such 

 a body and penetrating further into 

 the soil. 



The root further appreciates the in- 

 cidence of a lateral light. If the seed- 

 ling is cultivated in a glass vessel and 

 so placed that light reaches it only on 

 one side it very quickly modifies its 

 growth so that the apex becomes turned 

 away from the light. In the soil this 

 behaviour brings it closer to the par- 

 ticles of the soil, especially a little way 

 behind the tip. These three rudimen- 

 tary senses or sensitivities are supple- 

 nted by a fourth. It shows an 

 evident appreciation of the presence 

 of moisture, and grows towards the dampest parts of the 

 medium in which it is placed. 



If we revert for a moment to the young cress seedling 

 we find that when it has attained the length of about 

 half an inch a number of long delicate outgrowths of 

 its surface may be seen arranged in a broad band all 

 round the root at a little distance behind the apex (Fig. 

 9). So long as the root grows this band of outgrowths, 

 which are known as root hairs, is maintained. New ones 

 are formed on the side of the apex while the older ones 

 die and disappear on the hinder margin of the band. 

 As the root advances in the soil these hairs become so 

 closely attached to its particles that they cannot be 



