SENSITIVENESS. 5 ( J 



sequence of this is that the roots in such case turn towards 

 the obstructing substance, and, if it be of small dimensions, 

 coil themselves around it, or, if it be too large for this 

 purpose, creep over its surface. 



On the other hand, if the extreme tip of the root be 

 touched, the root bends away from the obstruction, becom- 

 ing convex on the side where contact is effected, concave on 

 the opposite side, the root sometimes making complete 

 loops by its continued curved growth. The object of this 

 sensibility to contact appears to be to enable the roots to 

 overcome the obstacles they meet with in the soil. Thus 

 " when a root meets with an obstacle in its way. the pressure 

 on one side of the tip causes the growing part of the root to 

 grow more rapidly on the side of the obstacle, and thus 

 curve away from it " (F. Darwin). 



It will be seen that the irritation from the various causes 

 above mentioned is not merely local in its effect, but that it 

 induces movement in adjoining parts, on which account the 

 parts so influenced are spoken of as " sensitive." 



Passage of Boots through the Soil Summary. The 



course followed by a root through the soil is, says Darwin, 

 " brought about and modified by extraordinarily complex 

 and diversified agencies by geotropism, acting, as has just 

 been explained, in a different manner on the primary, 

 secondary, and tertiary radicles ; by sensitiveness to contact, 

 different in kind in the apex and in the part immediately 

 above the apex; and apparently by sensitiveness to the 

 varying dampness of different parts of the soil. . . . The 

 direction which the apex takes at each successive period of 

 the growth of a root ultimately determines its whole course ; 

 it is, therefore, highly important that the apex should 



