SENSITIVENESS. 59 



When the tip of the root is covered with grease, the root 

 does not bend to the wet surface, on which account Mr. 

 Darwin and his sou infer that sensitiveness to moisture 

 resides specially in the tip. The relation these move- 

 ments and this growth bear to the processes of nutrition 

 carried on by the roots is too obvious to need further 

 comment. 



The Influence of Contact on Roots. The effect of 

 pressure such as that caused by the contact of any sub- 

 stance, even if it be very slight, is to produce movements 

 of curvature in the root, the direction of the curvature 

 varying according to the part of the root touched. Thus, 

 if the root be touched in the region where growth is 

 going on most actively, the root becomes concave on the 

 side which is touched, convex on the opposite side, prob- 

 ably because growth is arrested by the pressure on the 

 one side, while it is unrestricted on the other. The con- 

 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, 

 becoming 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. Dar- 

 win). 



It will be seen that the irritation from the various 

 causes above mentioned is not merely local in its effect. 



