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 son 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 gne 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 fiom it” eG Dar- 
win). 
ce will be seen that the irritation from the various 
causes above mentioned is not merely local in its effect, 
