392 VEGETABLE PHYSIOLOGY 
new position is due to the renewal of growth on the under- 
sides of the swollen nodes, which is excited by the stimulus 
and proceeds till the stem is again vertical. 
The way in which gravitation affects the sensitive part 
of the root is obscure, for we have no conception of the 
nature of the force. It has recently been suggested that 
the stimulation is brought about by the presence of movable 
starch grains in the cells of the sensitive area. When the 
root is pointing downwards these grains lie on the front 
walls; when it is displaced they fail to be symmetrically 
distributed on these, and may impinge on the lateral walls 
that should be vertical. In this way a stimulus due to the 
change of position may arise from such unusual contact 
with the movable grains. These, which may include 
other small bodies than starch grains, have been called 
statoliths. 
Contact witH A Forrtcn Bopy.—Many instances of 
sensitiveness to this form of stimulus have been observed. 
When a leaf of Mimosa pudica is handled, the leaflets all 
droop downwards with great suddenness, and if the handling 
ig very rough, all the leaves on the plant behave similarly. 
When a stamen of Berberis is touched at a point a little 
below the anther, the whole stamen bends forward towards 
the pistil. The stigma of Mimulus, which is composed of: 
two lobes normally extending outwards from each other, 
will close if either lobe is touched with a fine point, so that 
the upper surfaces come into contact with each other. 
When an insect alights on the surface of a leaf of Drosera, 
the tentacles with which it is furnished slowly curl over, so 
that their terminal glands are brought together at the 
exact point of irritation, and at the same time the glands 
are excited to pour out a viscid, slightly acid, secretion 
which is capable of digesting the proteins of the insect’s 
body. The leaf of Dioneea, the Venus’s fly-trap, which is 
normally widely expanded, closes with some rapidity when 
a touch is applied to one of the six sensitive hairs which 
spring from its upper surface, The leaf closes as if the 
