STIMULATION AND ITS RESULTS 383 
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 wirH a Forseran 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 hand- 
ling is 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 Dionea, 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 mid-rib were a hinge, bringing together the upper 
surfaces on each side so as to imprison the body which 
touches it. 
