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 WITH A FOREIGN BODY. 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 Dioncea, 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. 



