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 

 statolitUs. 



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 handling 

 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 thai 

 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 



