384 VEGETABLE PHYSIOLOGY 



This point will be discussed however more fully in a sub- 

 sequent chapter. 



The movements of geotropism and apogeotropism are 

 not confined to growing organs. When the haulm of a 

 grass is placed horizontally on the ground, as" is the case 

 when a patch of wheat or other cereal is beaten down by 

 wind or storm, it after a time again becomes erect. The 

 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. 



As in the case of heliotropism, the sensitiveness varies 

 very much in . different plants, and the response made is 

 not always the same throughout the life of an organ. 



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, if either lobe is touched with a fine point, 

 close, 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 proteids of 

 the insect's body. The leaf of Dionaa, 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. 



