496 MANUAL OP BOTANY 



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 forwards towards the pistil. The stigma 

 of Mimulus, which is normally composed of two lobes 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 point of 

 irritation ; and at the same time the glands pour out a viscid, 

 slightly acid secretion which is capable of digesting the proteids 

 of the insect's body. The leaf of Dionsea, the Venus's fly-trap, 

 which is normally widely expanded, closes with great rapidity 

 when one of the six sensitive hairs which spring from its upper 

 surface is touched. The leaf closes as if the midrib were a 

 hinge, approximating the upper surfaces on each side so as to 

 imprison the body which touches it. 



This form of sensitiveness is peculiarly characteristic of the 

 growing apex of young roots. If a seedling bean is taken, 

 and a small piece of cardboard is attached to one side of the 

 tip, a curvature speedily results which causes the root to bend 

 away from the irritating body. If the movement takes the 

 sensitive part away from the latter, the curvature is slight, but 

 if, as in the experiment, the foreign body accompanies it in its 

 displacement, the curvature will continue until the root is coiled 

 completely round. The stimulus in the case of this movement 

 must be prolonged, differing thus from the cases noted above, 

 where a mere touch is sufficient to bring it about. 



The cause of the curvature must be the sensitiveness of the 

 protoplasm to the stimulus of contact. The movement cannot 

 result from any injury done to the cells causing a hindrance to 

 growth, for if this were the case the curvature would be towards 

 the body touching the root, whereas it is in the opposite direction. 

 If the cells of the side of the root at some distance from the tip 

 are stimulated in this way, the curvature is round the stimulating 

 body and not away from it. This may very likely be due to 

 such a mechanical hindrance of the growth. In the first case, 

 moreover, the part which shows the curvature is not the part 

 irritated, but a region some little distance further back. 



Perhaps the best instance of this susceptibility to slight 

 contact is afforded by the behaviour of tendrils, the twining of 

 these organs round their supports being altogether due to it. 

 A very slight touch is sufficient to bring about a perceptible 



