NEKVOUS MECHANISMS 603 



course which serves the same purpose. The stimulus causing 

 the movement of hydrotropism serves to bring the root-hairs 

 into contact with the moist surface, thus enabling them to dis- • 

 charge their appropriate function. The behaviour of the tentacles 

 of Drosera is very interesting in this connection. The leaf is of 

 some size, and can therefore receive stimuli over a fairly large 

 area. When the tentacles bend over in response to the alight- 

 ing of an insect, they do not do so irregularly, but always place 

 their glandular apices directly upon the spot which is the centre 

 of the disturbance. This is very definitely purposeful, the 

 invader being captured and digested wherever it alights, as 

 all the tentacles are brought to bear upon it. The purposeful 

 character of heliotropic and diaheliotropic curvatures is also 

 very evident, the leaves being always placed thereby in the 

 most favourable position for the discharge of their functions. 



This special sensitiveness can be distinguished again from 

 the general state of irritability m the plant. So long as the 

 conditions remain favourable the latter is considerably prolonged, 

 but the power of responding to particular impressions disappears 

 much sooner. In some cases this power can be ascertained to 

 have been lost by an organ while it still retains its power of 

 circumnutating. The effect of a prolonged stimulation some- 

 times is to fail to produce a movement. Thus in the case of 

 Dionaea, repeated touching of one of the hairs, if the leaf be 

 prevented from closing, ultimately leads to such a touch having 

 no effect when the leaf is released. This is a result of exhaus- 

 tion or fatigue. At iirst it may seem doubtful whether or no 

 the mterference with the free response of the leaf may have so 

 injured the motor mechanism as to make it incapable of acting. 

 The exhaustion, however, is shown to be that of the hair and not 

 of the hmge, by the fact that touching another of the hairs at 

 once causes closiu-e. 



The nervous sensitiveness is by this and many other similar 

 experiments shown to be capable of fatigue. A similar suspen- 

 sion of power may be demonstrated by exposing the sensitive 

 parts to anaesthetics, such as the vapour of chloroform, ether, 

 &c. The effect of these drugs at once suggests a similar action 

 to that which they have on the nervous mechanism of an 

 animal. When the effect of the fatigue or the anaesthetic has 

 passed off, the organ agam becomes capable of responding. 



A nervous system generally' can be shown to possess three dis- 

 tinct parts : one means whereby external stimulation is received 

 and appreciated ; another whereby movements, &c., are caused ; 



