406 VEGETABLE PHYSIOLOGY 



in all directions through the plant, only certain cells can 

 be excited to change in response to them, or whether the 

 paths of the conduction of the impulses only take them 

 to the responding organ. But the fact remains that the 

 response bears a definite relationship to the stimulus, par- 

 ticularly to its locality, and to a less degree perhaps to its 

 intensity. If a root-tip is brought into contact with an 

 obstacle, the bending is invariably in such a direction as 

 to enable the root to pass it. When one is allowed to 

 impinge upon a small stone at right angles to its direction 

 of growth, the curvature continues till the root has turned 

 through a right angle, and can for a short distance, at any 

 rate, grow parallel to the opposing surface, till, passing it, 

 it can again respond to the influence of gravitation and 

 grow vertically downwards. The stimulus causing the 

 movement of hydrotropism serves to bring the root-hairs 

 into contact with the moist surface, thus enabling them to 

 discharge their appropriate function. 



The behaviour of the tentacles of Drosera rotundifolia 

 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 

 alighting 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 diahelio- 

 tropic curvatures is also very evident, the leaves being 

 always placed thereby in the position most favourable to 

 the discharge of their functions. 



The very rudimentary differentiation of any mechanism 

 for co-ordination suggests a very immobile condition of the 

 co-ordinating protoplasm. There are several considerations 

 which support this view. In many cases the movement 

 of heliotropism does not commence till a considerable time 



