416 VEGETABLE PHYSIOLOGY 



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 

 after the access of the lateral light, the actual time varying 

 in different cases. Similarly the apogeotropic curvature 

 of a stem placed horizontally may not be observable till 

 the stimulus has lasted for more than an hour. We have 

 what is generally called a long latent period before the 

 manifestation of the irritability. The time is taken up 

 in bringing about the response to the stimulus and not in 

 appreciating it. The power of appreciation is generally 

 rapid, as we should imagine when we remember the great 

 degree of sensitiveness as measured by the smallness of 

 the stimulus which is necessary to produce an effect. The 

 sluggish nature of the co-ordinating mechanism can be 

 seen from the fact that the removal of a stimulus before 

 any response to it has become evident does not prevent 

 that response from subsequently appearing. If young 

 roots are laid upon their sides for about an hour and a 

 half, and their tips are then carefully amputated so that 

 they no longer perceive the stimulus of gravitation, they 

 will nevertheless curve after a while towards the side 



