THE NEKVOUS MECHANISM OF PLANTS 407 



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 

 which was downwards during the first exposure. The same 

 curvature will be seen if they are placed in a vertical 

 position after the amputation. The long delay in the 

 response may no doubt be attributed partly to the disturb- 

 ance set up by the amputation, but the fact tHat the response 

 to the stimulus does eventually take place shows that the 

 delay is due to slowness of changes in the responding 

 protoplasm and not in the part which is sensitive. 



An even more striking instance of action after the 



