THE NERVOUS 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 
