398 VEGETABLE PHYSIOLOGY 
is that an extremely small stimulus is able to bring about 
a very considerable effect, and that there is no direct or 
simple ratio between the intensity of the stimulus and the 
extent of the response, whether this takes the form of move- 
ment or chemical change. The tendrils of Passiflora, 
already alluded to, can be caused to move by the contact 
with them of a small piece of thread, weighing not more 
than #, of a grain, and the resulting movement will be of 
considerable extent and prolonged for some time. The 
sensitive hair of the leaf of Dzon@ea needs only a touch to 
cause a rapid movement of the whole leaf-blade; the 
pricking of the staminal filament of Berberis causes a 
considerable movement of a relatively bulky body. The 
seedlings of Phalaris bend with some speed towards a 
light which is not sufficient to cause a visible shadow at 
the distance at which they are placed from it. 
It can hardly be imagined that such slight disturbances 
can act mechanically upon the parts that move. This point 
is illustrated by the observation made by Wiesner, that if a 
part which responds only to the stimulus of lateral light is 
exposed for some time to such an illumination, and then, 
before the heliotropic curvature has begun, is removed 
into darkness, it will slowly bend towards the side which 
has been stimulated. The same observation has been 
made by other observers in the case of the stimulus of 
gravitation. There is no explanation possible other than 
that the stimulus brings about changes in the protoplasm 
of the cells of the moving part, which slowly modify their 
relation to the water of their contents, so that a great 
alteration of their turgidity results. Moreover, the separa- 
tion of the part stimulated and the cells which are the 
seat of the resulting action, implies that there must be in 
the plant a means of more or less rapidly conducting such 
external impressions from one part to another. 
If, then, we admit that there is even a rudimentary 
neryous system in plants, we may proceed with an inquiry 
into the degree of its differentiation, and the completeness 
