THE NERVOUS MECHANISM OF PLANTS 407 
devoted especially to some definite object which is closely 
related to the stimulus. 
Another consideration which bears upon this question 
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 movement 
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 34 
of a grain, and the resulting movement will be of consider- 
able extent and prolonged for some time. The sensitive 
hair of the leaf of Dionwa 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 
bending 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 an 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. 
