STIMULATION AND ITS RESULTS 385 
affect more or less the whole plant, we find instances of 
special sensitiveness in various parts to influences which 
are not appreciated by the whole of the living substance, 
but are especially received by the young growing regions. 
Of these the most prominent are lateral light, gravity, contact 
with foreign bodies, moisture, and certain chemical stimult. 
The effects produced, however, must be regarded in all 
these cases as the action of the whole organism, which 
modifies and co-ordinates the behaviour of its several parts 
for the good of the plant. The usual form of response 
consists in various modifications of local growth. 
One or two other cases of special sensitiveness affecting 
only particular organisms may also be discussed. 
Laterat Ligut.—The effect of the lateral incidence of 
light may be studied very easily in the case of young seed- 
lings. When one of these is so placed that one side of its 
stem is more brightly illuminated than the opposite, a 
curvature soon appears in the part which is actively growing. 
This is of such a nature, and takes place to such an extent, 
as to cause the axis of the plant to take up a position in 
which it is parallel to the direction of the incident rays. 
It manifests itself in some cases very rapidly, in others 
more slowly. This response to the stimulus of a lateral 
illumination is not confined to the stems of seedlings, but 
may be seen to a greater or less degree in many adult plants. 
It is a matter of common observation that geraniums grown 
in a window all bend their stems and petioles towards 
the illuminated side. 
In other cases the same stimulus may produce an opposite 
effect. When certain young roots are exposed to it, they 
curve so as to place themselves in the same position with 
regard to the incident rays, but with their growing apices 
in the opposite direction. Stems are said accordingly to 
grow towards, and roots away from, the light-source. This 
behaviour is not, however, confined to roots; it is exhibited 
by the tendrils of Bignonia capreolata, the peduncles of 
Cyclamen persicum, and by many other organs. 
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