STIMULATION AND ITS RESULTS 377 
contact with foreign bodies, moisture, and certain chemical 
stimuli. One or two other cases of special sensitiveness 
affecting only particular organisms may also be discussed. 
Larerau ticut.—The effect of the lateral incidence of 
light may be studied very easily in the case of young 
seedlings. 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. 
Leaves in many cases show a similar sensitiveness, but 
the position they assume is different again. They place 
themselves so as to present their upper surfaces at right 
angles to the incident rays. 
‘These phenomena, thus associated with the incidence 
of a lateral light, are spoken of as heliotropism, aphelto- 
tropism, and diaheliotropism respectively. The purposeful 
character of the response is generally obvious; the 
heliotropism of a stem places its leaves in the most favour- 
able position for the action of the chlorophyll in the 
