STIMULATION AND ITS RESULTS 395 
arrangement of clockwork made to undergo a semi-revolution 
at intervals of thirty minutes. The force of gravity thus 
exerted its effect upon alternate sides for this interval of 
time, so that each side of the stem became slightly convex 
apogeotropically in turn. After a period of exposure upon 
the instrument the clockwork was stopped. Instead of the 
side which was then undermost increasing its convexity 
till the stem was vertical, the two sides continued to 
become alternately convex, as if the reversal of the instru- 
ment was stilltaking place. There was, in fact, an artificially 
induced rhythm manifested. 
While the movements of heliotropism show the super- 
position of an induced rhythm upon a natural one, a conflict 
between the two can be observed in many organs. The 
heliotropic curvature is not brought about by a direct move- 
ment of the bending organ, but by its describing a series of 
ellipses. The organ at the time of the incidence of the light 
stimulus is performing its ordinary circumnutation, the apex 
describing a circle. The effect of the stimulus is to turn 
that circle into an ellipse; when the rhythmic impulse 
coincides with the stimulus of the light, the movement is 
accelerated and the resulting curve takes the direction of 
the long axis of the ellipse; when the two act in the opposite 
direction to each other, the short curve of the same figure 
is described. The same result is obtained under the stimulus 
of gravity when the stem or root has by any means been 
inclined from the vertical. The ordinary rhythm of 
circumnutation is resumed when the new position: has been 
assumed and the stimulus consequently no longer acts. 
The slow response to the action of a stimulating force 
may frequently be explained in the same way. Often, 
however, the long delay is due to peculiarities in the 
protoplasm which will be discussed in the next chapter. 
The various positions which are assumed by the different 
sub-aerial organs of plants are evidently those in which 
they can react most advantageously with their environment. 
It must be borne in mind, however, that in every case 
