404 VEGETABLE PHYSIOLOGY 
and Pertz have described a very interesting experiment 
of this nature. A plant was fixed to a spindle placed 
horizontally, on a modification of the klinostat, and was by 
an 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 con- 
vexity till the stem was vertical, the two sides continued 
to become alternately convex, as if the reversal of the 
instrument was still taking 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 
movement 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 circum- 
nutation, 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 form of the greater curvature of the ellipse ; when the 
two act in the opposite direction to each other, the lesser 
curvature of the same figure is described. The same result ig 
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 proto- 
plasm, which will be discussed in the next chapter, 
