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 apogeo tropically 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 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 proto- 

 plasm, which will be discussed in the next chapter. 



