RHYTHM 511 



tendency to rhythmic change can be manifested by the produc- 

 tion of an altogether artificial rhythm, induced by submitting 

 the plant to intermittent stimulation. Darwin and Pertz have 

 described a very interesting experiment of this nature. A plant 

 was fixed to a spindle placed horizontally, which, by an ar- 

 rangement of clockwork, was ixiade to make a semi-revolution 

 at intervals of thirty minutes. The force of gravity thus exerted 

 its effect on alternate sides for this interval of time, so that each 

 side of the stem tended to become convex apogeotropically in 

 turn. After a period of exposure upon the instrument the clock- 

 work was stopped. Instead of the side which was then under- 

 most increasing its convexity 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. 



The conflict between the natural rhythm and the movement 

 due to stimulation can be observed in the heliotropic curvature 

 of many organs. The heliotropic position is not brought about 

 by a direct movement of the 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 stinmlus 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 geotropism. 



The slow response to the action of a stimulating force may 

 frequently be explained in the same wa^j. Often, however, the 

 long dela^^ in responding is due to the sluggish nature of the 

 protoplasm, a considerable time being taken up in inducing the 

 necessary change in its motility. AVe have then what is called 

 a latent pei'iod, before the manifestation of the irritability. 

 Similarly the results of the stimulation continue for some time 

 after the exciting cause is removed, giving what is called an 

 after effect. An example of this is afforded by some experi- 

 ments of Cisielski's ; he laid some geotropic roots horizontally 

 on moist sand, and allowed them to remain there for some time. 

 Before they began to show their geotropic curvature, he am- 

 putated the tips, which we have seen to be the sensitive parts 

 or sense organs. After a time the geotropic curvature never- 

 theless took place. The stimulus was received while the roots 



