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 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 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 



