396 VEGETABLE PHYSIOLOGY 



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 heliotropisni 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 geotropism 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 how- 

 ever 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 various 

 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 

 during natural life the plant is receiving coincidently several 

 kinds of stimulation, the effect of some being not infre- 

 quently antagonistic to that of others. It is not easy to 

 discriminate between these, nor to say how the influence of 

 each helps to determine the resultant response. This is the 

 more difficult as not only the stimuli themselves but their 

 relative potencies differ continually. 



