Chap. VIII. CIECUMNUTATION AND HELIOVKOPISM. 437 



The result is that the course is rendered more or less 

 zigzag and unequal in rate. Lastly, when the light 

 is very bright all lateral movement is lost ; and the 

 whole energy of the plant is expended in rendering 

 the circumnutating movement rectilinear and rapid in 

 one direction alone, namely, towards the light. 



The common view seems to he that heliotropism is 

 a quite distinct kind of movement from circumnuta- 

 tion; and it may be urged that in the foregoing 

 diagrams we see heliotropism merely combined with, 

 or superimposed on, circumnutation. But if so, it must 

 be assumed that a bright lateral light completely 

 stops circumnutation, for a plant thus exposed moves 

 in a straight line towards it, without describing any 

 ellipses or circles. If the light be somewhat obscured, 

 though amply sufficient to cause the plant to bend 

 towards it, we have more or less plain evidence of still- 

 continued circumnutation. It must further be assumed 

 that it is only a lateral light which has this extraor- 

 dinary power of stopping circumnutation, for we know 

 that the several plants above experimented on, and 

 all the others which were observed by us whilst grow- 

 ing, continue to circumnutate, however bright the light 

 may be, if it comes from above. Nor should it be 

 forgotten that in the life of each plant, circumnuta- 

 tion precedes heliotropism, for hypocotyls, epicotyls, 

 and petioles circumntitate before they have broken 

 through the ground and have ever felt the influence of 

 light. 



We are therefore fully justified, as it seems to us, in 

 believing that whenever light enters laterally, it is the 



1879), Sachs hns discussed the the organs of plants stand with 



manner ia which geotropism and respect to the direction of the 



heliotropism are affected by dif- incident force, 

 ferences in the angles at which 



