CHAP. VIII. CIRCUMNUTATION AND HELIOTROPIHM. -131 



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 circumimtating movement rectilinear and rapid in 

 one direction alone, namely, towards the light. 



The common view seems to be that heliotropisrn is 

 a quite distinct kind of movement from circuinnuta- 

 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 circurnnutate, 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 circumnutate before they have broken 

 through the ground and have ever felt the influence of 

 light. ' 



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

 beli(3ving that whenever light enters laterally, it is the 



1879), Ssichs hns discusser! the the organs of plants stand with 



manner in which jeotropisin find respect to the direction of th 



heliotropism are affected by dif- incident force, 

 ierences in Ihe angles ut \\Lich 



