138 MODIFIED CIECUMNUTATION. Chap. VUL 



movement of circumnutation which gives rise to, or la 

 converted into, heliotropism and apheliotropism. On 

 this view we need not assume against all analogy that 

 a lateral light entirely stops circumnutation ; it merely 

 excites the plant to modify its movement for a time 

 in a beneficial manner. The existence of every pos- 

 sible gradation, between a straight course towards a 

 lateral light and a course consisting of a series of loops 

 or ellipses, becomes perfectly intelligible. Finally, 

 the conversion of circumnutation into heliotropism or 

 apheliotropism, is closely analogous to what takes place 

 with sleeping plants, which during the daytime de- 

 scribe one, or more ellipses, often moving in zigzag lines 

 and making little loops ; for when they begin in the 

 evening to go to sleep, they likewise expend all their 

 energy in rendering their course rectilinear and rapid. 

 In the case of sleep-movements, the exciting or regu- 

 lating cause is a difference in the intensity of the 

 light, coming from above, at different periods of the 

 twenty-four hours ; whilst with heUotropic and aphe- 

 liotropic movements, it is a difference in the intensity 

 of the light on the two si des of the plant. 



Transversal-heUotropismus (of Frank *) or Biahelio- 

 troptsm. — The cause of leaves placing themselves 

 more or less transversely to the light, with their 

 upper surfaces directed towards it, has been of late 

 the subject of much controversy. We do not here 

 refer to the object of the movement, which no doubt 

 is that their upper surfaces may be fully illuminated, 

 but the means by which this position is gained. 

 Hardly a better or more simple instance can be given 



♦ 'Die naturliehe Wagerechte Prnge iiber Transver^alGeo-nnd 



Uichtung von Pflanzeiithi ilen,' Heliotropismus," ' Bot. Zcitung, 



18 '0 .See ylso sjme interusting 1873, p. 17 et seq. 

 ailiclLS by the same author, " Zur 



