Chap. VUl. MOVEMENTS EXCITED BY LIGHT. 419 



heliotropism, implies that a plant, when unequally 

 illuminated on the two sides, bends from the light, 

 instead of, as in the last sub-class of cases, towards it ; 

 but apheliotropism is comparativoly rare, at least in a 

 well-marked degree. There is a third and large sub- 

 class of cases, namely, those of " Transversal- Helio- 

 tropismus " of Frank, which we will here call diahelio- 

 tropism. Parts of plants, under this influence, place 

 themselves more or less transversely to the direction 

 whence the light proceeds, and are thus fully illumi- 

 nated. There is a fourth sub-class, as faj; as the final 

 cause of the movement is concerned ; for the leaves of 

 some plants when exposed to an intense and injurious 

 amount of light direct themselves, by rising or sinking 

 or twisting, so as to be less intensely illuminated. 

 Such movements have sometimes been called diurnal 

 sleep. If thought advisable, they might be called 

 paraheliotropic, and this term would correspond with 

 our other terms. 



It will be shown in the present chapter that all the 

 movements included in these four sub-classes, con- 

 sist of modified circumnutation. We do not pretend to 

 say that if a part of a plant, whilst still growing, did not 

 jircumnutate — though such a supposition is most im- 

 probable — it could not bend towards the light ; but, as 

 a matter of fact, heliotropism seems always to consist 

 of modified circumnutation. Any kind of movement 

 in relation to light will obviously be much facilitated 

 by each part circumnutating or bending successivelv 

 in all directions, so that an already existing movement 

 has only to be increased in some one direction, and to 

 be lessened or stopped in the other directions, in order 

 that it should become heliotropic, apheliotropic, &c., 

 as the case may be. In the next chapter some obser- 

 vations on the sensitiveness of plants to light, their 



