504 LECTURE XIX. 



either negative geotropism or with photo-epinasty, with 

 whichever of the two opposing forces is the weaker, to pro- 

 duce equilibrium ; in the third case, photo-epinasty is more 

 powerful than negative geotropism and diaheliotropism com- 

 bined. 



The relative sensitiveness of an organ to light and to 

 gravity is indicated by any change of the direction of growth 

 which may take place when an organ, previously growing 

 in darkness, is exposed to light. We have seen that in these 

 dorsiventral organs the relation of the dark-position and the 

 light-position is not the same in all cases, and we may there- 

 fore conclude that the relative sensitiveness to gravity and 

 to light is not the same in all cases. But the angle between 

 the two directions of growth cannot always be taken as an 

 accurate measure of the relative sensitiveness. For instance, 

 a diageotropic dorsiventral organ which, when exposed to 

 light falling vertically downwards upon its morphologically 

 superior surface, grows obliquely downwards, is clearly more 

 sensitive to light than to gravity. But the degree of its de- 

 viation from the horizontal cannot be taken as a measure of 

 this relative sensitiveness, for the effect of light upon it is 

 probably twofold, photo-epinastic and diaheliotropic, and the 

 tendency of the former to produce downward curvature, that 

 is, to increase the obliquity of the direction of growth, is 

 opposed by that of the latter to maintain the superior surface 

 horizontal. Similarly, in the case of a negatively geotropic 

 dorsiventral organ, the angle of deviation from the vertical is 

 only a measure of the relative sensitiveness to gravity and to 

 light when the epinastic and diaheliotropic effects of light 

 co-operate. This is only the case, in vertical light, when the 

 angle of deviation is less than a right angle ; when, under 

 these circumstances, the direction of growth is obliquely de- 

 scending, the angle of deviation below the horizontal is a 

 measure only of the epinastic effect of light as opposed to 

 negative geotropism and to diaheliotropism. 



With regard, now, to the relation between the plagiotropic 

 effect of light and the angle of incidence of the rays, it is 

 clear that only that portion of the plagiotropic effect which 



