440 LECTURE XVII. 



might be so constituted that their growth should be promoted, 

 instead of being retarded, by light. Were this the case de 

 Candolle's explanation of positive heliotropism would also 

 apply to negative heliotropism, and there would then be some 

 ground for accepting it. But the researches of Miiller- 

 Thurgau, of F. Darwin, and of Wiesner, shew that also nega- 

 tively heliotropic organs grow more rapidly in darkness than 

 in light. 



The endeavour which is usually made to explain helio- 

 tropic curvature by referring it to the direct action of light 

 upon the elongation of the two opposite sides of the curving 

 organ, is clearly unsatisfactory, and must be abandoned. If 

 we sum up all the facts with which we have now become 

 acquainted on the subject of heliotropism, we find that our 

 knowledge amounts to this, namely, that the curvature is de- 

 pendent upon the direction of the incident rays, and that the 

 organ tends to place its long axis parallel to the direction of 

 the incident rays, directing its apex sometimes towards and 

 sometimes away from the source of light. Light affects the 

 organ as a whole, and not merely the sides exposed to or 

 turned away from it. The curvature, too, is effected by the 

 organ as a whole, and is not merely the result of the direct 

 influence of light upon the side most exposed to it. The 

 difference in length of the convex and concave sides is the 

 result, and not the cause ; the organ, as a whole, is induced to 

 take up a certain position with reference to the direction of 

 the incident rays ; to do this it must curve, and curvature can 

 only be effected by heterauxesis, one side, the convex, becom- 

 ing longer than the other, the concave. 



It remains to explain the mechanism by which an organ 

 performs its heliotropic curvature. Without entering at pre- 

 sent into a full discussion of the subject, which we reserve for 

 a subsequent lecture, it is clear that the heterauxesis is due to 

 the greater turgescence of the cells of the convex side as 

 compared with that of the cells of the concave side. The 

 cells in question must necessarily be parenchymatous, and it 

 appears that they belong to the cortical tissue, a point to 

 which we shall revert when we are studying Geotropism, 



