HELIOTROPISM 119 



Through the process of bending, both sides of the stem come under 

 the influence of light, and this fact determines the extent of the bend- 

 ing. As soon as the tip of the stem is bent to such an extent as to ex- 

 pose the symmetrical sides or elements of the stem equally to the light, 

 the bending must cease ; and the tip of the stem must continue to grow 

 in this direction. The reason for this is obviously the fact that if the 

 symmetrical elements of the tip are struck by the ray of light at the 

 same angle, the photochemical effects in symmetrical elements must 

 be the same, and the tendency to contract or the resistance to elonga- 

 tion must be the same on both sides. In this case the tip or rather its 

 axis of symmetry will continue to grow in the direction of the rays of 

 light. It is, of course, taken for granted in this discussion, that the 

 plant is exposed to only one source of light. What has thus far been 

 said refers to positively heliotropic organs, e.g. stems, which bend 

 toward the source of light if illuminated from one side only. The same 

 reasoning applies also to negatively heliotropic organs, e.g. roots, with 

 the difference only, that in the latter case the photochemical effects 

 result in a relaxation or a decreased resistance to the stretching forces 

 on that side of the organ where the light strikes. It appears as if there 

 might exist a chemical or physical difference between stem and root; 

 it might be possible that while the light accelerates oxidation in one 

 organ it accelerates reduction in the organ with opposite heliotropism. 

 It might also be possible that the chemical effects of light are the same 

 in the stem and the root of a plant, but that the colloids in the root are 

 affected by these substances in the opposite sense from those of the stem. 

 We have no data which enable us to test these suggestions. 



Wortmann * has made sections through the tips of stems and roots 

 which were exposed to light from one side only. He found that the 

 cells on that side of the stem which was directed toward the light possess 

 denser protoplasm than the cells on the opposite side; in roots it was 

 the reverse. Wortmann concluded from this that the protoplasm 

 itself is heliotropic in the stem and that it creeps toward the illuminated 

 side, while in the root the reverse process takes place. Botanists have 

 raised the objection that a creeping of the protoplasm from cell to cell 

 could not occur so rapidly on account of the great resistance offered 

 to such a process. I wonder whether the changes which Wortmann 

 observed are not of a character similar to those observed by Darwin 

 in the basal cells of the tentacles of Drosera, an insectivorous plant, 

 which he designated as aggregation.! In the unstimulated condi- 

 tion these cells are filled with a homogeneous watery liquid of a pur- 



* Wortmann, Botanische Zeitung, 1887. 

 t Darwin, Insectivorous Plants. 



