V Movements of Plants 83 



And even if we accept the general conclusion that the 

 altered growth is due to a modification of turgescence in 

 the growing cells, to what is the change of turgescence due ? 

 — to a change in the elasticity of the cell-wall, as Sachs, 

 Pfeffer, Wiesner, and others suggest ; or to a change in 

 the osmotic properties of the cell-sap, the stimulus pro- 

 moting, according to De Vries, the formation of substances 

 which are osmotically active ; or to a change in the perme- 

 ability of the protoplasm, resulting directly from the influence 

 of light, as Vines believes ? Finally, according toWortmann, 

 growth depends on three factors — the osmotic force within 

 the cell, the extensibility of the cell-walls, and the relative 

 abundance of surrounding water. Or if we turn to a recent 

 Botanical Journal, we find Noll maintaining that curvature is 

 due to a greater extensibility of the cell-walls on the convex 

 side and to a diminished extensibility on the concave side — 

 opposite effects produced by the mysterious activity of the 

 parietal protoplasm ; and we find Wortmann denying this, 

 saying that the curvature is due to a movement of the 

 protoplasm which in the region of growth alters the thick- 

 ness and tension of the cell-wells, distributing its materials 

 so that the two sides are unequally thickened. 



Thus the student may see that as in regard to structural 

 problems, such as the real nature of the pitchers of pitcher 

 plants, so in regard to physiological problems, such as this 

 of the movement of plants in relation to light and other 

 stimuli, there is much difference of opinion. The reason 

 for this is perfectly obvious — the problem is so com- 

 plex, so many factors have to be considered. The history 

 of the investigation is, as in all other cases, one of pro- 

 gressive analysis. That plants move towards the light 

 in virtue of " hehotropism " is obviously a truism ; that the 

 " heliotropism " may be due to unequal growth expresses 

 the first step in the analysis ; the unequal growth is asso- 



