580 LECTURE XXI. 



for water exercised by the cell-sap, or to changes in the pro- 

 toplasm, in the cells. 



The former of these alternatives is strongly supported by 

 de Vries, who regards the differences in the turgidity of the 

 cells of the two sides of the curving organ as being due to an 

 increased formation of osmotically active substances in the 

 cells of the convex side. But to this view there are certain 

 fatal objections. In the first place, according to de Vries' 

 own observations it appears that the osmotically active sub- 

 stances of the cell-sap are organic acids. From the point oi 

 view which he takes up with regard to the intimate mechanism 

 of curvature it is to be expected that the cells of the convex 

 side of the curving organ should contain a relatively large 

 quantity of organic acids. As a matter of fact, however, it 

 appears from Kraus' researches that, in the case of geotropic 

 curvature, the absolute amount of organic acids in the cells of 

 the convex side diminishes, instead of increasing, as curvature 

 proceeds. In the second place, de Vries' view does not take 

 into account the diminution of the turgidity of the cells of the 

 concave side. It may be argued that the loss of water which 

 these cells undergo is due to the increased osmotic attraction 

 exercised by the sap of the cells of the convex side, in other 

 words, to a redistribution of the water in the cells of the 

 growing region. But de Vries' own observations prove that 

 this is not the case. He found, namely, that the curvature of 

 a tendril on stimulation is more rapid when its intercellular 

 spaces are injected with water. Under such circumstances 

 there can be no question of a redistribution of the water in 

 the growing region. De Vries' explanation, to be complete, 

 must include not only an increase in the amount of the 

 osmotically active substances in the cells of the convex side, 

 but also a diminution in the amount of these substances in the 

 cells of the concave side, inasmuch as the latter fail to become 

 turgid when water, as in the injection-experiment, is abund- 

 antly supplied to them. In view of these various facts it is 

 impossible to consider that curvature is due to differences in 

 the osmotic properties of the cell-sap of the cells of the 

 opposed halves of the curving organ. 



