IRRITABILITY. 579 



We will first take the case of multicellular organs. The 

 curvature of such an organ is due, in the first instance, to a 

 difference in the turgidity of the cells of the two opposite 

 halves of the growing region, those of the concave side being 

 less turgid than those of the convex, this difference being due 

 as well to a decrease in the turgidity of those of the concave 

 side as to an increase in that of the cells of the convex side. 

 The fact that the curvature is due to a difference in turgidity 

 of the cells of the opposed halves of the growing region is 

 established by the observations of de Vries, who found that 

 an organ which has begun to curve, whether heliotropically or 

 geotropically, or, as in the case of tendrils, in consequence of 

 contact, is more or less straightened out on plasmolysis. The 

 fact that the curvature is due as well to a decrease in the 

 turgidity of the cells of the concave side as to an increase 

 in that of the cells of the convex side is established by the 

 general observation that, whereas the rate of growth of the 

 convex side is more rapid than that of the organ when 

 straight, the rate of growth of the concave side is less rapid. 

 The slower growth of these cells may be due in part to 

 compression, but it is mainly due to a definite diminution of 

 turgidity. 



This being the mechanism of curvature of multicellular 

 growing organs, the question arises as to how the observed 

 differences in turgidity are brought about. It is inconceivable 

 that it should be due to differences in the rigidity of the 

 cell-walls of the cells of the two halves of the organ. If it 

 be suggested that curvature is due to an increased rigidity 

 of the walls of the cells of the concave side, such an assump- 

 tion must involve the further assumption that the rigidity of 

 the walls of the cells of the convex side is diminished. Such 

 changes in the physical properties of the cell-wall necessarily 

 involve modifications of structure of the nature of growth. 

 Such modifications of structure are, however, permanent. But 

 we have seen that the incipient curvature is obliterated on 

 plasmolysis, which would not be the case were the cell-walls 

 of one side of the organ more rigid than those of the other. 

 It must depend, then, either upon differences in the attraction 



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