550 PLANT RESPONSE 



sap of the convex side of the organ is more powerfully 

 osmotic than that on the concave. 



But various effects have been observed, of which the 

 occurrence of plasmolysis alone affords no explanation. 

 Thus, the diminution of curvature, which De Vries showed 

 to be a consequence of plasmolysis, was demonstrated by 

 Noll, in the case of recently curved organs, to be only the 

 second phase of the effect ; for at the beginning of the 

 operation in these cases, as he pointed out, the curvature of 

 the organ was actually increased. This opposition of effects, 

 the increase of curvature, followed by the flattening of the 

 curve, as the result of plasmolysis, has not hitherto met with 

 any satisfactory explanation. 



These obscurities in plasmolytic reactions have arisen 

 from the fact that the excitatory action of some of the 

 plasmolysing reagents has not hitherto been recognised. 

 That such an influence may be exercised, and sometimes in 

 opposition to osmosis, was, however, demonstrated in my 

 experiments on suctional response, where it was seen that a 

 strong solution of sodium chloride applied at the roots, 

 instead of arresting suction by the withdrawal of cell-sap, 

 actually enhanced the rate of suction for some time (p. 383). 

 It was also shown in a later chapter, dealing with the 

 effects of chemical reagents on growth, that when favourable 

 tonic conditions had been induced, fairly strong salt solutions, 

 instead of the usual retardation of growth, brought about a 

 temporary enhancement, followed by depression (p. 486). 

 We thus see that the responsive movement is modified 

 (1) by the condition of the tissue, and (2) by the duration of 

 the application of the solution. Now, in an already curved 

 organ, we have an induced anisotropy, or difference of con- 

 dition, on the two opposite sides. The effects of chemical 

 res gents, then, on such an organ will be complex, for they 

 will differ on the two sides, in intensity, and also in phase of 

 reaction. The observed responsive movement in increasing 

 or diminishing the existing curvature will thus represent the 

 algebraical summation of the effects on the two sides. 



