3:;2 



THE AMERICAN NATURALIST [Vol. XLIV 



to the untimeliness of the observations. It is said that 

 when the curvature is taking place the turgor of the cells 

 on the concave side is unchanged and on the convex side 

 lowered. But by the time curvature is well under way, 

 the turgor conditions immediately following excitation 

 may have undergone complete alteration, if we may judge 

 from similar changes in motor organs. Thus, the orig- 

 inal turgor, lowered on excitation (according to theory) 

 by the increased permeability of the under side, may 

 have been regained, while the. very growth itself of the 

 convex side may be responsible for the reduced turgor 

 then observed on that side. 



The responses of recoil and orientation in motile or- 

 ganisms are easily interpretable in terms of this concep- 

 tion, and though the application is purely hypothetical, 

 it is no more so than the current explanations. Just such 

 a change as results in changed permeability of the walled 

 protoplast to solutes of the great vacuoles might take 

 place on the passage of a naked protoplast into or out of 

 a stimulation zone. How such a change can reverse the 

 mechanism of movement is not explicable at present ; but 

 the new conception introduces no new difficulty. For ex- 

 ample: If a change in surface tension is predicated as 

 the result of excitation and the occasion of recoil, it is 

 just through some change in chemical constitution that 

 such an alteration in surface tension might come about. 

 If the organism be one which orients itself to one-sided 

 stimuli, orientation may as easily result from a local 

 change in constitution as in any of the ways assumed by 

 current theory. 



No one is more keenly aware than I that these all are 

 matters of speculation. They are presented as such. 

 My hope is that they will direct attention to this phase of 

 plant behavior and will stimulate thought, which should 

 first clarify our conceptions, and then suggest lines of 



