206 SCIENCE PROGRESS 



sponsible for the ionic distribution at the basis of the Donnan 

 Equilibrium. 



This free passage of all ions brings about their equal distribu- 

 tion and the disappearance of the electric charge at the excited 

 area. This area then becomes of different electric potential to 

 the rest of the surface of the organ, and this change is initially 

 caused by a change in the solid medium at the surface of the 

 living cell. Naturally this change is propagated along the solid 

 surface as a wave of compression is transmitted along a stretched 

 string, and then we have the transmission of a nervous impulse 

 with its accompanying E.M.F. detectable by suitable galvano- 

 metric methods. 



If in the plant cell we have a similar solid surface with the 

 unequal distribution of ions anticipated and explained by 

 Donnan, we may find the clue to the reception of tropic 

 stimuli in this ionic distribution and its accompanying electric 

 charge. 



This is in effect what is suggested by Small {New Phytolo- 

 gist, 19, pp. 49-62, 1920), when he associates the different 

 response of stem and root to tropic stimuli to a different re- 

 action of the cells of stem and root or to the different electric 

 charge at the protoplast surface in stem and root, a natural 

 consequence of the assumption of different sap reaction. 



If protoplasts have this different charge in stem and root 

 then the result of a stimulus which increases the permeability 

 will be that in one case the stimulated region will become 

 electro-positive, in the other electro-negative, to the surrounding 

 regions. It therefore would seem natural that the same direc- 

 tive stimulus should produce directly opposite effects upon 

 stems and roots. 



Recent controversy (Blackman, New Phytologist, 20, pp. 

 38-42, 1 92 1, Small, ibid., 20, pp. 73-81, 1921) has centred 

 around the adequacy of the " creaming " mechanism assumed 

 by Small. If it is remembered that the protoplast on which 

 the surface charge is distributed is a turgid structure with a 

 delicate solid surface, it would seem that the necessary difference 

 in permeability might readily be produced by the new strains 

 in this solid surface induced by changes in its position relative 

 to gravity, without reference either to emulsions or to statoliths. 

 To take this question further would be too speculative ; the 

 writer has only desired to show that in all directions Loeb's 

 work upon the behaviour of proteins to electrolytes at different 

 hydrion concentrations will be found most fertile in its appli- 

 cation to problems of plant physiology. 



Loeb's papers will be found in the Journal of General Phy- 

 siology, 1918-1921, as follows : Vol. I, pp. 81-96, 237-54, 

 363-86, 483-504, and 559-80. Vol. II, pp. 273-96, 387-408, 



