PLANT PHYSIOLOGY 205 



possible significance of Loeb's views in relation to current 

 problems of plant physiology. 



Permeability. — It is fairly clear that the semi-permeable 

 properties of the surface layer of the protoplast depend largely 

 upon the protein constituents of this layer. Both physical and 

 chemical properties of these protein constituents evidently 

 depend, in view of Loeb's work, upon the reaction of any sap 

 bathing this surface layer. 



Long series of anions and cations, comparable with the 

 Hofmeister series, given by various investigators from a study 

 of the effect of salts upon the properties of the protoplasmic 

 membrane, will obviously require revision from this standpoint 

 (see, for instance, Borowikow, Bioch. Zeitschr,, 48, p. 230, 191 3). 

 A difference in ionic penetrability may well be associated with 

 different ionic diffusion velocities, but, in so far as these series 

 are purely empirical, they may be expected to disappear on 

 further analysis. 



Staining Reactions. — ^The very difficult problems of vital 

 staining will possibly be partly elucidated by a reconsideration 

 of the behaviour of acid and basic stains in relation to the 

 hydrion concentration of the medium, and this will be true 

 even if Rubland's view {Bioch. Zeitschr., 54, pp. S9-77, 191 3* 

 and Roll. Zs., 12, pp. 1 13-124, 1913) is correct that penetration 

 of these stains is essentially a question of pore diameter in the 

 living membrane. Another possible application of Loeb's 

 work appears to lie in the fact that different protein constituents 

 of protoplasm probably have different iso-electric points. It 

 may therefore be possible, by destaining stained material at 

 different but definite pH values, to discriminate between one 

 protein member of the protoplasmic complex and another. 



Thus, a protein with iso-electric point at pH4 will not 

 release a basic stain if washed in a medium with pH 4-5, whilst 

 a protein with iso-electric point at pHs will release a basic but 

 retain an acid stain when destaining in the same medium. The 

 possible significance of these considerations, in interpreting the 

 phenomena of double or differential staining, will be obvious to 

 every cytologist. 



Reception of Tropic Stimuli. — ^The Donnan Equilibrium will 

 be in existence at every protoplast surface, and as a result of the 

 unequal distribution of ions on the two sides of the protoplast 

 membrane an electric charge may be expected at each such 

 surface. Reference to Bayliss {Principles of General Physiology, 

 191 5 ed., pp. 643-50) will show that a similar charge is assumed 

 to exist along the surface of the living cells of nerve or muscle. 

 The local excitation of this tissue by a stimulus is assumed to 

 cause a momentary increase in permeability, permitting the 

 passage of the ion previously restrained and therefore re- 



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