122 PLANT PHYSIOLOGY 



theory of antagonism which seems to fulfill at least some of the 

 requirements of a satisfactory theory in this particular field. 



Attention has already been called to the fact that protoplasm 

 is colloidal. It probably bears an electrical charge on its colloidal 

 particles, and this charge, according to various investigators, is 

 negative. The blood is slightly alkaline and sea water is also 

 slightly alkaline, which facts furnish reasonable grounds for the 

 assumption of a negative charge on the surface of protoplasm. 

 Schaede (1924), Heilbrunn (1925), and Robbins (1926) have 

 brought forward additional evidence in support of the negative 

 charge on the surface of protoplasm. The cell sap on the other 

 hand is generally slightly acid, forming with the protoplasm what 

 is called an " electric double layer." 



The second assumption in this explanation is that although a 

 salt may be "neutral," its effect upon the protoplasm may not 

 be, but will depend rather upon the individual effects of the ions 



+ - 

 into which the salt dissociates. In NaCl a univalent positive ion 



(the Na cation) is associated with a univalent negative ion (the 



CI anion). In CaCl 2 the two positive charges of the bivalent 

 cation are concentrated upon one ion while the two negative 

 charges of the salt are divided between two anions. The dominant 

 effect of the salt is that produced by the cation with its double 

 charge and the CaCl 2 is thus a "positive" salt, as are in general 

 all salts whose cation has a greater valence than the anion. Sodium 



sulphate (Na2S0 4 ) wou ld similarly be a "negative" salt. When 



a plant is placed in a solution of calcium chloride the negatively 

 charged particles of the surface of the protoplasm are at first 

 drawn together as the "positive" salt strikes them. This causes 

 a decrease in permeability as the membrane "tightens," but later 

 as more positive salt diffuses in, the surface particles become 

 positively charged and repel each other. This repulsion causes the 

 later effect of calcium, i. e., the decrease in resistance or increase 

 in permeability. 



A "negative" salt will only add to the negative charges already 

 present and will cause'a continuous increase in permeability from 

 the very start. A salt like NaCl in which both ions have the same 

 valence, for reasons which cannot be taken up in an elementary 



