140 PLANT PHYSIOLOGY 



cells require for their growth the influence upon their growing 

 organs of various electrolytes, in the first place, of cations such 

 as potassium, magnesium, calcium, and of several others. The 

 electrolytes necessary for plant growth will be discussed in greater 

 detail in the chapter devoted to the mineral nutrition of plants. 

 At present, there will be mentioned only the fact, of great impor- 

 tance, that each of the salts that in combination with other salts 

 is indispensable for plant growth becomes toxic to plant cells 

 when it is supplied alone in a pure solution containing no other 

 admixtures. 



This poisonous effect of single salts was first observed in experi- 

 ments on mice (Ringer, 1883) and on sea animals (Loeb, 1898). 

 Solutions of sodium chloride, corresponding in concentration to 

 sea water, were found to have a pronounced toxicity if very pure 

 salt was used. But small quantities of calcium and magnesium 

 prevented this toxic effect. In a pure solution of sodium 

 chloride, for instance, not a single embryo developed from the 

 fertilized eggs of the sea urchin. But the addition of 1 cc. of a 

 very dilute solution oi CaS04 caused the development of 3 per 

 cent of the embryos; 2 cc, 20 per cent; and 4 cc, 75 per cent. 

 Pure solutions of calcium salts also are toxic, but this can be 

 counteracted by the addition of sodium salts. Thus, different 

 salts destroy the toxic effect of others. This phenomenon has 

 been termed the ''antagonism of ions." 



Ion antagonism can be easily observed in the development of 

 roots of plants in water cultures. It has been studied in detail 

 by Osterhout. Figure 36 shows the results of one of his experi- 

 ments. In pure solutions of CaCl2 and of NaCl, the roots have 

 developed very poorly. Much better growth was secured in 

 solutions where Na+ was balanced by Ca++, but the best develop- 

 ment was obtained when a third cation was added, viz., K+. 

 The greatest antagonism is usually observed between univalent 

 and bivalent cations, but ions of the same valence may be antag- 

 onistic. In general, the higher the valence of the ion, the smaller 

 the concentration required for antagonistic effect. 



By choosing different concentrations of the various ions, a 

 combination may be obtained that will suit best the development 

 of an organism. Such an optimal combination has been termed 

 a physiologically balanced solution. For animals, as well as for 

 marine algae, a balanced salt solution corresponds closely to sea 



