64 THE BIOLOGY OF FLOWERING PLANTS 



when supplied to the plant singly, even in small concentra- 

 tions. This is notably the case with the salts of magnesium, 

 which are highly toxic. A pea seedling grown in 0*04 per 

 cent, magnesium nitrate is injured in a few days ; the 

 leaves turn brown and wither, and the roots become slimy 

 and cease growing. Sodium salts and ammonium salts are 

 also highly toxic, potassium salts to a less extent. Loew 

 (1892) showed that the toxic action of magnesium salts was 

 entirely removed when calcium salts were added to the 

 nutrient solution, and this antitoxic effect of calcium has 

 since been demonstrated against salts of sodium, potassium, 

 and ammonium. Calcium salts produce no ill effects 

 unless supplied in relatively high concentrations. It should 

 be noted that both the toxic and antitoxic effects are due to 

 the metallic cation and not to the anion of the salt. 



Much work has been done on this relation in recent 

 years, especially in America. Osterhout (1906, 1907) 

 showed the toxic effect of single salts applied to salt- 

 water plants, such as Zostera, fresh-water plants, such as 

 Vaucheria, and land plants in general. Solutions of a single 

 salt have an injurious effect, and growth in these is worse 

 than in distilled water ; with pairs of salts growth is better ; 

 it is normal only when all the necessary salts are supplied 

 in the appropriate concentrations. Osterhout describes 

 such a solution as physiologically balanced ; sea water is a 

 balanced solution for marine plants. 



In nature and in agricultural practice the toxic effect may 

 be due, in some soils, to excess of sodium, magnesium, or 

 ammonium. It may be prevented by application of calcium 

 carbonate. The proportion of calcium to the toxic salt 

 required to obtain antitoxic action is a matter of interest ; 

 it is in general very small. It is different for different 

 plants, and also varies with the concentration of the nutrient 

 (soil) solution and the stage of development of the plant 

 (Shive, 1915). 



The mechanism of the antitoxic effect is not fully under- 

 stood. Loew sought an explanation in a definite combina- 

 tion of calcium with proteins which is destroyed by excess 



