Toxicity of Various Cations 



205 



Toxicity of magnesium 

 Magnesium has long since been considered as injurious to plants when 

 calcium is absent from the media or present in relatively small amounts. 

 It is to be noted that the lethal concentration of magnesium ions in dis- 

 tilled water with respect to the roots of seedlings is at a dilution several 

 times greater than that of either sodium or potassium. Wheat seedlings 

 are more resistant to this element than are pea seedlings. No root growth 

 occurs with peas in solutions composed of N '500 ]MgCl2, but slight elon- 

 gation of the roots of wheat seedlings takes place. The following data 

 verify the above statements: 



TABLE 4. Experiments with Pea and Wheat Seedlings, Continued for Thirty Days. 



Data for Ten Plants 



Composition of solution 



Pea 



Increase 



in length 



of tops 



(centimeters) 



Increase 



in length 



of roots 



(centimeters) 



Wheat 



Increase I Increase 



in length ! in length 



of tops I of roots 



(centimeters) (centimeters) 



Distilled water as the solvent 



N/oOMgCU 



N/100 MgCh 



N/300 MgClo 



N/500 MgCl2 



N/1,000 MgCl2 



Distilled water 



Full nutrient solution as the 

 solvent 



N/4MgCl2 



N/5 MgCl2 



N/6MgCl2 



N/10MgCl2 



Nutrient solution 



2.0 

 2.0 

 3.0 

 3.5 

 5.0 

 6.0 





 





 

 Slight-dead 

 7.0 



0.5 

 4.0 



6.0 

 7.0 

 7.0 

 7.0 





 







2.0 

 6.0 

 6.0 







2.0 



3.0 



6.0 



7.5 





 Slight 

 1.5 

 7.0 







SUght 

 3.0 

 7.0 

 9.0 







1.0 



4.0 



8.0 



10.0 



In working with several varieties of wheat, Harter (1905) ascertained 

 that considerable differences exist between the varieties in their power to 

 resist the poisonous action of various ions. In determining the limit for 

 any one variety, he took as the lethal concentration the one in which 

 about one half of the seedlings lived and one half died after remaining in 

 the solutions for twenty-four hours. The average lethal concentration 



