24 



roots survived in better condition in 0. 0075 normal sodium sulphate than 

 in 0.02 normal sodium chloride solution. It was found much easier to 

 determine sharply the limit of endurance for sodium carbonate and 

 sodium bicarbonate than for other salts, as in 0.005 and 0.02 normal 

 solutions of the two carbonates, respectively, all, or nearly all, roots 

 survived in apparently perfect condition, while in 0.0075 and 0.025 

 normal, respectively, all roots were killed and symptoms of advanced 

 disorganization were apparent after twenty-four hours. 



CONCENTRATION MINIMUM PROHIBITING ELONGATION OF ROOTS. 



A comparison of the seven salts above enumerated in regard to the 

 degree of concentration of each in which absolutely no elongation of 

 the roots occurred during twentj^-four hours is interesting, as illus- 

 trating how far this point is removed from that of the minimum con- 

 centration which is still toxic. It will be seen that the position of the 

 salts in this scale does not at all correspond with their sequence in 

 the table of limits of endurance. In many cases, especially when the 

 solution was still more concentrated, not only no inci-ease of length 

 but a positive shrinkage of 0.5 to 2 mm.^ was detected. 



Table II. — Concentrations ivhich absolutely prevent growth. 





Name of salt. 



Concentration of 

 solution. 





Parts per 

 100,000 of 

 solution. 



Normal. 





260 

 417 

 960 

 1,160 

 1,410 

 1,652 

 1,680 



0.05 



Sodium bicarbonate 



.05 



Magnesium chloride .. 



.3 



Sodium chloride a 



.3 



Sodium sulphate 



,3 



Calcium chloride 



.3 



Magnesium sulphate 



.3 







a According to Pfeffer (Pflanzenphysiologie, Ed. 2, 1, 414) a culture solution to which enough 

 potassium nitrate or sodium chloride is added to render it isosmotic with a 2 per cent potassium 

 nitrate or 1.7 per cent sodium chloride solution causes a cessation of growth m ordinary plants 

 while an increase to 3 per cent is necessary to prevent growth in halophytes. 



It is impossible to reconcile this sequence, as compared with that of 

 Table I, with the notion, which still appears to find advocates, that the 

 injurious effect of these salt solutions is merely a function of their 

 osmotic pressures. If any fresh evidence were needed to disprove this 

 assumption it is afforded hj the fact, veiy cleai'ly brought out in the 

 present investigations, that marked toxic effects frequently appear 

 long before loss of turgor has manifested itself or cessation of growth 

 has occurred. It is certain that no useful conclusions as to the degree 

 of toxicity of a solution can be drawn from its osmotic pressure. 



True [Bot. Gazette, 26, 407 (1898)] calls attention to the difficulty of 

 distinguishing the purely chemical from the merely osmotic (plasmo- 



^ In some solutions this loss of length due to plasmolysis was as great as that found 

 by Sachs in roots which were exposed for thirty minutes to the dry air of a room. 

 (Arb. bot. Inst. Wiirzburg, 1, 396; Qesammelte Abhandl., 2, 784, 785.) 



