43 



chloride three times. Calcium chloride ^ mixed with an equal volume 

 of a magnesium or a sodium salt raises the limit of the latter as fol- 

 lows: Magnesium sulphate, one hundred and sixty times; magnesium 

 chloride, forty times; sodium carbonate, fiftj' times ;^ sodium sulphate, 

 twenty-seven times, and sodium chloride, ten times. 



The most effective of the calcium salts tried was, however, calcium 

 sulphate. This, when added alone in solid excess, increases the 

 maxima of concentration endurable bj' the roots as follows: Magne- 

 sium sulphate, four hundred and eighty times; magnesium chloride, 

 eighty times; sodium carbonate, six times; sodium sulphate, sixty-six 

 times, and sodium chloride, ten times. Here we have probablj^ the 

 greatest effect of one kind of ion in neutralizing the effect of another 

 kind that has yet been obtained in experiments with plants. 



It is noteworthy that the effect of the calcium ions upon different 

 salts having a common basic ion differs greatly. Thus plant roots 

 can endure three times the concentration of a solution containing 

 magnesium cathions and sulph-anions to which calcium sulphate is 

 added than they can of a solution containing magnesium cathions and 

 chlor-anions idIus calcium sulphate. Yet the former solution in the 

 absence of calcium salts is endurable in concentration only one-half as 

 great as is the latter without a calcium salt. Here the effect maj^ be 

 parth' due to differences of dissociation in the two solutions. But it 

 appears necessary to attribute the greater part of it to an adverse 

 influence, presumabh^ exerted b}' chlorine ions, upon the physiological 

 action of calcium ions in the presence of magnesium ions. Similar 

 problems are suggested by the wide differences in the degree to which 

 calcium sulphate can neutralize the toxic action of each of the three 

 sodium salts. 



That the phenomena exhibited bj' the roots of plants in their reac- 

 tion to these various mixed salt solutions are not to be regarded as 

 mere functions of chemical changes in the solution itself is patent. 

 The problem is undoubtedly a much more intricate one, involving 

 chemical reactions of great comiilexity in the i3rotoplasm of the plant 

 itself. In this connection it is important to call attention to the 

 strikingly similar results obtained by Loeb -^ as to the relative toxic 

 effect upon animals of pure and of mixed solutions. 



A pure salt solution, e.g., of sodium chloride, Avas found to be 



' Loew (Bui. No. 18, Div. Veg. Phys. and Path., U. S. Department of Agriculture, 

 p. 33), referring to an experiment made by Boehm, appears to doubt the value of 

 calcium in the form of the chloride as a plant nutrient, owing to the formation 

 of hydrochloric acid in the assimilation of calcium by the plant. Here is another 

 suggestion as to the reason for the inferiority of calcium chloride to calcium sul- 

 phate in neutralizing the toxic action of salts of other bases. 



-As has already been noted (under Table VI). a heavy precipitate of calcium 

 carbonate is formed in this mixture, so that it becomes in great part a solution 

 of sodium chloride plus a solid excess of calcium carbonate. 



"Seethe papers by this author cited in the Bibliography (p. 58). 



