40 



amount of each salt absorbed by a plant from a mixed solution. 

 Especially interesting, as compared with the toxicological phenomena 

 of pure and mixed solutions, respectively^, are Wolf's results as to 

 the effect of calcium sulphate in stimulating the absorption of other 

 sulphates. 



GENERAL SIGNIFICANCE OF RESULTS WITH MIXED SOLUTIONS. 



To enter into a discussion, from the i3urely chemical point of view, 

 of the widely accepted hypothesis of the dissociation of electrolytes 

 in solution would be to exceed the proper limits of this paper. ^ It 

 is sufficient to sa}^ that salts such as those with which we are here 

 dealing are held to dissociate in- dilute solutions, more or less com- 



(Na, NH^, Mg, Ca) in absolutely greater amount than from a simple solution. 

 From a solution containing 0.025 gram each of potassium nitrate and calcium 

 nitrate the plants absorbed 100 per cent of the former and 88 per cent of the latter. 

 From an equivalent solution of potassium nitrate plus magnesium nitrate, 100 per 

 cent of the former and 88 per cent of the latter. The stimulation of the plant by 

 the presence of calcium to take up greater quantities of potash is referred by 

 Loew (1. c, p. 44) to the mcreased development of root hairs induced by the calcium. 

 But if the presence of magnesium has exactly the same effect, as would appear 

 from the experiment j ust quoted , we must look further for an explanation. Absorp- 

 tion of ammonium nitrate is decreased by the presence of other nitrates, wliile 

 that of calcium and of magnesium nitrates is stimulated thereby. It is remarkable 

 that ivhile neither of these last two salts is readily absorbed from a simple solution, 

 both are easily absorbed when mixed together. 



Plants could be grown in mixtures of potassium and calcium sulphate 

 (K2SO4 + CaSO^) and of calcium and magnesium sulphate (CaSO^ + MgSO^) , but 

 never in mixtures of sulphates of potassium and sodium (K^SO^ + Na2S04), of 

 potassium and ammonium (K.^SO^ + (NH^).^ SO4) , nor of potassium and magnesium 

 K^SO^ + MgS04), even when the solutions were very dilute. 



Potassium and ammonium salts were taken up much more readily in the pres- 

 ence of a calcium salt than from a pure solution. This was notably the case with 

 the sulphates, which are absorbed with difficulty from unmixed solutions. Gypsum 

 (calcium sulphate) is absorbed in very small quantity in the presence of a potas- 

 sium salt, but greatly stimulates absorption of the latter. From a mixture of 

 calcium and magnesium siilphate little of either salt is taken up, but the presence 

 of magnesium nitrate considerably increases the amount of calcium sulphate 

 drawn from a solution. 



From mixtures of a sulphate and a phosphate, the latter is always taken up in 

 greater quantity. Ma'^nesium sulphate is taken up in greater quantity in the 

 presence of a phosphate than are other sulphates. 



De Saussure's principle of the absorption of salts in solution by plant roots— 

 i. e,, that the salt is taken up in smaller proportion to the water absorbed than it 

 occurs in the culture solution; in other words, that the residual solution becomes 

 more concentrated — applies to the absorption of sodium chloride in the. presence 

 of a nitrate (KNO3, NH^NOg, Ca(N03)2), but does not hold as to the absorption 

 of the nitrate itself. 



^For the presentation of the subject in simple terms the reader is referred to 

 a former publication by one of us. (Rep. No. 64, U. S. Department of Agricul- 

 ture, p. 144r, 1900.) 



