ABSORPTION OF SALTS 67 



concentration. Among the substances which produce 

 plasmolysis are included the common salts which are 

 important plant nutrients, such as potassium nitrate and 

 potassium sulphate ; it has been assumed that the penetra- 

 tion of such salts, which necessarily takes place, is possible 

 in the very dilute solutions in which they normally occur 

 in the soil. As potassium nitrate, for example, plasmolyses 

 a root hair having an osmotic pressure of 10 atmospheres 

 in a 2 "9 per cent, solution, and as the concentration of this 

 salt in the soil must be less than o"oi per cent., it is clear 

 that no conclusions as to permeability in the latter case 

 can be drawn from the behaviour in the former. Fitting 

 (19 1 5) has recently shown by careful measurements that 

 penetration does, in fact, take place to an appreciable 

 extent even in high concentrations, and that, moreover, 

 the exposure of a cell to a highly concentrated salt solution 

 makes it less permeable. Stiles and Kidd (1919 a and b) 

 have studied the course of absorption of various salts, using 

 the delicate electrical conductivity method, and, besides 

 demonstrating absorption of various salts in all the con- 

 centrations used, obtained the interesting result that the 

 final concentration of the salt inside the cell is many times 

 that outside if weak solutions (N /5000) are used, while it 

 is less than that outside in strong solutions (N/io). N/5000 

 calcium chloride (= O'ooi per cent.) gives equihbrium with 

 the internal concentration 15 "3 times the external, while for 

 N/io calcium chloride (= 0*55 per cent.) the ratio is 0*24. 

 This is important, for it shows that from the weak soil 

 solution the plant is able to absorb relatively large amounts 

 of salts. 



The absorption of salts as such does not, however, take 

 place. In the soil, as in any other solution, salt molecules 

 are ionised, and, as the soil solution is very dilute, the degree 

 of ionisation is great. Taking any one salt, there are present 

 two kinds of ions and undissociated molecules. That the 

 ions pass into the cell independently follows from the fact 

 that the two ions of a salt are absorbed to unequal extents. 

 This is shown by the extensive investigations of Pantanelli 



