THE FUNCTIONS OF ROOT-HAIRS. 53 



absorbed : the rate of absorption of this particular 

 substance, and the quantity absorbed, up to the 

 hypothetical point of equilibrium chosen, would 

 then depend simply on the ease with which 

 its molecules traversed the living protoplasmic 

 membrane, and the degree of their solubility in 

 the sap. 



But now suppose the following new factor to 

 come in. Suppose that calcium sulphate under- 

 goes decomposition in some one of the internal 

 cells of the system of absorbing cells, or that it is 

 even merely crystallised out in such a cell, or in 

 any other way removed from solution {e.g. by 

 deposition in cell-walls). This alters the state of 

 affairs considerably. The separation of the mole- 

 cules from the sap-solution is itself a cause for the 

 flow of more of the solution to the cell concerned, 

 and such causes of diffusion are very common in 

 the plant. 



The importance of this principle consists in that 

 it lies at the base of the whole question of selective 

 absorption, application of manures, and the rotation 

 of crops ; and those who are acquainted with the 

 excellent analytical results of De Saussure, Boussin- 

 gault, Wolff, Trinchinetti, Godechen, etc., and the 

 water-culture experiments of Sachs, Nobbe, and 

 others, will understand what an illuminating effect 

 on these points was produced by the above 

 generalisation, which we owe especially to Pfeffer's 

 splendid researches into the nature of osmotic 

 phenomena. 

 . It will now be clear, I hope, why we regard 



