64 LECTURE IV. 



Wolff analysed the ash of various plants grown in poor soil with the 

 addition of certain salts ; the example taken is the Buckwheat. 



From Wolffs figures we learn further that the influence 

 of the composition of the soil upon the absorption by the 

 roots is not necessarily a direct one. In some cases we see 

 that it is ; thus, the addition of potassium-salts, magnesium- 

 salts, and calcium-salts to the soil increased the amount 

 of potash, magnesia and lime, respectively, in the ash : 

 but in others it is not, for the addition of sodium chlo- 

 ride to the soil did not lead to an increase in the sodium 

 of the ash. But the indirect influence, which, as we have 

 seen, depends upon double decompositions going on in the 

 soil, or at the surface of the root-hairs, is often very important ; 

 thus, in the case before us, the addition of sodium chloride 

 to the soil brought about the absorption of a considerable 

 quantity of potassium. 



The composition of the ash of a plant depends therefore 

 upon two factors, (i) the specific absorbent capacity of the 

 plant, (2) the composition of the soil in which it is growing. 

 It has been attempted to account for the specific absorbent 

 capacity of plants by attributing to the roots a selective 

 power. Such an assumption is as erroneous as it is unneces- 

 sary. We have learned that the absorption of a substance, 

 depends in the first instance upon the physical relation 

 which exists between its molecules and the cell-wall and 

 primordical utricle of the absorbing cell. If it is presented 

 to the cell in such a form that it can pass through these, its 



