56 LECTURE IV. 



fact that the surface of the marble is corroded only at those 

 points at which it was in contact with the roots shews that 

 the corrosion is not due to the action of carbonic acid, for that 

 is everywhere present in the soil, but to the acid sap in the 

 walls of the root-hairs. Similar experiments made with dolo- 

 mite, magnesite, and osteolith, give the same results. When 

 a slab of gypsum is used, the result is different In this case 

 it is the general surface of the slab which is corroded, the 

 parts in contact with the roots being preserved and forming 

 therefore projections : the roots appear to protect the gypsum 

 from the solvent action of the water in the soil. 



It has been pointed out that insoluble salts present in the 

 soil may be converted into soluble compounds by decompo- 

 sitions taking place between them and soluble salts which 

 the soil already contains or which have been supplied to it, 

 and there is reason to believe that the action of the acid sap 

 of the cell-walls is of this nature also. It has been found, 

 for instance, that when the roots of a plant are made to grow 

 in a solution in which calcium phosphate and potassium 

 nitrate are present, the solution becomes alkaline (Boussin- 

 gault) : if, however, the solution be one of the chloride of an 

 alkaline metal or earth, the solution becomes strongly acid 

 (Rautenberg and Kiihn). The most probable explanation 

 of these facts is that the salts are decomposed by the acid 

 sap of the roots, the acid being absorbed in larger proportion 

 in the first case, the base in the second. We learn also from 

 this that the chemical elements are not necessarily absorbed 

 by roots in the combinations in which they are present in 

 the soil. 



We see, then, that in all cases the salts which are absorbed 

 by the roots of plants are absorbed in solution, and we have 

 now to enquire into the relation between the amount of salts 

 and the amount of water absorbed, to enquire, that is, if a salt 

 in solution is absorbed in the same proportion as the water in 

 which it is dissolved. For example, let us suppose the roots 

 of a plant to be immersed in a solution which contains say OT 

 per cent, of a particular salt ; the question is, will the solution 

 which remains after absorption has been going on for some 



