ABSORPTION OF WATER. 



55 



These statements are borne out by the experiments of Beyer, to which 

 the following figures refer. In each case one kilogramme of felspar was 

 treated with 2^ litres of water, or of a watery solution of one of the salts 

 enumerated : the figures give the amount, in grammes, of the substances 

 dissolved. 



Finally the constituents of insoluble salts are brought into 

 solution by means of the acid sap which saturates the 

 cell-walls of the root-hairs. That the root-hairs are acid is 

 easily demonstrated by means of litmus-paper, or, as Sachs 

 suggested, by placing a root in a solution of potassium per- 

 manganate, when the salt is decomposed and a precipitate 

 of the hydrated dioxide is formed upon the surface of the 

 root, a reaction which does not take place when a leaf or a 

 stem is placed in the solution. This acid reaction is shewn 

 not to be merely due to carbonic acid, by the fact that the 

 reddening of the litmus-paper is permanent. 



The important part played by this acid sap in the ab- 

 sorption by roots of salts which are insoluble in pure water 

 is indicated by means of the following experiment devised 

 by Sachs. If a well-polished slab of marble be placed in 

 a flower-pot and a Bean or a Sunflower be planted in the soil 

 above it, the roots will penetrate through the soil until they 

 reach the surface of the marble. They will then grow along 

 it, in close contact with it. If, after some time, the slab of 

 marble be taken out, its surface will be found to be corroded 

 wherever the roots have been in contact with it ; in fact, 

 in a successful experiment, the root-system is etched upon it 

 This corrosion is the expression of the fact that the acid roots 

 have dissolved and removed particles of the marble, and the 



