ABSORPTION OF WATER. 49 



Another important property of the soil is that it can with- 

 draw from their solutions salts, and other substances, and 

 can retain them. Way found, for instance, that when solutions 

 of the chloride, nitrate or sulphate of potassium were poured 

 over portions of clay-soil, the water which drained off contained 

 a smaller proportion of the base than the original solution, 

 whereas the proportion of the acid was the same, the acid being 

 combined with a new base, generally lime. He also found that 

 free potash and ammonia are thus absorbed by soil, as also 

 phosphoric acid. This property is one of great practical 

 importance, inasmuch as it is in virtue of it that the soluble 

 salts of nutritive value which the soil contains are not entirely 

 washed out of it by excessive rain. Further, in consequence 

 of this retention of soluble salts by the soil, the solutions of 

 them which are presented to the root-hairs are rendered very 

 dilute, a condition which is favourable to the process of 

 absorption. 



The following results of Henneberg and Stohmann will serve to 

 illustrate the preceding statements. In each case 100 grms. of soil were 

 treated with 200 c.c. of a solution of ammonium phosphate for six hours. 



1. Amount of H 3 PO 4 in the solution, 0*144 g rm - amount absorbed, 



0*072 grm. 



2. 0720 : 0-244 

 3- i '440 : 0-396 



Sachs has pointed out that it is the hygroscopic water 

 in the soil which is of importance to plants, and that any 

 water which may be present in the interspaces and which 

 is not affected by the force of adhesion free water, we may 

 term it is not only of no use to plants but may be even 

 hurtful, in that it prevents the free access of air to the roots. 

 It is in fact the object of draining, to remove from the land 

 the free water. And yet there are plants whose roots are 

 permanently immersed in water, and these do not appear 

 to suffer. This leads us to consider the modifications which 

 are presented by roots, and which are an expression of the 

 adaptation of plants to their environment. We may dis- 

 tinguish, from this point of view, four kinds of roots ; land- 

 V. 4 



