THE CONSTITUENTS OF THE ASH OF PLANTS 189 



in a soil, can be made soluble in the hygroscopic water 

 permeating that soil, and can dialyse through the semi- 

 permeable membrane of the root-hair, absorption of a 

 certain quantity of it will take place. How much is ul- 

 timately absorbed is a question of the power of the plant 

 to decompose or utilise it after absorption. Many sub- 

 stances which are useless or even deleterious to the 

 plant which takes them up are absorbed continuously 

 until a very large percentage of them is present, because 

 other constituents of the plant decompose them, or because 

 their power of dialysis is such that they are easily removed 

 from the absorbing cells. The possibility of the dialysis 

 by which they are originally taken up is perhaps a ques- 

 tion of relationship between the size of their molecules 

 and that of the meshes of the plasmatic precipitation 

 membranes which bound the cytoplasm on its two 

 faces, abutting on the cell-wall and the vacuolar cavity 

 respectively. This possibility of penetrating these mem- 

 branes, and the power of subsequently removing the sub- 

 stances from the absorbing cells, are the special features 

 of the so-called selective power of the plant, and it is 

 evident that this power is particularly associated with the 

 disposition of the materials after absorption, more than 

 with the absorption itself. 



We may now turn to the consideration of these varied 

 constituents of the ash, and examine them in detail. The 

 first group, we have seen, is composed of sulphur and 

 phosphorus. Its importance lies in the fact that these 

 elements enter into very close relationship with protoplasm, 

 the former being in all probability a constituent of its 

 molecule. 



Sulphur is only taken up by the higher plants in the 

 form of sulphates of some of the metals of the other groups 

 or of ammonia. Fungi can also utilise salts of sulphurous 

 and hydrosulphurous acids when they are presented in 

 dilute solutions. 



Phosphorus is associated with the nucleus rather than 



