176 VEGETABLE PHYSIOLOGY 
in a soil, can be made soluble in the hygroscopic water 
permeating that soil, and can dialyse through the semi- 
permeable membrane of the roodt-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 protoplasmic membranes 
which bound the cytoplasm on its two faces, abutting on 
the cell-wall and the vacuolar eavity respectively. This 
possibility of penetrating these membranes, and the power 
of subsequently removing the substances 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 particu- 
larly 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 at any rate 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 
( with the cell-protoplasm. It is contained in the substance 
