180 VEGETABLE PHYSIOLOGY 
that this cannot be the only or even the main cause of the 
disturbance of nutrition, as the salts are not interchange- 
able, and a salt of sodium in concentration quite sufficient 
to maintain the condition of turgor is unable to replace 
the salts of potassium normally required. Moreover, tur- 
gescence can be maintained by organic acids in the total 
absence of the normal constituents of the ash. 
We can divide the latter into four groups which subserve 
different purposes. Of these the members of the first are 
essential, because they enter into the constitution of the 
living substance. They are sulphur and phosphorus. All 
analyses of proteins show that sulphur is an essential con- 
stituent of them, and as proteins are immediately applied 
to the construction of protoplasm, there can hardly be any 
doubt that sulphur is contained in living substance. Phos- 
phorus does not seem to be present in the ordinary cyto- 
plasm, but is undoubtedly associated with the nucleus. 
The nature of the connection is not very clear, but all nuclei 
contain a constituent which bears the name of nuclewn. 
This can be extracted from it by appropriate treatment, 
and analysis shows that phosphorus enters into its molecule. 
Nuclein occurs also in the substance of many cells, either 
as nucleic acid, or associated with certain protein bodies. 
The second group comprises certain metals which are 
essential to the development of a plant, but which appa- 
rently do not ever form part of the living substance. 
There is some little doubt about this, as the fact cannot 
be ascertained by analysis. The members of this group 
are potassium, magnesium, calcium, and iron. 
The third group includes several elements which are 
not absolutely essential, but which are useful in many 
cases, and which are very widely distributed, although not 
universally present. Among them are sodiwm, silicon, 
manganese, chlorine, bromine, and todine. 
The fourth group includes many other elements which 
are only occasionally present, and which probably play no 
part in the metabolic processes. They appear to be absorbed 
