THE CONSTITUENTS OF THE ASH OF PLANTS 175 
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 
constituent 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 cytoplasm, 
but is undoubtedly associated with thenucleus. The nature 
of the connection is not very clear, but all nuclei contain 
a constituent which bears the name of nuclein. 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 tron. 
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 sodium, 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 because they are present in the particular soil in 
which the plant happens to be growing, and have the 
power of osmosing through the walls of the root-hairs, and 
passing their plasmatic membranes. Many of them have 
only been found in a few plants. Among them may be 
mentioned aluminium, zinc, copper, cobalt, nickel, zirco- 
nium, fluorine, and lithium. 
What is frequently spoken of as the selective power of 
plants is often misunderstood. If a substance is present 
