ADDITIONAL NUTRIENTS 83. 
The use of free nitrogen by certain bacteria will be 
discussed further on. 
120. In addition to the substances mentioned in the 
preceding paragraph, silicon (Si) is taken up by many 
plants (as silicates of various kinds) and adds to their 
hardness but can be dispensed with except by the 
diatoms whose cell walls are composed largely of silica. 
Sodium can take the place of potassium for many pur- 
poses, e.g. neutralizing acids, but cannot be substituted 
for it entirely. Similarly an excess of calcium can replace 
part but not all of the magnesium, while barium (Ba) and 
strontium (Sr) can replace part of the calcium. Chlorine 
(Cl) in the form of chlorides is useful to many plants but 
apparently can be dispensed with by almost all. The 
various other salts present in the soil solution may be 
taken up by the plant in greater or less degree, but 
appear either to have no use whatever or to be used only 
incidentally without being indispensible. Such are salts 
of copper (Cu) aluminum (Al) manganese (Mn) zine 
(Zn), ete. 
121. The réle that the various substances mentioned 
in the foregoing paragraphs play in the plant economy 
is not certain in all cases. It is probable. that calcium 
and potassium, perhaps also magnesium and iron, are 
essential parts of the protoplasm molecule. Sulphur is a 
component of proteins while phosphorus is found in some 
proteins, especially in the nucleus. Carbon, hydrogen 
and oxygen are the components of the carbohydrates 
which are the chief building materials of the plant (e.g. 
cellulose) and of the proteins out of which protoplasm is 
built up. In the absence of iron the chlorophyll seems 
impossible of formation although it does not contain iron 
itself. Mention must be made of the principle of 
antagonistic action by various salts. Thus it has been 
