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) zinc 

 (Zn), etc. 



121. The role 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 



