50 THE STORY OF PLANT LIFE 



beneficial, if not essential, as above. Calcium added 

 to any other salt reduces the toxic effect. 



For the supply of nitrogen nitrates are the best 

 source for plants and can be artificially added. 

 Plants cannot themselves fix the free nitrogen in the 

 atmosphere, of which this gas and oxygen are the 

 main constituents. But certain bacteria are able to 

 do so. It is also obtained from ammonia. Some 

 plants require nitrates, some ammonium salts, but 

 the former are generally more beneficial. The 

 addition of nitrogen may not only increase the con- 

 centration of nitrogenous food in the soil, but also 

 the amount of root or absorbing surface, or number 

 of root-hairs, and of the leaf or assimilating surface, 

 as pointed out by Russell, and so cause the v^hole 

 plant to increase in bulk by accession of carbon, 

 etc. The amount of water, temperature, etc., may 

 affect the increase in growth which otherwise would 

 result from the supplying of nitrogen. 



Phosphorus is supplied as phosphates to plants 

 and promotes root development, being most useful 

 on clay soils. Phosphates are unnecessary for sandy 

 soils, in which plants usually develop abundant roots 

 or rootlets. In dry regions they cause the young roots 

 to seek the moister layers. They tend to accelerate 

 the ripening of the fruit or seed. Absence of phos- 

 phates, however, is not so detrimental as absence of 

 nitrates, at any rate in so far as cereals are concerned. 

 For mitotic cell-division phosphoric acid seems to be 

 an essential, mitosis not taking place in its absence. 



