Chemical Processes in Plants. 325 



which may be increased to 0'5 per cent, for those in more 

 advanced stage of growth. If any of the necessary 

 elements is omitted from this solution, the growth of the 

 plant will be stunted if not entirely arrested. From 

 solutions containing nitrogen, sulphur, and phosphorus in 

 the simple forms given above, the plant can elaborate com- 

 plex organic products containing these elements. Practically 

 nothing is known of the chemical mechanisms by means of 

 which this result is accomplished. 



In the normal method of cultivating plants, the roots 

 are embedded in the soil, in which the greater part of the 

 nitrogen is contained in the form of complex organic 

 substances, which are often added to it in such forms as 

 that of farmyard manure, when it has been impoverished 

 by the growth of crops. From the soil the growing plant, 

 with the exception of some of the lupines, which will be 

 discussed later, draws the whole of its nitrogen supply. 

 The majority of plants are quite incapable of utilizing 

 in any way the nitrogen of the air for the building 

 up of complex nitrogenous substances. 



The question next arises In what form is the nitrogen 

 withdrawn from the soil by the plant ? Is it possible for 

 the plant to utilize directly the complex organic nitrogenous 

 compounds of the soil, or must these undergo some trans- 

 formation in the soil before utilization by the plant ? 



The soil, as has been already stated, generally contains 

 the greater part of its nitrogen in the form of organic 

 compounds, but these are also accompanied by smaller 

 amounts of nitrogen in the form of ammonium salts and 

 nitrates. Now it was shown by Boussingault in 1871 

 that nitrates can be formed from the organic nitrogenous 

 compounds of the soil. Some soil, in which the organic 

 nitrogen and nitrates had been estimated, was allowed to 

 remain for twelve years in very large closed glass bottles. 



