GENERAL VIEW 389 



With the exception of those plants which are able to assimilate free 

 nitrogen, a supply of a suitable nitrogen-compound is in all cases necessary, 

 although the specific requirements of different plants vary widely. Thus 

 most higher plants and many lower ones prefer inorganic compounds of 

 nitrogen such as nitrates or ammonium salts, whereas many saprophytes 

 and parasites either must be supplied with proteids, or develop best when 

 nitrogen is supplied in the form of organic compounds such as peptones, 

 amides, &c. Many fungi, however, grow normally when ammonium nitrate 

 affords the sole source of nitrogen, provided the nutrient fluid contains the 

 essential inorganic salts together with sugar or glycerine. Such fungi 

 (Penicillium glaucum, Aspergillus, &c.) form proteids by synthesis, and 

 as growth progresses the total amount of proteid substance continually 

 increases. Most chlorophyllous. plants have the same power, and since 

 the sugar is produced by photosynthetic assimilation, nothing more than 

 a supply of inorganic salts, including nitrates, sulphates, and phosphates, 

 is necessary to render the construction of proteid possible. 



All grades of transition exist between organisms which require peptone 

 and those which can obtain all their nitrogen in the form of inorganic salts, 

 and hence the nutritive values of a series of nitrogen-compounds differ 

 according to the plant examined. When a plant is fed solely with peptone 

 or asparagin the same substance must serve as a source both of nitrogen 

 and of carbon. A certain amount of energy can always be obtained by 

 the oxidation of organic nitrogen-compounds, and nitrate-bacteria obtain 

 the whole of their energy from the oxidation of ammonia or of nitrites, 

 i. e. from compounds which at the same time afford part of the material 

 for the synthesis of proteid (Sect. 63). 



The fact that a plant requires a supply of nitrogen-compounds shows 

 that it is unable to assimilate the free nitrogen of the air which is present in 

 dissolved form in every living cell. Certain soil-bacteria and Leguminosae 

 with root-nodules are, however, able to assimilate free nitrogen, and the 

 fact that various transition forms are known, renders it possible that 

 other plants may develop a feeble power of assimilating free nitrogen 

 under special circumstances (Sect. 69). 



The assimilation of free nitrogen by the bacteria in question is not 

 dependent upon light or upon the presence of chlorophyll. This is here 

 also the case of the assimilation of nitrogen-compounds and the synthesis 

 of proteids, as is shown by the fact that fungi are able to complete their 

 life-cycle in darkness. On the other hand, the growth of green algae in 

 inorganic nutrient solutions exposed to light 1 shows that chlorophyllous 

 cells can also synthesize proteids, and no doubt the non-chlorophyllous cells 

 of green plants are also capable of constructing them. This is rendered 



1 Bineau, Ann. d. chim. et d. phys., 1856, iii. s<r., T. XLVI, pp. 60, &c. 



