NUTRITION 



251 



combustion processes, including those of plants and animals, water being taken 

 into the molecule in the process. Water and salts are also readily available and 

 suffer no degradation of energy in passing through the organism. Nitrogen, on 



B 



D 



E 



FIG. 70. ROOT TUBERCLES OF LEGUMINOS^E. About natural size. 



A, Lupin. External view. 



B, Diagram of longitudinal section of lupin tubercle. 



C, Diagram of transverse section. 



D, Cracca minor. 



E, Clover. 



(A, B, C from Woronine ; D and E from Vuillemin. 

 See Lutz, 1904, pp. 70 and 71.) 



the other hand, must be presented to the green plant in the form of nitrate, 

 in order that it may be further synthesised into a suitable form for the needs of 

 the animal. Atmospheric nitrogen is useless for this purpose, and although 

 ammonia, which is formed from animal excreta and from the debris of plants, 

 chiefly by bacterial agency, can be utilised by the higher plant as a source of 

 nitrogen, it is by no means the normal and efficient one (see Russell, 1912, pp. 

 30-31); moreover, in the conversion of the residues from 

 plant and animal into ammonia, a certain amount is 

 always lost in the form of free nitrogen. It is therefore 

 a matter of fundamental importance for the continued 

 existence of life on the earth that some means should 

 be present for the conversion of nitrogen gas into a form 

 available for the growth of the green plant, and also 

 that an effective mechanism should exist for the trans- 

 formation of ammonia into nitrates. 



The diagram given in Fig. 69 will serve to elucidate 

 the process by which these requirements are met and 

 will enable a shorter verbal account to be given than 

 would otherwise be necessary. Additional details may 

 be found in the monograph by Russell (1912, chap. iv.). 



Following the direction of the arrows in the figure, 

 and starting from atmospheric nitrogen, we notice that 

 there are two ways in which this is "fixed" in a form 

 available for the use of plants. In the first place, there 



are bacteria in the soil which are able to obtain their nitrogen from the atmos- 

 phere. Their existence was clearly shown by Vinogradsky (1895). The chief 

 forms are a Clostridium, anaerobic,|isolated by the observer named, and Azoto- 

 bacter, aerobic, discovered'by Beijerinck (1901). 



FIG. 71. MICRO-ORGANISMS 

 FROM TUBERCLE OF 

 DESMODIUM GYRANS. 



Pure culture. Stained with 

 methylene blue. Size of 

 organisms, 1'3 A 1 x 37 fJ- 



(Miss Dawson, 1900, 

 Fig. 6.) 



