252 PRINCIPLES OF GENERAL PHYSIOLOGY 



Vinogradsky recognised that the process required energy to be supplied, since it is 

 endothermic ; in experimental work, glucose is added, and a considerable amount is consumed ; 

 each milligram of nitrogen fixed requiring the oxidation of 500 mg. of sugar. In the soil, 

 decomposition products of cellulose apparently take the place of the glucose as sources 

 of energy. 



The second process is peculiar to the leguminous plants, together with a few 

 others. Russell points out (1912, p. 84, footnote) that it was known to the 

 Romans that the growth of vetches (a leguminous plant) on ground afterwards 

 used for wheat caused an increased crop of this latter. In Vergil's " Georgics," 

 Book I., lines 73 and following, the farmer is recommended, "before sowing his 

 yellow wheat, to take off a crop of beans, with their rattling pods, or of the frail 

 offspring of the vetch, or of lupins, with their brittle stalks and rustling straw." 

 All of these are leguminous plants, be it noted. 



The word translated " straw" in this passage is " silva" ; but it is difficult to see in what 

 sense a field of lupins could be called a " wood." 



The reason of the beneficial effect of such plants was discovered by Hillriegel 

 and Wilfarth (1888). They showed, in the first place, that, adding together the 

 nitrogen of the soil in a particular culture pot to that of the plants grown in it, 

 there is, in the case of oats, always a little less than that originally present, but, 

 in the case of peas, always more. This could only come from the nitrogen of the 

 atmosphere. At this time it was already known that the nodules on the roots of 

 leguminous plants contain bacteria, and the hypothesis was a natural one that these 

 organisms were able to fix nitrogen and hand it over to the plant in some way. 

 Beijerinck (1888) isolated the organisms from the root nodules, but, although they 

 must be present in the soil, since extracts of soil on which leguminous plants have 

 been grown will infect the roots of other plants of the same order, it was found 

 impossible to discover them therein. After entry into the root hairs, they multiply 

 rapidly and presently form a nodule on a part of the root. Inside the nodules 

 they change to Y-shaped "bacterioids." Fig. 70 shows roots with nodules and 

 Fig. 71 (from the paper by Miss Dawson, 1900) gives the appearance of the 

 bacteroids. The chemistry of the process is unknown. The final product is 

 supposed to be soluble protein, which is passed on to the plant. From what we 

 know as to the nitrogen supply to the tissues in animals,' - it seems more likely 

 that it is an amino-acid or amide. In any case, the facts are of great practical 

 importance, since leguminosae are among the commonest plants, and the process 

 is independent of organic matter in the soil. The carbohydrate required to afford 

 energy for the work of the micro-organisms is obtained from the plant on which 

 it grows. The growth of these plants, then, always leads to increase of organic 

 nitrogen in the soil. 



Owing to the two processes named, the green plant has been enabled to form 

 proteins. If eaten by an animal, these proteins serve as nitrogen food for it. 

 The waste products containing nitrogen, from both animal and plant, some of 

 them of simple composition, such as urea, others more or less insoluble solids, on 

 return to the soil, are converted into ammonium salts, mainly by the agency of 

 bacteria, although it is said that the process may take place slowly in the presence 

 of antiseptics. The reaction probably consists, in the case of the more complex 

 compounds, in the production of amino-acids and subsequent hydrolysis or oxida- 

 tion of these. During the process, however, a considerable loss of nitrogen in the 

 gaseous form occurs, as presented by the thin line in the diagram. This loss is 

 supposed to be due to oxidising bacteria, but the question is not yet decided. 



The ammonium salts thus formed are capable of serving to a certain extent 

 as nitrogen food for the green plant, indicated by the interrupted line in the 

 diagram leading back to plant proteins ; but they are not efficient in this respect 

 and, according to Russell (1912, p. 31), plants fed only on ammonium salts as 

 source of nitrogen, really suffer from nitrogen starvation. 



A means of converting ammonia into nitrates is clearly an essential require- 

 ment. This is actually provided in the following way. The first step is the forma- 

 tion of ammonium carbonate, by simple chemical reaction with alkaline carbonates, 

 so far as not already present in this form. This ammonium carbonate is rapidly 



