ASPECTS OF AN OLD AGRICULTURAL QUESTION. 261 



Hitherto there were four sets of conflicting opinions in 

 the air as to the explanation of the allegation that leguminosae 

 can accumulate nitrogenous substance in soil so poor in 

 nitrogenous compounds that even if all possible sources of 

 combined nitrogen were taken into account, there is an 

 unexplained balance in favour of the plant. 



First, some assumed that the richly-foliaged leguminous 

 plant assimilates free nitrogen direct from the air by the 

 leaves : " They obtain their carbon from the air — why not 

 their nitrogen ? " 



Second, others suggested that the abundant foliage 

 enables these plants to gather up more completely than 

 can other plants, the traces of nitrogen compounds always 

 existing in the air, and thus to concentrate this obscure 

 supply. 



Third, others pointed to the deep and abundant root 

 system so characteristic of many leguminosae, and said : 

 " These roots can obtain supplies from the subsoil far 

 beyond the reach of the roots of ordinary plants ". 



And fourth, many people denied that any difference as 

 to power of obtaining nitrogen really existed between 

 leguminosae and other plants, but assumed that these richly- 

 rooted plants in some way favour processes of nitrogen 

 accumulation in the soil itself — processes quite independent 

 of vegetable, but in some way produced by physical or 

 chemical fixation in the soil. 



The researches of Boussingault and of Lawes, Gilbert 

 and Pugh, already referred ix>, had refuted the first of these 

 suggestions, and, properly interpreted, may still be regarded 

 as a complete reply to the assertions of Ville, their chief 

 opponent. But we must not forget that the conditions of 

 these experiments excluded the infection of the leguminous 

 plant, and that the results obtained were got with plants 

 devoid of nodules on their roots. 



As regards the second hypothesis, Hellriegel and Wil- 

 farth showed clearly (and others have shown the same) 

 that unless there is some compound of nitrogen in the air 

 not detected by the methods of analysis to date, the gains in 

 analysed peas and other leguminosae, growing in soils free 



