12 GENERAL CONSIDERATIONS 



organisms or on their death it is available for the 

 growth of the higher plants after j^assing through 

 the usual nitrifying changes. This ability to draw 

 on the vast reserve of free nitrogen existing in the 

 atmosphere is a matter of the utmost importance. 

 The study of these biological activities in the soil 

 which play so important a part in its fertility and 

 consequently are so vitally important in the world's 

 progress has only recently begun to receive atten- 

 tion. In the past few years many attempts have 

 been made to propagate and artificially distribute 

 these nitrogen-fixing bacteria and to learn the con- 

 ditions which control their natural growth and in- 

 crease in the soil. The problems involved are very 

 complex and difficult and so far but few results 

 of practical value have been obtained. The field 

 remains an open one, and it is one of the most attrac- 

 tive and important in the whole range of the scien- 

 tific investio^ations bv means of which this asfe is 

 striving to explain and to improve upon the ancient 

 agricultural practices of the world. 



There is one immensely important group of these 

 nitrogen-fixing bacteria, however, whose activities are 

 now fairly well understood and whose services are 

 being utilized on a large and rapidly increasing scale. 

 These organisms are parasitic on the roots of the 

 numerous species of plants belonging to the bean 

 and pea family or, as it is usually called, the Legumi- 

 nosce. Their presence causes the growth of little 

 galls or swellings known as nodules or tubercles, 

 and in this respect they are to some extent an incon- 

 venience to the plant. The favor of being fur- 



