XVIII. TITLES OF LITERATURE CONCERNING 



THE FIXATION OF FREE NITROGEN BY 



PLANTS. 



D. T. Mac Dougal. 



The relations sustained by plants to the nitrogen compounds 

 of the soil and water, and to the free nitrogen of the air form a 

 subject of gfeat biological import, and since aside from its 

 purely scientific aspect certain phases of the main question 

 are of vast practical interest they have attracted the attention 

 of the agriculturist and chemist as well as of the botanist. 



The res alts of the investigations, from these various points 

 of view, which have been in progress for a century, form a mass 

 of literature which is scattered through the journals and pro- 

 ceedings of the various branches of natural science in such 

 manner as to be very difficult of access to the student with 

 ordinary facilities. 



Among this rich and withal unwieldy mass of literature the 

 part of especial interest to the botanist is that which concerns 

 the fixation of free nitrogen by the leguminous plants and the 

 organism found in the tubercles which characterize this group, 

 and the fixation of free nitrogen by green plants which do not 

 sustain mutualistic relations to the lower organisms. 



The large number of controversies resulting from the attain- 

 ment of radically different conclusions from similar experiments 

 along certain lines of the work, in the hands of various investi- 

 gators, leads to the belief that safe generalizations can be made 

 from the restricted groups of facts thus obtained only when 

 confirmed by extended and parallel researches. To meet this 

 idea the references given below concern the points of central 

 interest to the botanist, beside a number of titles to "nitrifica- 

 tion," and to cases of mutualism and symbiosis which may 

 offer a comparison however distant with the relations existing 

 between the leguminous plant and the tubercle organism. 



