346 The Physiology of Plants BOOK in 



and Cruciferae. Vines showed in 1888 that if compounds 

 of nitrogen are present in the soil in which leguminous 

 plants are growing, the development of the root-tubercles 

 is inhibited. He observed a certain approximate inverse 

 proportion between the tubercles and the amount of com- 

 bined nitrogen present. 



Lawes and Gilbert took up the question at Rothamstead 

 in 1887 and conducted their researches for three years. 

 They obtained results confirmatory of Hellriegel and Wil- 

 farth in every particular. 



A number of investigations by different observers in 

 different countries at about this period also helped to estab- 

 lish the accuracy of the early results detailed. Among 

 these workers may be mentioned Atwater in America and 

 Breal in France. Schloesing and Laurent a little later, 

 in 1890, added an additional proof by cultivating some 

 leguminous plants in closed vessels and showing by analysis 

 of the air before and after growth that free nitrogen dis- 

 appeared in quantity closely corresponding to that gained 

 by the plants. 



A discordant voice was raised by Wolff, whose experi- 

 ments, extending over the years 1883-6, were interpreted 

 by him as showing that the fixation is effected by the soil 

 under the influence of its porous and alkaline constituents. 



The striking results that had been obtained led Marshall 

 Ward to the view that the phenomenon is one of sym- 

 biosis; the organism, possibly a parasite at first, having 

 ultimately modified its behaviour so as to serve as a stimulus 

 to the vegetative processes of the plant. Investigations 

 during the remainder of the century were directed towards 

 obtaining a clearer knowledge of its structure and life- 

 history and the part it plays in the symbiosis. They 

 modified Ward's view but slightly, but up to 1900 very 

 little information was forthcoming as to the process of 

 chemical change accompanying the fixation of the gas. 



