ASSIMILATION OF FREE NITROGEN 401 



root-tubercles, owing to the fact that the bacteria do not remain living in the soil 

 for longer than a certain time. The Soja plant cultivated in our gardens usually 

 forms no root-tubercles, whereas in its normal habitat in Japan these are produced 

 in abundance, a difference which may be due either to the influence of the rich 

 garden soil, or to the absence of the specific bacterium form by which it is infected 

 in Japan 1 . 



For the formation of root-tubercles and the maintenance of the symbiosis 

 certain specific interactions are necessary, as is indicated by the fact that the 

 tubercles are formed only upon certain plants, and that even then certain essential 

 external conditions must also be fulfilled. Thus Zinsser found that an injured 

 root lost the power of forming root-tubercles until the process of regeneration was 

 completed, that is until less heavy demands were made upon its available plastic 

 materials, while the roots of typical terrestrial plants do not form tubercles so 

 easily or abundantly when grown in a water-culture as they do in the soil 2 . 

 The presence of large amounts of saltpetre also prevents the formation of 

 tubercles to a more or less marked extent 3 . 



Under optimal conditions the formation of root-tubercles occurs so rapidly 

 that even in soil free from combined nitrogen a seedling pea or lupine experiences 

 no starvation for want of nitrogen. All tubercles seem to have the same functional 

 importance, and Frank's 4 conclusion that those of Phaseohis and Robinia are 

 inactive has proved to be incorrect. It is, however, not surprising that the power 

 of forming root-tubercles should not be possessed by all leguminous plants : thus 

 no tubercles have been observed upon the roots of Gleditschia triacanthus^ nor 

 could any be produced by artificial infection. A power of fixing nitrogen may be 

 obtained by union with other organisms, and the possibility of the existence of 

 simple contact-symbiosis with bacteria has already been mentioned. 



History and methods. De Saussure's gasometric experiments showed, in con- 

 tradiction to the opinion prevailing in his time, that green plants are unable to 

 make use of atmospheric nitrogen, but Boussingault was the first to clearly prove 

 that particular plants are unable to assimilate this gas s . He grew plants in 

 bell-jars filled with air filtered through sulphuric acid, using either a non- 



1 Kirchner, Cohn's Beitrage z. Biologic, 1895, Bd. vn, p. 214. 



' 2 Hiltner, Versuchsst., 1896, Bd. XLVI, p. 161. The formation of tubercles in water-cultures 

 was observed long ago. See Prazmowski, Versuchsst., 1891, Bd. XXXVIII, p. 41. 



3 Nobbe u. Hiltner, Versuchsst., 1893, Bd. XLII, p. 477 ; Hiltner, 1. c. Additional observations 

 upon the influence of the external conditions are given by Beyerinck, Bot. Zeitung, 1888, p. 743; 

 Prazmowski, Versuchsst., 1890, Bd. xxxvil, p. 189; Laurent, Ann. d. 1'Inst. Pasteur, 1891, T. v, 

 p. 133; Zinsser, I.e. 



* Cf. Prazmowski, Versuchsst., 1891, Bd. xxxvin, p. 60; Nobbe, ibid., 1891, Bd. xxxix, 



PP- 339- 35- 



5 Saussure, Rech. chim., 1804, p. 206. Saussure (Mem. d. 1. Soc. d. Phys. d. Geneve, 1833, 



T. vi, p. 570) also concluded that decomposing seeds evolved large quantities of nitrogen, although 

 in some cases a slight fixation of nitrogen occurred. The errors of the gasometric method were at 

 the time so great that the experiments cannot be regarded as proving the latter fact, although they 

 suffice to firmly establish the former. Boussingault, Agron., Chim. agiic. et Physiol., 1860, T. I, p. i. 

 These works are partly contained in Ann. d. sci. nat., 1854, iv. ser., T. I, p. 241, and 1855, iv. ser., 

 T. iv, p. 32. 



PFEFFER D d 



