39 6 



THE FOOD OF PLANTS 



soil, and frequently observed the appearance of algae in it. Petermann has, 

 however, recently shown that barley is unable to fix any nitrogen in a sterile 

 soil, although in earlier researches he found an energetic fixation of nitrogen 

 occurred when the plants were grown in soil containing algae and bacteria. 



According to Schlosing and Laurent the 

 simple suppression of the algae suffices to 

 prevent any apparent fixation of nitrogen 

 by oats, mustard and cress, and Aeby, 

 Pfeiffer, and Franke, all conclude that hemp 

 plants are unable themselves to fix free 

 nitrogen. These results all agree with Hell- 

 riegel's conclusion that only Phanerogams 

 bearing root-tubercles are able to assimilate 

 free nitrogen. Whether certain algae have 

 the same power when isolated must remain 

 at present an open question, for entirely 

 negative results have hitherto been obtained 1 , 

 and the importance of algae for the fixation 

 of nitrogen in the soil may be simply due to 

 their symbiotic relationship to the above- 

 mentioned bacteria 2 . 



Lcginninosac. Hellriegel's 3 re- 

 searches first established the fact that 

 certain Leguminosae are able to assimi- 

 late free nitrogen, and proved that this 

 power is due to the presence of root- 

 tubercles formed by infection with micro- 

 organisms from the soil. These latter 



are specific in nature (root-tubercle bacteria, Bacillus radicicola, Beyerinck ; 

 Rhizobium legiiminosantm, Frank). ' Similar root-tubercles may be formed 

 in the roots of the alder 4 , and, according to Nobbc 5 , Eleagnus and 



FlG. 59. Root-tubercles of Litpinus ttleus 

 (J nat. size). 



1 Kossowitsch, 'Bot. Zeitung, 1894, p. 112; Molisch, Sitzungsb. d. Wien. Akad., 1895, Bd. civ, 

 Abth. i, p. 793. Frank (Bot. Zeitung, 1893, p. 146) supposes that algae can also fix nitrogen. 

 - 2 Frank, I, Bot. Zeitung, 1893, p. 139; II, Unters. iiber d. Ernahrung d. Pflanze mit Stickstoff, 

 1888; III, Pilzsymbiose d. Leguminosen,, 1890; IV, Die Assimilation des freieu Stickstoffs, 1892 

 (Sep.-abdr. a. Landw. Jahrb.). Cf. also Frank, Lehrb. d. Bot., 1892, p. 563; Petermann, Bull. d. 

 1'Acad. roy. d. Belgique, 1893, iii. se"r., T. xxv, p. 267 ; Rech. d. chim. et d. physiol., 1894, T. II, 

 p. 265; ibid., 1889 and 1890; Bouilhac, Compt. rend., 1896, T. cxxin, p. 828; Schlosing et 

 Laurent, Ann. d. 1'Inst. Pasteur, 1892, T. vi, pp. 115, 827; Aeby, Versuchsst., 1896, Bd. XLI, 

 p. 438; Pfeiffer und Franke, ibid., p. 117, and 1887, Bd. XLVIII, p. 418, where Liebscher's view 

 concerning mustard plants is given ; Hellriegel, Unters. iiber d. Stickstoffnahrung, &c., 1888. 



3 Hellriegel, Unters. iiber d. Stickstoffnahrung d. Gramineen u. d. Leguminosen, iSSS; Ber. d. 

 Bot. Ges., 1889, p. 138. 



' Hiltncr, Versuchsst., 1896, Bd. XLVI, p. 160. 



5 Nobbe, Versuchsst., 1894, Bd. xi.v, p. 155, and 1892, Bd. XLI, p. 138. On the root-tubercles 



