348 FIXATION OF FREE NITROGEN BY BACTERIA. 



bacteroidal tissue already mentioned (Fig. 62). The plasma of 

 these cells, with its fungoid enclosures, has been termed myco- 

 plasma by A. B. FRANK (VI. ). 



Before tracing the career of the bacteria any further, a few 

 explanatory words must be added concerning their mode of 

 arrangement (already referred to as a " sac ") at the time of pene- 

 tration into the cells. These branching bacterial colonies en- 

 veloped, as has been stated, in a membrane, were for a long time 



misunderstood. Beyerinck at 



\ \ first considered them as the 



surplus matter from the divi- 

 sion of the nucleus of the 

 nodule-cells (Kerntonnenfdden). 

 By other workers they were 

 styled mucus threads, fungoid 

 hyphse, plasmodial cords, &c. 

 Frank, for a long time, held 

 them to be the mycelium of 

 an independent higher fungus, 

 differing from the nodule 

 bacteria and belonging to the 

 genus Schinzia, and conse- 

 quently named them Schinzia 

 leguminosarwn. Subsequently 

 Frank, by the new name in- 

 fection threads, indicated their 

 true nature, first recognised by 

 Prazmowski. Frank, moreover, 

 in his studies on the immigra- 

 tion of the nodule bacteria in 

 the root, arrived at results dif- 

 fering in several points from 

 those of Prazmowski. We can- 

 not, however, go further into 

 this matter, which is one more 

 of botanical than mycological 

 interest. He also discarded 

 the name bestowed on these 

 root - dwelling organisms by 



Beyerinck in favour of the term Rhizolnum leguminosarum. 

 The integument of the threads is not, as erroneously opined by 

 Frank, a product of the plasma of the nodule-cells, but is formed 

 by the union of the swollen membranes of the outer layers of the 

 bacteria constituting these filamentous colonies (or zoogloea). As 

 was shown by A. KOCH (IV.) and M. W. BEYERINCK (XVI.), it is 

 stained blue by zinc iodo-chloride, and therefore consists of a sub- 

 stance allied to cellulose. The structure and progress of the in- 

 fection threads in the nodule-cells can be readily recognised in 



FIG. 62. Section through the bacteroidal 

 tissue of Lathyrus silvestris. 



Nuclear staining with i per cent, chromic 

 acid and methylene blue ; k. the nucleus 

 with its nucleolus (lying in a vacuole) ; 

 /. the infection-threads ; mi. the micro- 

 somes of the cytoplasma; am. starch 

 granules ; v. large central vacuole ; 

 bact. the bacteroids. Magn. 400. 

 (After Beyerinck.) 



