THE BACTEROIDS 267 



words must be added concerning their mode of arrangement (already referred to 

 as a " sac ") at the time of penetration into the cells. The.-e branching bacterial 

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

 misunderstood. Beyerinck at first considered them as the surplus matter from 

 the division of the nucleus of the nodule-cells (Kerntoimenfaden). By other 

 workers they were styled mucus threads, fungoid hyphte, 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 consequently named them /Schinzia leguminosarum. Subsequently Frank, 

 by the new name infection threads, indicated their true nature, first recognised 

 by Prazmowski. Frank, moreover, in his studies on the immigration of the 

 nodule bacteria in the root, arrived at results differing in several points from 

 those of Prazmowski. We cannot, however, go further into this matter, which 



FIG. 63. Development of bacteria to bacteroids from the FIG, 64. Keticulated band of bac- 



meristem of a root-nodule ol Vicia sativa. Explanation teroids from the nodules of Vicia 



in the text. Faba. 



Magu. 700. (After Beyerinck.) Magn. 700. (After Beyerinck.) 



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 

 Rhizobium 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 con- 

 stituting these filamentous colonies (or zooglcea). 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 substance allied to cellulose. The structure and progress 

 of the infection threads in the nodule-cells can be readily recognised in sectional 

 preparations, stained by a solution of equal parts of fuchsine and methyl violet 

 in i per cent, acetic acid. This colours the plasmal contents and membrane of 

 the nodule-cells blue, the bacteria of the infection threads being stained red, 

 whilst the membrane of the latter remains uncoloured. It should be mentioned, 

 in conclusion, that these threads of capsuled bacterial colonies are but rarely 

 found in the nodules of lupins. 



The bacteria which have gained access now develop in the cells of the 

 bacteroidal tissue, and, finally, under the influence of the surrounding protoplasm, 

 become modified into involution forms termed bacteroids, rich in albumen and no 

 longer capable of reproduction. This morphological change is represented in 

 Fig. 63, in an example taken from the nodule of Vicia satii-a shown in Fig. 60. 



A section cut near the lower extremity meets the youngest meristem, where 

 only long rods (u) in a high state of development are to be seen. A little higher 



