440 



Prof. M. Ward. On the 



[Nov. 21, 



enveloped by membrane. The top of the root-hair coils round the 

 knob, and the latter then puts forth a tube filled with bacteria and 

 surrounded by a brilliant membrane. 



From this stage onwards, til] the tubercle is developed, this tube 

 behaves like a hypha, growing at the apex, and putting out branches 

 beneath the apex which behave like the original filament. 



The tube now grows from the root-hair, through the cortex, and 

 even as far as the endodermis, boring through the walls of the cortex 

 cells, splitting them mostly into two lamella?, so that a swelling full 

 of bacteria and bounded by lamellae is formed. In the cells the tube 

 grows towards the nucleus, and usually applies itself so close that the 

 latter is indistinguishable unless stained. Hence, probably, the 

 reason why Beyerinck regarded the tube as " schleim " debris remain- 

 ing over from nuclear division. So far, we have no free bacteria in 

 cells ; they are all in the tube. 



As soon as the tube reaches the deeper layers of the cortex the cells 

 begin to divide, at first slowly and irregularly, then quickly ; and this 

 is especially true of the four or five innermost layers of the cortex. 

 Then also numerous thin branches are developed from the tubes, 

 enter the new cells, and branch in them. The result is the meristem 

 of the tubercle. The middle of the tubercle consists of a parenchyma 

 of larger cells, penetrated by tubes in all planes, and filled later with 

 bacteria freed from the dissolving tubes ; the outer layers consist of 

 smaller and more flattened cells with poorer contents, the membranes 

 of which are suberised later. Between the bacteroid tissue and the 

 latter (cortex of tubercle) is a small-celled meristem, free of bac- 

 teroids ; this produces vascular bundles further back, and these fork 

 and are joined to those of the root. Between the vascular bundles 

 and the bacteroid tissue is further a layer of starch-bearing cells, free 

 of bacteria. 



The place where the tubercle forms is predetermined by the infect- 

 ing tube, and since this enters anywhere, the tubercles arise irregularly, 

 i.e., opposite or not to xylem or phloem. Pericambium has nothing 

 to do with it, and so Yan Tieghem, Beyerinck, and others are wrong 

 in regarding these tubercles as modified lateral roots. 



After the tissues of the tubercle are differentiated and the bacteria 

 are set free (parts of the tubes do not burst and dissolve), the latter 

 fuse with protoplasm, multiply by growth and fission, become forked, 

 and subsequently form the bacteroids. The further fate of these 

 depends on their role in the economy of the plant. 



As did Hellriegel, so also Prazmowski put some plants in soils 

 with, others in soils without, nitrogen, and he confirms Hellriegel \s 

 results — infected plants require no nitrogen at their roots ; non- 

 infected plants pass into a state of hunger and die if not supplied 

 with nitrates. 



