Jan. 3 , i 9 2i Nodule Bacteria of Leguminous Plants 549 



phism of these organisms. Before their complete life history can be 

 given much additional material will have to be collected, especially with 

 regard to the form of gonidangia, regenerative bodies, and the various 

 cells developing from the symplastic stage. At present we intend only 

 to bring out as clearly as possible those points which are important 

 for a correct differentiation and diagnosis. As far as one may judge 

 from the microscopic appearance, it is the inclination of Bacillus radio- 

 bacter to form stars which is most characteristic (Pi. 68, L), and this was 

 used, therefore, by Beijerinck for its denomination. With B. radicicola 

 the frequent occurrence of the clear-cut, compact Y forms is the most 

 conspicuous feature (Pi. 68, H) ; whereas the bacteria of the cowpea- 

 soybean group present themselves in most cases, when stained with aqueous 

 aniline dyes in the usual manner, as short or long, unstained sheaths 

 with one or more darkly stained granules (Pi. 68, J). Of course Y forms, 

 as well as unstained sheaths with darkly stained gonidia, can be ob- 

 served not infrequently with the other organisms, too, and the star for- 

 mation is by no means solely to be found with Radiobacter; but we feel 

 sure that those pictures, as shown on Plate 68, G-L, will be found most 

 valuable for diagnostical purposes. 



The flagellation is the same with Radiobacter (PI. 68, C) and Radici- 

 cola (PI. 68, B), while the bacteria of the cowpea-soybean group are 

 characterized by one coarse, fairly straight polar flagellum ( Pi. 68, A). 

 Just before fission one cilium may be seen at each end; as a rare excep- 

 tion a tuft of polar flagella was observed occasionally. Frequently a 

 darkly stained body becomes visible within the rod just at that point 

 where the flagellum springs forth, which may be considered to be a flag- 

 ellated, not yet liberated, gonidium, such as can be seen occasionally 

 with many other bacteria, especially with Bacillus radicicola, too. When 

 liberated this becomes the monotrichic small "swarming body" described 

 by Beijerinck in 1888 (4). 



The growth on mannite-nitrate agar, as well as on beef agar slants, as 

 described in Table I, is quite characteristic, and after the eyes have been 

 sufficiently trained, one seldom makes a mistake in guessing the group 

 to which a culture presented foi inspection may belong. But it must be 

 admitted that occasionally and temporarily a strain of the cowpea- 

 soybean group can show the flat, transparent growth characteristic of 

 Radicicola, whereas it is a very rare occurrence that a member of the 

 last-named group simulates the former one. The growth of Radiobacter 

 is always very typical, except when a very weak strain is encountered, 

 which does not frequently occur within this group. Plate 69, A, demon- 

 strates the characteristic differences noticeable on mannite-nitrate agar 

 as clearly as they can be shown in a photographic reproduction. 1 



1 As was the case with Azotobacter, for which the mannite-nitrate agar was first used (13, p. 686), so also 

 the nodule bacteria and Bacillus radiobacter grew very readily on this substrate. Allen (/, p. jj) asserted 

 recently that he could not get any growth of Azotobacter on a dextrose agar, which he erroneously called 

 "Lohnisand Smith's medium." But not even the formula used by us has been quoted correctly by Allen, 

 and it is, of course, quite obvious that on account of the alterations made by Allen his agar must indeed 

 have been quite unsuitable. 



