ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 687 



of transforming the nitrogen contained in the nitrogenous compounds 

 into alhumen. 



They do not thrive under strictly anaerohic conditions. All the 

 seven kinds coagulate milk, some within 48 hours, others less quickly. 

 Several form a greenish fluorescing pigment on agar and gelatin. Some 

 darken the surface of potato, while others form thereon a yellow shining 

 deposit. Some form a surface scum on liquid media. Details are given 

 of the experiments on which the general conclusions are founded. 



Branching of Bacteria.* — Prof. A. Meyer records observations on 

 and experiments with Bacillus cohserens made for the purpose of testing 

 the vexed question of the branching of bacteria. He concludes that the 

 sjiecies of the genera Bacillus and Bacterium, and also very likely of 

 the genus Spirillum, have inherited from their ancestors a potentiality 

 to branching. The formation of branches occurs however only rarely, 

 and then in a rudimentary manner. It takes place as a rule when the 

 species is young, in a stage of the development of the species in which 

 apparently the formation of the branched mycele in the ancestors of 

 the bacteria arose. These retrograde formations are called forth by 

 special external exciting causes, and apparently act by prolonging the 

 youthful stage of the species ; but about the exact nature of the momenta 

 nothing accurate is known. 



Oligonitrophllous Microbes and the Genus Azotobacter.f — By 

 oligonitrophilous, Prof. M. W. Beijerinck understands those micro-organ- 

 isms which, in free competition with the rest of the microbic world, 

 develop in nutrient media to which no nitrogenous compounds have 

 been intentionally added, or from which no special care has been taken 

 to remove the last traces of such compounds. They are able to make 

 use of the free atmospheric nitrogen and adapt it to their own nutrition. 

 There are two principal series, one developing in the light at the cost 

 of atmospheric carbonic acid and being coloured green by chromophyll ; 

 the other developing in the dark and being devoid of colour. After 

 describing observations and experiments relative to the oligonitro- 

 philism of the Cyanophycese, he passes on to oligonitrophilous bacteria, 

 of which there is one genus Azotobacter and two species, A. chroococcum 

 and A. agilis. Azotobacteria exist as large diplococci or short rodlets, 

 4-6 [j. about, but may be shorter or much longer. Their contents are 

 hyaline, they sometimes including a vacuole, and their mucous wall is of 

 variable thickness. When young they are motile. The flagella are 

 polar and about the same length as the bacteria. They do not form 

 spores. They are capable of growing in media containing suitable 

 carbon compounds but very little nitrogen, the nitrogen being assimi- 

 lated from the air. Pure cultures grow on the most different media, 

 but best on those which contain very little nitrogen. The temperature 

 optimum is not far from 28° C. 



A. chroococcum forms a membranous scum when tap-water with 2 p.c. 

 mannite and 0*02 p.c. K 2 HP0 4 is infected with garden earth. Only a 

 few of the youngest are motile, most of the rodlets being at rest. Young 

 membranes answer to the description given under the genus, while 



* Centralbl. Bakt., 1" Abt., xxx. (1901) pp. 49-60 (2 pis.). 

 f Op. cit., '2" Abt., vii. (1901) pp. 501-82 (1 pi. and 6 figs.). 



