37 



Four species of green algae were grown in pure culture on six 

 media which had as a common basis a solution of mineral salts 

 devised by Schramm, but differing- in that the nitrogen was 

 supplied as ammonium nitrate, calcium nitrate or ammonium 

 sulphate ; for each of these sources of nitrogen there were two 

 media, one without added sugar and the other containing 1% 

 glucose. The cultures were aerated daily with sterile air free from 

 combined nitrogen. The initial nitrogen-content of the medium in 

 each flask was ascertained from check analyses of that medium, 

 and the nitrogen-content after six months' growth was determined 

 by chemical analysis of the whole of the contents of the flask. 



In practically all cases a good growth of algae was obtained, 

 and in a large number the growth was luxuriant. Nevertheless 

 the analytical results afforded no evidence whatever that any 

 fixation had occurred. In fact, those cultures the growth of which 

 had been most luxuriant had a final nitrogen-content that was, if 

 anything, slightly lower than that of the medium originally. 



This result differs from that obtained by Wann (Amer. Jour. 

 Bot., 1921., Vol. VIII.) Investigation showed, however, that the 

 method by which he estimated nitrogen breaks down in presence 

 of nitrate. The results give the appearance of nitrogen fixation 

 even when none occurred. 



The chemical methods used by the present authors were free 

 from these sources of error and, as already stated, no fixation could 

 be detected. While it is quite conceivable that green algae might 

 under certain conditions, as yet unknown, assimilate atmospheric 

 nitrogen, there is so far no trustworthy evidence that they can do so. 



BACTERIA. 



XIX. H. G. Thornton. "On the Developmeyit of a 

 Standardised Agar Medium for Counting Soil 

 Bacteria, with especial regard to the Repression of 

 Spreading Colonies." Annals of Applied Biology, 

 1922. Vol. IX. pp. 241-274. 



For counting bacteria by the plating method it is a first essen- 

 tial to accuracy that the plating medium should give uniform 

 results. The medium should be exactly reproducible, i.e., different 

 batches should give similar results. In the medium here develcped, 

 this has been achie\ed by using pure chemical compounds as food 

 constituents, selecting those compounds that did not alter the re- 

 action of the medium during sterilisation. 



Further parallel platings of a suspension of organisms made on 

 a single batch of medium should develop the same number of 

 colonies (within the limits of random sampling variance). This 

 necessitates the independent development of each colony on the 

 plate, which on agar media is frequently prevented by the develop- 

 ment of bacteria that form rapidly spreading colonies which 

 interfere with the development of other bacteria. 



A special study was therefore made of a common ''spreading" 

 organism with a view to limiting its growth. It was found that 

 the organism spreads over the agar surface by active motility and 

 that the factors controlling its spread were (i.) the existence of a 



