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CURRENT LITERATURE. 181

Evans Alexander W- A Taxonomic Study of Dumortiera Bulletin of the Torrey Botanical Club, Vol. 46, No. 5, {May 1919).

The writer gives a historical account of the genus, and fully examines critically the various characters which have been used by various writers on this genus. Stephani recognised three species in his species Hepaticarum in 1899 and Campbell published last year the description of a fourth species. The writer however recognises only two species on the basis of the char- acters drawn from the structural features of the vegetative thallus, and says that the characters "drawn from the size and method of branching of the thalus seem especially unreliable, Those drawn from the female receptacle and the spores are scarcely more satisfactory." The writer has examined a very large number of specimens and gives a full list of synonyms and the places from which the plants came. The following is the key which he gives : —

Upper surface of the thallus smooth or nearly so throughout (although often showing vestigial air chambers) D. hirsuta.

Upper surface of the thallus with crowded papilliform cells, at least in certain portions (always showing vestigial air chambers). D. nepalensis.

S. R. K.

Allen, E. R., Soma Conditions Affecting the Growth of Azotobac- ter chroococcum. Annals of the Missouri Botanical Garden, Vol. VI. No. l,p.I (Feb. 1919).

The Nitrogen-fixing bacterium Azotobacter has since its discovery by Beijerinck in 1901 been the subject of considerable study; and it has been found by many workers that growth in ordinary synthetic culture media is poor, but that an aqueous extract of soil or even tap — instead of distilled water causes a marked improvement. Kzyemieniewski, in 1908, found that the humus was the important constituent of soil, and several suggestions as to the meaning of this have been made. One is that the beneficial result is due to the presence of iron and aluminium-silico-phosphates, but as the optimum quantity of the former lies at 10 mg per 100 cc, and is thus considerably above that which would be required for nutrition, H. Fischer suggested that the role of humus or of the Fe 2 3 is that of an oxygen carrier. Bottomly in 1914 suggested that certain bodies, analagous to vitamines and which he called • auximones ', are liberated from peat treated with certain anoerobic bacteria, and that there auximones are the cause of the action of humus in cultures of Azotobacter. In the paper under notice the author describes experiments which point to the need of phosphorus and the absence of all acid in the culture-medium as the chief requirements, and that it is unneces- sary to postulate the action of colloidal oxygen carriers or of any special and rare or supposed constituents or products of the soil-humus such as

anximones.

P. F. F.

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