RHIZOBIUM LEGUMINOSARUM 



501 



Slightly alkaline soils support vigorous growth without altering the morphology. 

 Relatively high temperatures, 30-37° C, prevent or postpone the change into the 

 pre-swarming stage. 



Inside the root nodules the organisms assume curious Y-shaped, pyriform, and 

 racket-like forms, known as bacteroids. Filaments may likewise be formed. In 

 culture the bacteroids develop into rods (Fremlin 1898). 



The properties of Rliiz. leguminosarum vary in accordance with the species of 

 plant from which they are derived. Fred and Davenport (1918), who studied 

 21 strains from different Leguminosce, found a variation in their resistance to 

 acids. Thus the strains from alfalfa and sweet clover had a limiting pH for growth 

 of 4-9 ; for the garden pea and vetch of 4-7 ; for red clover and common beans 

 of 4-2 ; for soya beans and velvet beans of 3-3 ; and for lupins of 3-15. All strains 

 had much the same resistance to alkali. Incidentally their alkali tolerance was 

 much greater than their acid tolerance. 



Whether the strains from different plants should be regarded as varieties of 

 one species is doubtful. Klimmer and Kriiger (1914), who examined a number 

 of strains from eighteen different species of LeguminoscB by means of the agglu- 

 tination, complement-fixation, and precipitation tests, were able to classify them 

 into 9 different groups. Bushnell and Sarles (1939) likewise found a large number 

 of serological types. In their experience little relationship was noted between 

 the ability of strains from different plants to cross-agglutinate and to cross-infect 

 (see also Fremlin 1898, Kleczkowski and Thornton 1944). 



Rhiz. leguminosarum is an obligate aerobe, capable of fixing atmospheric 

 nitrogen when grown in a medium free from combined nitrogen, but containing 

 a fermentable carbohydrate. Such a medium is composed of : 



Inoculated into the root of a leguminous plant it gives rise to a nodule, from 

 which it can be recovered in pure culture. Each variety is apparently specialized 

 to attack its own or closely related species of plants ; thus a strain isolated from 

 a nodule on a pea {Pisum sativum) will produce nodules also on vicia, Lathyrus 

 and Lens, but not on other legumes (Russell 1923). 



We append a detailed description of Rhiz. leguminosarum. 



Rhizobium leguminosarum Frank 



Synonyms. — Bacillus radicicola ; Ps. radicicola ; nodule organism. 



Isolation. — By Beijerinck in 1888 from root-nodules of leguminous plants. 



Ecology. — Soil ; found also in water. 



Morphology. — Has a life-cycle with gross changes in morphology. Shows non-motile 

 coccoid forms ; very small, highly motile, elMpsoidal form (swarmer), 0*9 fi X 0-2 

 /x ; motile rods, 2-3 /j, X 0-5 ;* ; large " vacuolated " rod and Y-forms. In root 

 nodules shows filamentous forms, and Y-forms (bacteroids). Swarmer forms motile 

 by single, long, polar flagellum. Vacuolated forms show bands of chromatin. Non- 

 sporing ; may be capsulated (see below). Gram-negative. Non-acid-fast. 



