41 



juslilit'cl. This uhjfctlon does not appl) lo the dilution method 

 di'scril)ed above. 



W 111. W. 1'. BbUi.Lv and H. B. Hltchinson. " On the 

 OiiUi^es through which the Xodule Organism (Ps. 

 riidicicohi) passes under Cultural Conditions.'' 

 Journal of A^^ricultural Science, 1920. Vol. X. pp. 

 144-16'2. 



Lnder oerlain cullural tDuditions the nodule organism from 

 the roots of red clover, broad bean, lucerne and lupin exhibits a 

 tenden(^y towards j^ranular disintet^ration of the cell with the for- 

 mation of small non-molile coccoid bodies, about 0.4/^ diameter. 



In the culture media ordinarily in use these coccoid bodies iire 

 not formed extensively, but cultivation on soil extract media 

 rapidly leads to their production, until finally they constitute the 

 j)red()minant type in the culture. 



A life-cycle consistinj^' of five siaj^cb i>. described : — 



1. — 'The hrc-sitHiriuer form (non-motile). When a culture of 

 the orj^anism is |)laced in a neutral soil solution, it is converted 

 after four or five days into the pre-swarmer form. 



2. — Second slagCy larger non-motile coccus. In presence of 

 saccharose, certain other carlK)hydrales and phosphates, etc., the 

 pre-swarmers undergo a chanj^e. The original coccoid pre- 

 swarmer increases in size until its diameter has been doubled, but 

 still remains a non-motile coccus. 



3. — Swarmer stage, motile. The cell then becomes ellipsoidal 

 and develops high motility. This form is the well-known 

 *' swarmer " of Beijerinck. 



4. — Rod-form. Proceeding in an *' up-grade " direction, the 

 swarmer becomes elongated and gives rise to a rod-form, which is 

 still motile but decreasiiigly so. So long as there is sufficient avail- 

 able carbohydrate in the medium, the organism remains in this 

 form. 



5. — Vacuolated stage. When, however, the organism is 

 placed in a neutral soil extract or the available carbohydrate 

 becomes exhausted, it becomes highly vacuolated and the chromatin 

 divides into a number of bands. F'inally, these bands become 

 rounded off and escape from the rod as the coccoid pre-swarmer. 



The formation of the coccoid bodies (pre-swarmers) may also 

 be induced by the addition of calcium or magnesium carbonates to 

 the medium or by placing the organisms under anaerobic condi- 

 tions. Of a considerable number of compounds other than carbo- 

 hydrates, calcium phosphate alone was capable of bringing about 

 the change from pre-swarmers to rods. 



The organism also appears to be affected greatly by the re- 

 action of the soil. In the main, the normal rod rapidly changes 

 into the pre-swarmer form in calcareous soils; acid soils cause the 

 production of highly vacuolated cells and eventually kill the 

 organism, while a slightly alkaline soil was found to be capable 

 of supporting vigorous growth without altering the form of the 

 cells. 



The eft'ect of various temperatures on the rapidity of pre- 

 swarmer formation has been studied. Relatively high tempera- 

 tures (30° and 37"^) either prevent or postpone the entrance of 

 down-grade changes. 



