The cytology of bacteroids from root nodules of subterranean clover (Trifolium subterraneum L

SUMMARY: Bacteroids from effective nodules of subterranean clover were ovoid, pear-shaped, or slightly branched and measured 5-7 x 4-6p. In the fully developed bacteroid the nuclear material was central, divided into discrete bodies and showed signs of degeneration. Surrounding the nucleus was a perinuclear region, probably derived from it, and this was surrounded by cytoplasm which contained several mitochondria-like granules. The development of bacteroids from rod-shaped cells is described. Bacteroids from the ineffective nodules studied possessed no perinuclear region, and, in contrast to those from effective nodules, accumulated varying amounts of glycogen, depending on the length of time for which the bacteroids survived in the host cell. The symbiotic nitrogen-fixing nodule bacteria have been the subject of much research, and it is surprising that the structure of the organisms within the host cells has not received more attention. The term ‘ bacteroid ’ has been given to the swollen irregularly shaped bacteria found in the cytoplasm of infected host cells in nodules on legume roots. Morck (1891) made a careful study of bacteroids, and his drawings show cells with clear central areas surrounded by granular cytoplasm. Uher (1937) described spindle- and globiform-shaped nuclei in bacteroids from Trifolium species, and Schaede (1939, 1941) demon-strated a diffuse Feulgen-positive reaction in bacteroids from Lupinus and Neptuniu nodules but not in those from Vicia fuba. Baylor, Appleman, Sears & Clark (1945) described nuclear granules visible in electron micrographs of nodule bacteria from cultures and excised nodules, and Bisset (1952) included some observations on bacteroids in a paper on life cycles in Rhixobium species