The Structure and Development ofTrifolium subterraneumL. Root Nodules

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Abstract
The structure of effective nodules on Trifolium subterraneum L. grown at a root temperature favourable for nitrogen fixation was examined by light microscopy. Rhizobium were released into host cells from vesicles on infection threads which were closely associated with the host cell nucleus. Enlargement into bacteroid forms was rapid and synchronous in similarly-aged host cells, but groups of rhizobia close to the nucleus sometimes did not develop into bacteroids. Rhizobium changed in shape from small rods to more elongate and then to irregularly swollen forms before finally becoming coccoid. On degeneration clusters of bacteroids became un evenly stained before coalescing at the centre of the host cell and finally disintegrating. Small vegetative rods multiplied amongst the degenerating bacteroids and in the greatly enlarged intercellular spaces; host cell walls became folded and distorted and sometimes broke before collapsing onto each other.
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