The structure of infection threads,bacteria and bacteroids in pea and clover root nodules

Summary The bacteria and infection threads of the pea root nodule were examined by light and electron microscopy. The bacteria in the infection thread are enclosed in a microcapsule. This capsule disappears when the bacterium is released into the host cytoplasm. The membrane envelope which surrounds the bacteria in the host cell is shown to be derived from the plant cell membrane. The infection of the host cell is by means of a process akin to pinocytosis and the bacteria are confined to vacuoles in the host cytoplasm. As each membrane envelope contains only one bacterium, the envelopes must divide with the bacteria. The bacteria increase 40fold in volume from the infection thread to the stage of the mature bacteroid. The mature infected host cell contains few organelles. The mitochondria become confined to the periphery of the cell. Differences of membrane structure in gram negative bacteria found by other workers have been attributed to fixation artifacts.