The fine structure of the leaf nodules of Ardisia crispa (Thunb.) A.DC. (Myrsinaceae)

The fine structure of the leaf nodules of Ardisia crispa is described. The bacterial endophyte in mature nodules is extracellular, forming a compact mass which is encompassed within a 5–6 cell thick sheath of modified mesophyll cells. Processes from these sheath cells invade the bacterial mass providing a host surface across which exchange of metabolites may take place. Considerable pleomorphism is exhibited by the bacteria, paralleling that found in other symbiotic associations in which the host can influence the morphology of the microsymbiont. The spherical, highly granate chloroplasts, rich in starch, which are characteristic of the spongy mesophyll leaf cells, are replaced, in the modified sheath cells, by a degenerate plastid form containing membrane whorls. Lipid reserves appear to replace the starch in the sheath cells. The significance of these changes is discussed.