Examples of endosymbiotic interactions. (a) An arbuscule (fungal hyphal ingrowth) within a root cortical cell (in this case of Glomus sp.). (b) An infection thread and bacteroid within a root nodule cell. Note that the perimicrobial membrane (symbiosome membrane; red) is of plant origin. This figure was prepared by Eva Wegel (John Innes Institute, UK) and is reproduced with permission from Curr Opin Plant Biol 2000, 3:320-328.