Woody papilionoid legumes show varied nodule morphology. Two forms have particular interest for nodule evolution. The aeschynomenoid type, commonly found in the tribes Dalbergieae and Aeschynomeneae, is associated with lateral roots and follows infections at the lateral-subtending root junction. Infection threads are rarely, if ever, found and the infected tissue does not contain uninfected (interstitial) cells. Some primitive genera of the tribes Dalbergieae, Robinieae and Millettieae have woody, indeterminate nodules in which infection threads are formed only when the bacteria attempt to penetrate host cells. These threads proliferate within the host cell and persist throughout the active nitrogen-fixing period. Similar infected cells have been found in all eight genera so far examined in the Caesalpinioideae and are considered to be a primitive case. They have not been found in mimosoid nodules. Primitive nodules often contain numerous massive vascular bundles with many thick-walled xylem elements. Many extant legumes lack root hairs; this is also thought to be a primitive feature. A hypothesis is developed for the evolution of legume nodules from an ancestor which lacked root hairs and which did not contain bacteria within infection threads in the early stages of infection. Infection of legumes via root hairs is an advanced character. An attempt is made to accommodate the data and hypothesis into a taxonomic framework.
In: Advances in legume biology, monographs in systematic botany from the Missouri Botanical Garden 29. Stirton, C.H., Zarucchi, J.L. (Eds.). pp. 559-578.