The ontogeny of pea (Pisum sativum L.) flowers, as in many legume and nonlegume plant species, proceeds through a very different sequence of events from the same process in Antirrhinum majus and Arabidopsis thaliana. Using scanning electron microscopic analysis, we have characterized the early development of wild-type pea flowers and selected morphological characters or markers to break it down into different developmental stages. We used these markers as tools to characterize early alterations in flower development of several pea floral homeotic mutants. These mutants display phenotypes resembling those of: (1) floral meristem identity mutations, frondosus (brac); (2) class A mutations, calix carpellaris (cc); (3) class B mutations, stamina pistilloida (stp-1 and stp-2); and (4) class C mutations, petalosus (pe). According to the homeotic transformations observed in the pea floral mutants, it would appear feasible that the identity and developmental pattern of the four organ types in pea flowers are governed by at least the same three developmental functions, A, B, and C, proposed for the two model systems. However, our results suggest that, in pea, although these functions do have a similar role in the specification of organ identity shown by their counterparts in Arabidopsis or Antirrhinum, they may differ in the control of other processes, such as floral determinacy, organ number, or leaf development. The more remarkable features of pea flower ontogeny were the existence of four common primordia to petals and stamens, the early carpel primordium initiation, and the abaxial-adaxial unidirectional initiation of organ primordia within each different floral whorl, in contrast to the centripetal and sequential floral ontogeny in other plants. Organ differentiation within each of these common primordia appears to be a complex process that plays a central role in the ontogeny of pea flowers. Analysis of flower developmental pea homeotic mutants suggests that A, B, and C functions are necessary for the correct differentiation of organs from common primordia and that, in addition to its role in the specification of petals and stamens, B function, would be involved in conferring common primordia identity. Copyright 1999 Wiley-Liss, Inc.