Early ovule development in Papaver rhoeas

Abstract

Adjacent placentae in the youngest floral buds that were studied were seen to be in contact, possessing axial procambial cells in a mass of ground tissue and two layers of anticlinally dividing cells at their surfaces. Nucelli are initiated by periclinal divisions in the second layer; normally two or three adjacent cells are involved. Initiation sites are four to six cells apart. Nucelli develop into spheres with single protoderm layers and ground tissue. At this time localized divisions in placental procambium and adjacent ground tissue produce procambial strands, each strand representing an incipient ovule trace. Such strands propagate acropetally from the placental procambium. Primary xylem and phloem within these strands appear to differentiate acropetally. Ovular integuments are produced by the nucellar protoderm in which the division planes are variable but periclinal divisions predominate. The megasporocyte differentiates interior to the protoderm at the apex of the nucellus. Based on the time of appearance of ovule trace procambial strands and their spatial relationship to individual nucelli, it appears that each nucellus induces the development of its prospective ovule trace.