Ontogenetic anatomy of Streptocarpus grandis (Gesneriaceae) with implications for evolution of monophylly

The monophylly of Streptocarpus grandis was examined ontogenetically and anatomically. When the seed is shed, the embryo is composed of a hypocotyl and two equal-sized cotyledons, lacking root and shoot apices. During germination, cell division and subsequent cell enlargement occur in the hypocotyl and cotyledons. The hypocotyl soon produces a primary root from its distal tip; this involves surface and subsurface cells at the point of attachment of the suspensor remnant. In the cotyledons, cell enlargement and differentiation occur basipetally, leaving small meristematic cells at the bases. These small cells give rise to the basal meristem in one of the two cotyledons, which contributes to an accrescent cotyledon. The groove meristem, which later differentiates into an inflorescence. arises in place of shoot apices when the cotyledons become visibly unequal in size. It later exhibits a tunica-corpus like configuration and differentiates directly into an inflorescence meristem. The evolution of this unique growth of one-leaved Streptocarpus is discussed with regard to morphogenetic data. (C) 2000 Annals of Botany Company.