Evolutionary study of a unique morphology, anisocotyly of Gesneriaceas

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The monophylly of Streptocarpus grandis, Monophyllaea horsfieldii, and M. singularis was examined ontogenetically and anatomically, and compared with the ontogenesis of the caulescent species, S. kirkii, and of rosulate species, S. rexii. In all species examined, the embryo in the seed shedding stage 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, involving surface and subsurface cells at the attaching point 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 cotyledon, which contributes to an accrescent cotyledon, but do not in the other cotyledon. Due to long-erm activity of the basal meristem in one cotyledon, the apparent anisocotyly, that is, the size difference between two cotyledons is established. The groove meristem, which later differentiates into an inflorescence, arises in place of shoot apices when the cotyledons become apparently unequal. It later exhibits a tunica-corpus like configuration and differentiates directly into inflorescence meristem in monophyllous and rosulate species, but into vegetative shoot-apex in caulescent species. The evolution of the unique growth of one-leaved Streptocarpus is discussed on the basis of the morphogenetic data.