Developmental Morphology of the Caulescent SpeciesStreptocarpus pallidiflorus(Gesneriaceae), with Implications for Evolution of Monophylly

The evolutionary pathway of monophylly characterized by a sole conspicuous leaf (macrocotyledon) and a cotyledonary inflorescence in Gesneriaceae is uncertain. This article describes the developmental anatomy of the caulescent speciesStreptocarpus pallidiflorusand compares it with unifoliate species. During and after seed germination, the root apical meristem arises exogenously and develops into the primary root, while the shoot apical meristem (SAM) arises in the actual center of the region between the anisocotyledons and produces the primary shoot. The macrocotyledon, like the foliage leaves, grows determinately because of the disappearance of the basal meristem during development. The developmental manner in which multiple axillary meristems form basipetally only on the side facing the subtending leaf or the macrocotyledon is similar to the basipetal initiation of secondary inflorescence on the primary inflorescence in the axil of the macrocotyledon in unifoliate congeners. We propose an evolutionary scenario for monophylly in which the embryonic SAM has been lost or suppressed in association with the establishment of the indeterminate basal meristem in the macrocotyledon and transfer of the vegetative axillary meristem at the macrocotyledon into an inflorescence meristem.