Leaf Development in Isophyllous and Facultatively Anisophyllous Species of Pentadenia (Gesneriaceae)

Abstract

Shoots of Pentadenia orientandina exhibit varying degrees of anisophylly, ranging from pairs of equal‐sized leaves to pairs of large ventral and small dorsal leaves. In this study we compare phyllotaxis, leaf expansion, and accompanying histological changes in extremely anisophyllous shoots of this species and in isophyllous shoots of the related species, P. crassicaulis. In P. orientandina, decussate phyllotaxis is modified at leaf initiation, and angles of leaf insertion appear to be further changed during leaf expansion. In both species, leaf primordia of a pair are not distinguishable at inception, suggesting an equivalent developmental potential. In P. orientandina, size differences between ventral and dorsal leaves become significant at the P2 or P3 stage, coincident with lamina initiation. Minute dorsal leaves are arrested in their development at the P3 stage and mature without differentiation of multiple epidermis, stomata, mesophyll and most vascular tissue. Variation in dorsal leaf structure in P. orientandina emphasizes the plasticity of leaf development in this facultatively anisophyllous species.