VASCULAR ARCHITECTURE IN ISOPHYLLOUS AND FACULTATIVELY ANISOPHYLLOUS SPECIES OF PENTADENIA (GESNERIACEAE)

The organization and differentiation of primary vascular tissue in isophyllous shoots of Pentadenia crassicaulis and facultatively anisophyllous shoots of P. orientandina (Gesneriaceae) were compared using serial reconstructions and quantitative methods. Despite clear differences in shoot symmetry, both species are vascularized by four sympodia, with trilacunar, split-lateral nodal anatomy. Leaf trace tracheary element number and diameter reflect leaf size differences in P. orientandina: these parameters are significantly greater in the large ventral leaves than in the small dorsal leaves. The median and lateral traces of ventral leaves of this species have a similar number of tracheary elements of equal diameter, while there are significantly more tracheary elements in the median than lateral traces of dorsal leaves. The pattern seen in P. crassicaulis is similar to that seen in the dorsal leaves of P. orientandina. In both species, protoxylem development anticipates differences in mature shoot vasculature. Changes in tracheary element number during ontogeny precede or are approximately coincident with changes in leaf size. These results suggest that the facultative expression of leaf size differences in P. orientandina is associated with opportunistic development and differentiation of the lateral trace.