Comparison of shoot vascular organization in isophyllous (Pilea cadieri) and anisophyllous (Pellionia daveauana) species of the Urticaceae

Primary vascular organizations in isophyllous shoots of Pilea cadieri and in anisophyllous shoots of Pellionia daveauana were compared using computer-assisted reconstruction of serial cross sections of mature stems and shoot tips. Stems of P. cadieri are characterized by a closed vascular system with six sympodia and trilacunar three-trace nodes. In contrast, shoots of P. daveauana have an intermediate system with four sympodia and trilacunar four-trace nodes on the ventral side of the stem and unilacunar one-trace nodes on the dorsal side of the stem. This highly modified vascular architecture reflects the extreme anisophylly and dorsiventral symmetry of shoots. In P. daveauana, the dorsal leaf trace differs from the ventral leaf traces in phloem cross-sectional area and in tracheary element number and diameter; in P. cadieri, all traces are similar in size. Leaf-trace procambium develops simultaneously into both leaves of a pair in P. cadieri, but in P. daveauana the median trace of the ventral leaf differentiates one plastochron before the dorsal leaf median trace. These features indicate a strong developmental interaction between expansion of the leaf primordium and differentiation of procambial tissue.