EVOLUTION OF COTYLEDONARY AND NODAL VASCULATURE IN THE JUGLANDACEAE

Cotyledonary nodal patterns of the Juglandaceae range from 1-gap, 2-trace to multi-gap, multi-trace. The development of increased nodal complexity is associated with at least two independent evolutionary shifts from epigeal to hypogeal germination. The taxa with epigeal germination such as Engelhardia sect. Engelhardia, Engelhardia sect. Psilocarpeae, Platycarya, and Pterocarya all have 1-gap, 2-trace nodes. The change to hypogeal germination in Engelhardia sect. Oreomunnea and Alfaroa is correlated with the development of 1-gap, 3-trace cotyledonary nodes. The second line has led to large, heavy-fruited members with hypogeal germination and complex cotyledonary nodes ranging from 2–6 gaps. The diversity of nodal patterns is the result of variation on a common theme; five basic vascular strands in the cotyledon, undergoing variations in dichotomy, fusion, and separation, are associated with one to many gaps. Presumably the complex development of the cotyledonary node is a response to increased functional demands of hypogeous cotyledons.