Stem and leaf anatomy of the arborescent Cucurbitaceae Dendrosicyos socotrana with comments on the evolution of pachycauls from lianas

Abstract.

Stem and leaf anatomy of Dendrosicyos socotrana, the only arborescent Cucurbitaceae, are examined for correlations with life form and ecology and are used to test hypotheses regarding features adaptive in scandent plants. The stem consists mainly of ray and conjunctive parenchyma with small strands of xylem forming an anastomosing net throughout the trunk. Xylem strands bear vascular cambia that produce secondary phloem, representing the first report of successive cambia in Cucurbitales. Some features characteristic of lianas, such as very wide vessel elements with thick walls, are absent from Dendrosicyos. Other features, such as very wide rays and abundant axial parenchyma, are present in both Dendrosicyos and lianas but appear to serve differing roles in these different life forms. It is suggested that lianas have numerous features that are readily co-opted in the evolution of pachycaul trees and that the evolution of pachycauls from lianas has happened repeatedly in the core eudicots.