Stem Anatomy of Climbing Palms in Relation to Long-distance Water Transport

Abstract

Palms lack secondary growth so their primary vascular system is long-lived and must be minimally vulnerable to dysfunction. For water movement, the axial xylem must be well defended against cavitation. Climbing palms can be very long and represent a maximum solution to transport problems. How is this demonstrated in their anatomy? This article contrasts stem vascular anatomy in a cane-like tree (Rhapis excelsa) with that in the American climbing palm Desmoncus and the Old World rattan genus Calamus. Rhapis, representing the basic classical palm vasculature, has a continuously integrated vascular system determined by branching of the axial (stem) system to produce leaf traces, bridges, and continuing axial bundles. Axial transport is favored over appendicular structures because leaves are irrigated solely by narrower protoxylem tracheids. Maximum stem vessel length is inherently limited by the leaf contact distance (LCD). Desmoncus is very similar except that interconnections involve more numerous bridges and axial continuity is less obvious. Both Rhapis and Desmoncus retain scalariform perforation plates in their stem vessels. However, Calamus lacks axial continuity because axial bundles extend distally into leaves as leaf traces but end blindly in a basipetal direction. The only interconnection is via narrow transverse commissures (not bridges). Calamus stem metaxylem vessels have simple perforation plates. Resistance to water transport can be calculated, based on axial changes in metaxylem vessel diameter and is very high in the climbing palms. The unique features of Calamus may relate to safety of the hydraulic system as much as its efficiency, with Desmoncus an intermediate condition more clearly based on the classical model of palm vasculature. Calamus may have also evolved to mitigate the limitation of vessel length determined by LCD. Further anatomical solutions to the climbing habit in palms are briefly discussed.