Stem vascular architecture in the rattan palm Calamus (Arecaceae-Calamoideae-Galaminae)

Climbing stems in the rattan genus Calamus can reach lengths of well over 1 00 m, are long-lived, and pet their vascular tissue is entirely primary. Su ch a combination suggests that stem vasculature is efficient and resistant to hydraulic disruption. By means of an optical shuttle and video recording of sequential images we analyzed the stem of a cultivated species. The ste m has vascular features that are unusual compared with those in arborescent palms and seemingly inefficient in terms of long-distance water transport. Axial bundles are discontinuous basally because leaf traces, when followed downwards, always end blindly below. Furthermore, there is no regular dist al branching of each leaf trace at its level of departure into a leaf, so t hat neither a continuing axial bundle nor bridges to adjacent axial bundles are produced as in the standard palm construction. Instead, the axial bund les in the stem periphery are connected to leaf traces and to each other by narrow and irregular transverse or oblique commissures that are not the de velopmental homologues of bridges. As in other palms, metaxylem within a le af trace is not continuous into the leaf so that the only connection to a l eaf is via protoxylem. Within the stem, protoxplem (tracheids) and metaxyle m (vessels) are never contiguous, unlike in other palms, which suggests tha t water can only move from metaxylem to protoxylem, and hence into the leaf , across a hydraulic resistance. We suggest that this minimizes cavitation of vessels and/or may be associated with an unknown mechanism that refills embolized vessels. Also, the metaxylem can be significant in stem water sto rage in the absence of abundant ground parenchyma.