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Wood anatomy of Rauvolfioideae (Apocynaceae): a search for meaningful non-DNA characters at the tribal level¹

² Laboratory of Plant Systematics, Institute of Botany and Microbiology, Kasteelpark Arenberg 31 Box 2437, K.U.Leuven, BE-3001 Leuven, Belgium ³ Institute of Systematic Botany, University of Zurich, Zollikerstrasse 107, 8008 Zurich, Switzerland ⁴ Nationaal Herbarium Nederland—Leiden University Branch, P.O. Box 9514, NL-2300 RA Leiden, The Netherlands ⁵ Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3DS, UK

ABSTRACT

Wood anatomical studies in the economically important Apocynaceae or dogbane family are fragmentary. This study represents a first attempt to unravel the phylogenetic significance and major evolutionary trends in the wood of the family, using existing and new microscopic wood observations within the large subfamily Rauvolfioideae. On the basis of LM and SEM observations of 91 species representing all 10 currently recognized tribes, we found that most of the tribes are characterized by a unique combination of wood characters, such as vessel grouping, vessel element length, fiber type, frequency of uniseriate rays, and fused multiseriate rays. Climbing rauvolfioid taxa can generally be distinguished from erect species by their wider vessels, tendency to form paratracheal axial parenchyma, presence of tracheids, and occurrence of laticifers in rays. With respect to the entire family, there is a general phylogenetic trend toward shorter vessel elements, a higher proportion of vessels in multiples and more vessels per multiple, higher tracheid abundance, more paratracheal parenchyma, and fewer cells per axial parenchyma strand in the more derived Apocynaceae. Most of these evolutionary trends are likely to be triggered by drier environmental conditions and/or shifts from an erect to a climbing habit.

Key Words: Apocynaceae • APSA clade • climbing vs. nonclimbing habit • Rauvolfioideae • systematic wood anatomy • tribal classification

Received for publication 5 May 2008. Accepted for publication 14 August 2008.