Flora 206 (2011) 595–600 Contents lists available at ScienceDirect Flora journal homepage: www.elsevier.de/flora Vessel elements present in the secondary xylem of Trochodendron and Tetracentron (Trochodendraceae) Hong-Fang Li a,1 , Shu-Miaw Chaw b , Chun-Mei Du a , Yi Ren a,* a College of Life Sciences, Shaanxi Normal University, Xi’an 710062, China b Biodiversity Research Center, Academia Sinica, Taipei 115, Taiwan article info Article history: Received 30 September 2010 Accepted 24 November 2010 Keywords: Vessel elements Fiber-tracheids Tracheids Fibers Primitive angiosperms Xylem anatomy abstract For almost 150 years, the two monotypic genera Trochodendron and Tetracentron (Trochodendraceae) have been considered to share an unusual and primitive feature in angiosperms – the lack of vessels in their wood. Therefore, they have been classified in a basal position in the angiosperms. Our observations by light microscopy, low-vacuum environmental scanning electron microscopy (ESEM) and high-vacuum scanning electron microscopy (SEM) both in fresh and FAA-fixed materials consistently showed the pres- ence of tracheary elements differentiated into two types in both genera. In Trochodendron, the tracheary elements can be divided into perforate vessel elements and imperforate fiber-tracheids and tracheids. The vessel elements show end and lateral walls. The pits on the end walls are elongate- broadened and do not have membranes or only a few remnants of them forming the perforation plates. The fiber-tracheids show crossfield pit pairs and sharp ends, and the tracheids show bordered pits. In Tetracentron, the tracheary elements comprise vessel elements and fibers. The vessel elements are similar to those of Trochodendron, whereas the fibers have no crossfield pit pairs but, rather, elliptical pits and sharp ends. Thus, both Tro- chodendron and Tetracentron are vessel bearing rather than vesselless, although their vessel elements are primitive. © 2011 Elsevier GmbH. All rights reserved. Introduction The Wheel Tree family, Trochodendraceae Eichler, is a fam- ily of flowering plants endemic to eastern Asia and includes only two monotypic genera, Trochodendron and Tetracentron (Endress, 1993). These genera were considered to have a close relation- ship morphologically (Cronquist, 1981; Endress, 1986, 1993) and molecularly (APG, 1998, 2003, 2009). For almost 150 years, the two genera were considered as lacking vessel elements in the secondary xylem (Bailey and Nast, 1945; Bailey and Thompson, 1918; Carlquist, 1996; Eichler, 1864; Harms, 1897; Smith, 1945; Thompson and Bailey, 1916; Zhang and Gao, 1990), which resulted in the classification of Trochodendraceae in a basal position in angiosperms. However, recent phylogenetic studies have shown Trochodendraceae to be an early lineage in the eudicots rather than to be placed in the basal position of extant angiosperms (APG, 1998, 2003, 2009). This suggests that the absence of vessel ele- * Corresponding author. E-mail addresses: hflee@126.com, hflee@stu.snnu.edu.cn (H.-f. Li), bochawsm@gate.sinica.edu.tw (S.-M. Chaw), 156314976@qq.com (C.-m. Du), renyi@snnu.edu.cn (Y. Ren). 1 Present address: Weinan Vocation and Technical College, Weinan, Shaanxi 714000, China. ments might be a secondary evolved character, not a primitive one. Nonetheless, recent scanning electron microscopy (SEM) inves- tigations revealed vessel elements in the secondary xylem of Tetracentron (Ren et al., 2007), which raised the question of whether vessels are really lacking in the xylem of Trochodendron. There- fore, we aimed to investigate whether Trochodendron bears vessels in the secondary xylem and to compare the patterns of tracheary elements in Trochodendron and Tetracentron. Materials and methods We collected 3- to 5-year-old branches of Trochodendron aralioides Eichler and Tetracentron sinense Oliver in Yangming Mt., Taipei, Taiwan (voucher: Shu-Miaw Chaw s. n., SANU (Herbarium of Shaanxi Normal University)) in May 2006 and in Xunyangba, Ningshan County, Shaanxi province (voucher: Hai-ning Li 200408, SANU) in May 2004, respectively. The branches were cut into pieces of 1–1.5 mm in diameter and 1 cm long and fixed by FAA (Forma- lin:acetic:ethanol:water = 10:5:50:35) before sectioning. The fixed materials were examined by three methods. (1) Mate- rials were treated with Jeffrey’s Fluid (10% chromic acid:10% nitric acid = 1:1) at room temperature for 72 h, spread on microscope slides, and examined under a Leica DM 5000B light microscope. 0367-2530/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.flora.2010.11.018