Lianas usually have two stages of development, the autologous support stage and the climbing stage (Schnitzer and Bongers 2002). Internally, the autologous support stage is characterized by a few narrow vessels and many thick fibers. The climbing stage involves secondary growth of the stem, which can be a standard or anomalous (Isnard and Silk 2009). Morphological variation in liana stems is primarily associated with the geometry of the phloem and xylem, and irregular shapes within the stems are classified into cambial variants (Angyalossy et al. 2015). The diversity of liana stem shapes, structures, and cambial variants has been recently reviewed (Angyalossy et al. 2012), including those of Piperaceae.
Members of Piperaceae represent about five genera and 3700 species around the world (Christenhusz and Byng 2016), and many are economically important because of their medicinal and culinary uses. They are erect or scandent shrubs, small trees, or succulent, terrestrial herbs, with nodose stems. The leaves are entire, alternate to opposite or verticillate, petiolate or infrequently subsessile, palmately nerved or penninerved, pellucid dotted, and sometime aromatic (Souza et al. 2004). The genus Piper L. is represented by about 1050 species distributed primarily in the tropics (Mabberley 2008). The most outstanding anatomical character in the Piperaceae is the nature of the vascular bundles in the stem. In all species of Piperaceae studied to date in the genera Piper, Manekia Trel., and Zippelia Blume, vascular bundles are organized in two or more concentric rings, a characteristic that is not present in other genera in Piperaceae (Trueba et al. 2015).
Several studies have characterized cambial variants in certain Piper species. Piper obtusilimbum C. DC. has a typical arrangement of tissues (Tepe et al. 2007), with a parenchymatous pith, medullary vascular bundles, a sclerenchymatous cylinder, peripheral vascular bundles, and a vascular cambium. The stem of Piper betle L. had an inner irregular circle of primary vascular bundles interspersed with a large mucilage canal in the pith, more mucilage canals in the inner cortex, an undulating wall of sclerenchyma, and an outer ring of smaller cortical bundles. The primary vascular (medullary) bundles occupied the pith (Beck 2011). Raman et al. (2012) showed that the stem of P. betle has a ring of mucilage canals located between cortical and medullary rings of primary vascular bundles, a central mucilage canal in the pith, secretory cells in the cortex, cortical fibers, and an endodermis with a Casparian strip. The descriptions of P. betle here do not match each other owing to the age of the stem. Beck (2011) showed that the stem of P. excelsum G. Forst. had two medullary bundles that occupied the very center of the pith. The species had two cylinders of vascular bundles. The central, irregularly grouped primary vascular bundles were enclosed by a thick cylinder of secondary xylem capped by phloem that is separated by wider medullary rays of primary parenchyma.
Raman et al. (2012) studied P. sarmentosum Roxb. The stem of Piper sarmentosum has a large central mucilage canal, medullary vascular bundles surrounded by parenchyma in the pith, a ring of cortical vascular bundles, and a discontinuous ring of collenchyma in the outer cortex. Saraswathy et al. (2013) indicated that the stem of P. retrofractum Vahl. was circular in cross section, with a large central mucilage canal surrounded by individual bundles scattered in a parenchymatous cortex, which was encircled by a wavy ring of sclerenchyma followed by a ring of vascular bundles and medullary rays. A pericycle and collenchyma formed the outer cortex. In a broader study of woodiness in Piperales and Trueba et al. (2015) presented images of transverse sections of P. gorgonillense Trel. and Yunck. and P. nudibracteatum C. DC. Piper gorgonillense produced wide, lignified rays, with solitary or clustered vessels, but the center and edge of the stem were not included. A growth ring was visible in the rays and vascular tissue. The peripheral vascular bundles in Piper nudibracteatum gradually elongated through secondary growth, exceeding the medullary vascular bundles, while the growth of the medullary bundles was minimal, and wide rays were formed by primary parenchyma cells. Santos et al. (2015) showed that in P. amalago L., a large parenchymatous pith is surrounded by an inner circle of approximately nine medullary bundles, a ring of sclerenchymatous fibers, and an outer circle of approximately 30 vascular bundles, some of which have bundles of fiber capping the adjoining phloem.
According to these studies and others, Piper typically has an inner series of vascular bundles separated from an outer series by a sclerenchymatous ring. This pattern has been observed in P. nigrum L. and P. colubrinum Link (Ravindran and Remarshree 1998); P. diospyrifolium Kunth (Souza et al. 2004); P. amalago, P. betle, Piper excelsum, Piper gorgonillense, P. nudibracteatum, Piper obtusilimbum, P. retrofractum, P. sarmentosum, P. regnelli (Miq.) C. DC. (Pessini et al. 2003); P. gaudichaudianum Kunth (Albiero et al. 2005b); P. crassinervium Kunth (Albiero et al. 2005a); P. hispidum Sw. (Albiero et al. 2006); P. arboretum (Souza et al. 2009); and P. mikanianum (Kunth) Steud. (Duarte and Siebenrock 2010).
Taiwan is located in the subtropical zone, with a warm and humid climate. Piperaceae are represented in Taiwan by two genera, Peperomia Ruiz and Pavon and Piper. Eight scandent species and two herbs are recorded in the genus Piper (Lin and Lu 1996; Hsu and Chung 2016). However, data regarding the internal stem anatomy of seven of the climbing species are lacking. Cambial variations are diverse within the family, so the present study attempts to (1) provide detailed photographs of features discussed, (2) specify the developmental stages observed, and (3) provide a key based on anatomical characters of transverse sections to facilitate the identification of irregular cambial activity in lianescent species of the genus Piper in Taiwan.