IAWA Journal, Vol. 33 (4), 2012: 391–402 STEM ANATOMY AND DEVELOPMENT OF SUCCESSIVE CAMBIA IN THE NEOTROPICAL LIANA SECURIDACA RIVINIFOLIA (POLYGALACEAE) Kishore S. Rajput1,*, Marina B. Fiamengui2 and Carmen R. Marcati2 SUMMARY The pattern of secondary growth and structure of secondary xylem was studied in the stem of the Neotropical liana Securidaca rivinifolia A. St.-Hil. (Polygalaceae). Increase in thickness of the stem was achieved by formation of successive cambia, from which initially two or three successive rings formed complete oval to circular cambia. Thereafter, the successive cambia were always crescent-shaped and never formed a complete cylinder, resulting in dumbbell-shaped cross-sectional outlines of the stems. The first successive cambium originated in the pericyclic parenchyma located outside the crushed protophloem. Prior to the devel- opment of cambium, pericyclic parenchyma formed a meristematic band of radially arranged cells. From this band, cells located in the middle of the band became the new ring of cambium. Cells on the inner face of the xylem produced by newly formed cambium differentiated into conjunctive tissue. The first elements to be differentiated from the newly developed cambium were always xylem fibres but differentiation of ves- sels was also observed occasionally. The xylem was diffuse porous with relatively distinct growth rings and composed of mostly solitary vessels with simple perforation plates, fibres with bordered pits, paratracheal axial parenchyma, and exclusively uniseriate rays. Key words: Cambial variant, Polygalaceae, secondary phloem, secondary xylem, Securidaca, successive cambia. INTRODUCTION Climbing plants differ from trees and shrubs in a number of characteristics; the most notable is the mechanical properties of the stem (Isnard et al. 2003; Lopes et al. 2008). This variation in the mechanical properties of wood is related with a shift from the self-supporting habit to the climbing habit. The liana habit is often (but not exclusively) associated with the presence of successive cambia, interxylary phloem, dissected or compound xylem, and furrowed xylem (Rajput et al. 2012). These phenomena are often referred to as cambial variants or anomalous secondary growth (Carlquist 2001). 1) Department of Botany, Faculty of Science, The M.S. University of Baroda, Vadodara 390002, India. 2) Depto. Recursos Naturais–Ciências Florestais, Faculdade de Ciências Agronômicas (FCA), Universidade Estadual Paulista (UNESP), Campus de Botucatu, CP237, CEP 18603-970 Botucatu, SP, Brazil. *) Corresponding author [E-mail: [email protected]]. Downloaded from Brill.com09/27/2021 12:31:54PM via free access 392 IAWA Journal, Vol. 33 (4), 2012 Variant secondary growth is widespread in lianas and it is considered to increase stem flexibility, to protect the phloem, and to some extent also to increase storage paren- chyma (Carlquist 1991, 2001; Patil et al. 2011). It also limits physical disruption of vascular tissues during twisting and bending, and promotes wound healing (Dobbins & Fisher 1986; Fisher & Ewers 1992; Lopes et al. 2008). A supposedly major benefit of the variant secondary growth in lianas is the adaptation to the climbing habit from the self-supporting one. Anomalous secondary thickening is, however, by no means restricted to lianas. In Amaranthaceae (including Chenopodiaceae) and other members of the Caryophyllales successive cambia characterize virtually all, largely erect, self- supporting members of the order (cf. Heklau et al. 2012). Although woody climbers (lianas) are very common in tropical forests, anatomical studies have been rather few (Bamber & Ter Welle 1994; Lopes et al. 2008; Pace et al. 2009, 2011), and very little attention has been given to the cellular composition of xylem in scandent plants (Carlquist 1985; Araujo & Costa 2006; Rajput et al. 2010a; Pace et al. 2011). However, lianas are important subjects for the study of cambial variants, and their development (Obaton 1960; Carlquist 1991; Araujo & Costa 2006; Patil & Rajput 2008; Rajput et al. 2008, 2010a; Pace et al. 2011). The family Polygalaceae comprises herbs, scandent shrubs, shrubs or rarely trees (Xanthophyllum has some large tree species in it) and climbers/lianas (rarely) that occur in both temperate and tropical regions. Secondary xylem of herbs and shrubs in the Polygalaceae is normal, but presence of included phloem of the concentric type is recorded for the different climbing species of the Polygalaceae including Securidaca (Record & Hess 1943; Metcalfe & Chalk 1950) but the development of successive cambia remains to be studied. Characteristically, stems of this species are lobed or dumbbell-shaped as seen in cross-sectional view. The present investigation is, therefore, aimed to elucidate the pattern of secondary growth, the development and structure of xylem and phloem, and correlation of cambial activity with growth ring formation (if any) and stems shape in Securidaca rivinifolia. Materials AND METHODS Eight to ten 40–60 mm long segments of the main stems, ranging in diameter from 5–30 mm were collected from two plants of Securidaca rivinifolia A. St.-Hil. (Polygalaceae) growing in Botucatu municipality, in Rio Bonito, São Paulo state, Brazil. Samples were collected from the base (30 cm above the ground), middle (c. 2 metre above ground) and top portion about 8–10 mm thick (5–6 metre above ground) in September 2008 and June 2009. Samples were fixed immediately in FAA70 (Berlyn & Miksche 1976) and transferred to 70% alcohol after 12 hrs of fixation. The samples were cut into smaller pieces (8–10 mm2) and embedded in PEG (30%, 50%, 70%, 90% followed by 2×100% pure PEG) and processed as described by Rupp (1964), modified by Richter (1981) and adapted by Barbosa et al. (2010). Transverse, radial and tangential longitudinal sections of 12–15 µm thickness were obtained with a sliding microtome and stained with a 1% aqueous solution of safranin (Bukatsch 1972) and Astra Blue (Roeser 1972) and mounted permanently in synthetic resin Entelan®. Downloaded from Brill.com09/27/2021 12:31:54PM via free access Rajput et al. — Successive cambia of Securidaca (Polygalaceae) 393 Small pieces of secondary xylem adjacent to the outermost cambium ring were macer- ated according to Franklin’s method (Franklin 1945, modified by Kraus & Arduin 1997) at 55 to 60 °C for c. 8 hrs, and stained with an ethanolic solution of safranin (Bukatsch 1972) to study general morphology and dimensional details. Length and width of the sieve tube elements were measured directly from the tangential longitudinal sections. Thirty measurements were chosen randomly to obtain mean and standard deviation for each cell type. Wood anatomical terminology follows the IAWA Committee (1989) and Carlquist (2001). Results Development of successive cambia The first ring of the vascular cambium remained active for longer than a complete growing season to produce a pencil-thick (8–10 mm in diameter) stem. In the next growing season, when the plants were with new leaves and the apical shoot was growing actively, two small arcs of cambium originated on opposite sides of the stem (Fig. 1, 2A). The pericyclic parenchyma cells in the outer region of the protophloem became meri- stematic and formed a group of cells arranged in radial rows like cambium (Fig. 2B). From these meristematic bands, cells located in the middle of the band remained meristematic and gave rise to the small segments of the cambium. The cells on the inner face of the newly developed cambial zone formed the conjunctive tissue while cells external to this cambial zone served as site for the origin of future cambium Figure 1. Overview of Securidaca rivinifolia young stem in cross section. Encircled portion of the stem indicates the position of first successive cambial origin. — Scale bar = 200 µm. Downloaded from Brill.com09/27/2021 12:31:54PM via free access 394 IAWA Journal, Vol. 33 (4), 2012 Figure 2. Transverse view of young and mature stem of Securidaca rivinifolia showing different stages of cambial origin. – A: Enlarged view of Fig. 1 showing newly originated first successive cambium and few xylem cells. Note the parenchymatous derivatives of pericycle on the upper side of newly developing cambium (arrowheads). X = xylem. – B: Enlarged view of newly develop- ing cambium. Note the wide band of parenchyma cells that develops into cambium and conjunc- tive tissue (arrow). Arrowhead showing newly differentiated phloem element. – C: Enlarged view of newly developing cambium. Arrowheads indicate newly formed sieve tubes. – D: Parenchyma cells located externally to the phloem formed by previous cambium (arrowhead). Parenchyma cells that undergo dedifferentiation and give rise to a new ring of successive cambium. – E: Tan- gential spreading of the newly developed cambium. Note the differentiated fibres on the xylem side (arrowhead). – Scale bars for A = 125 µm; for B–E = 75 µm. Downloaded from Brill.com09/27/2021 12:31:54PM via free access Rajput et al. — Successive cambia of Securidaca (Polygalaceae) 395 (Fig. 2C, E). Inception of the second ring of cambium began as small segments, which gradually extended tangentially and formed crescent-shaped arcs. These arcs ultimately met each other to form a complete cylinder. Functionally this cambium was bidirectional and formed secondary xylem centripetally and secondary phloem including tangential bands of phloem fibres centrifugally. These fibre bands were interrupted at regular intervals by 2–3 parenchyma cells (Fig. 1, 2D, E). The second