FORMATION of SUCCESSIVE CAMBIA in the MENISPERMUM TREE COCCULUS LAURIFOLIUS (MENISPERMACEAE) Kishore S. Rajput1,* and Sangeeta
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400 IAWAIAWA Journal Journal 36 (4), 36 2015: (4), 2015 400–408 FORMATION OF SUCCESSIVE CAMBIA IN THE MENISPERMUM TREE COCCULUS LAURIFOLIUS (MENISPERMACEAE) Kishore S. Rajput1,* and Sangeeta Gupta2 1Department of Botany, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390 002, India 2Wood Anatomy Discipline, Forest Research Institute, Dehradun 248006, India *Corresponding author; e-mail: [email protected] ABSTRACT Successive cambia are often associated with the climbing or shrub habit, and is less common in trees. We studied formation of successive cambia and structure of secondary xylem in young stems of Cocculus laurifolius DC., a tree species of Menispermaceae. Cell division in the vascular cambium ceased in pencil-thick stems. Subsequently, parenchyma cells located outside the perivascular fibre cap re-differentiated and gave rise to several small segments of meristematic cells, of which the central cells divided repeatedly to initiate the first successive cam- bium which produces secondary xylem centripetally and phloem centrifugally. Cells located on the inner side of the newly initiated cambium differentiated into conjunctive tissue while cells on the outer side of it divided further and dif- ferentiated into sclereids. Xylem was diffuse porous and composed of vessels, fibre tracheids and ray parenchyma cells, and only differed in vessel diameter from wide-vessel climbing relatives. Keywords: Cambial variant, multiple cambia, secondary phloem, xylem, tree habit. INTRODUCTION Menispermaceae comprise 71 genera and approximately 520 species (Jacques & De Franceschi 2007). Most of them achieve secondary growth by forming successive cambia (Carlquist 2007; Ortiz et al. 2007; Jacques & De Franceschi 2007) while few show normal secondary growth (Tamaio et al. 2010). It is a cosmopolitan family of mainly vines or lianas whereas trees, shrubs or self-supporting herbs are rare (Ortiz et al. 2007). Stem anatomy of the Menispermaceae has been studied extensively in the past (Schenck 1893; Metcalfe & Chalk 1950; Mennega 1982; Carlquist 1996; Rajput & Rao 2003; Jacques & De Franceschi 2007; Tamaio et al. 2009, 2010). Successive cambia have been interpreted as adaptive to the climbing habit (Fisher & Ewers 1991; Carlquist 2001; Patil et al. 2011; Rajput et al. 2012), but have also been reported in a few tree species such as Avicennia marina, Dalbergia paniculata, Gallesia integrifolia, Phyto- lacca dioica, Salvadora persica (Studholme & Philipson 1966; Wheat 1977; Kirchoff & Fahn 1984; Schmitz et al. 2008; Longui et al. 2011; Robert et al. 2011; Rajput et al. 2012). However, in erect shrubs successive cambia can be of common occurrence, © International Association of Wood Anatomists, 2015 DOI 10.1163/22941932-20150110 Published by Koninklijke Brill NV, Leiden Downloaded from Brill.com10/05/2021 07:09:00AM via free access Rajput & Gupta – Successive cambia in Cocculus 401 e.g. in Amaranthaceae (including the former Chenopodiaceae, cf. Heklau et al. 2012), Combretaceae (Van Vliet 1979) and several other woody families. Successive cambia in different members of the Menispermaceae have been reported to originate from four different types of tissue, viz. cortical parenchyma, the endoder- mis, the pericycle, and from irregular activity of the vascular cambium itself (Maheu 1902; Jacques & De Franceschi 2007; Tamaio et al. 2009). When studying the wood anatomy of the Menispermaceae, Jacques and De Franceschi (2007) reported that formation of successive cambia in Cocculus laurifolius does not fit into any one of the four origins proposed by Maheu (1902). Therefore, they suggested that other origins for those successive cambia are possible and that careful developmental studies are needed to clarify their precise origins. The present study, therefore, investigates i) the precise origin of successive cambia in C. laurifolius, and ii) any possible differences between the xylem of this self-support- ing species and climbing species of the same family. MATERIALS AND METHODS Young stems (3–10 mm thick) of Cocculus laurifolius (Menispermaceae) were col- lected from three plants growing in the Tropical Botanical Garden Research Institute (TBGRI), Thiruvananthapuram (India). They were fixed in FAA (Berlyn & Miksche 1976) and transferred in 70% alcohol after 12 hrs for further storage and processing. After suitable trimming into smaller pieces (3–4 mm), they were dehydrated through tertiary butyl alcohol (TBA) and processed by routine paraffin embedding (Johansen 1940). Transverse, radial and tangential longitudinal sections of 12–15 µm thickness were cut with the Leica rotary microtome and stained with safranin-fast green combi- nation (Johansen 1940). Subsequently, slides were dehydrated through ethanol xylene series and embedded in DPX. To study the structure of secondary xylem, 15–20 µm thick sections were prepared from wood blocks deposited in the xylarium of the For- est Research Institute (FRI), Dehra Dun, India. These wood blocks were collected from 8-year-old C. laurifolius growing at the Botanical Garden of the Forest Research Institute (Uttarakhand State, Acc. No. DDw 4497, 4643). Vessel lumen diameter and vessel frequency was obtained from transverse sections while dimensional details of ray height, ray width and ray cell diameter was measured from the tangential longitudinal sections. Measurements (50 per feature) were carried out only from the slides prepared from mature stems (xylarium samples) while fresh samples were used only to study the origin of successive cambia. Values in parentheses indicate standard deviation. Important results were micro-photographed with the Leica DME 2000 trinocular research microscope. Wood descriptions follow the IAWA Com- mittee (1989) and Carlquist (2001). RESULTS Structure of the young stem – In the young stem, the epidermis is composed of thin-walled oval to polygonal cells of varying sizes and covered with a thick cuticle (Fig. 1A, B). The hypodermis is 1–2-layered and poorly differentiated. The cortex is Downloaded from Brill.com10/05/2021 07:09:00AM via free access 402 IAWA Journal 36 (4), 2015 4–6 cells wide and composed of thin-walled parenchyma cells. The endodermis is indistinct; the pericycle is composed of dome-shaped pericyclic fibre caps opposite each of the vascular bundles (Fig. 1B), while 3–4 layers of parenchyma cells are present between the protophloem and the fibre caps (Fig. B1 ). As growth progresses, one to two Figure 1. Transverse view of young stem of Cocculus laurifolius showing initiation of succes- sive cambium. – A: Young stem showing first normal ring of vascular cambium in the early stage of secondary growth. PD = pericyclic derivatives, P = pith. – B: Enlarged view of young stem. Arrowhead indicates pericyclic fibre cap. PD = pericyclic derivatives, C = cortex. – C: Initiation of first successive ring of cambium. Arrow shows one of the vascular bundles formed by the newly initiated successive cambium while arrowhead indicates pericyclic fibre cap. Note the marginal pith cells that differentiate into sclerenchyma (small arrow). P = pith. – D: Enlarged view of young stem showing initiation of first successive ring of cambium (arrow- heads). Arrow shows sclereids. PFC = pericyclic fibre cap, P = pericycle cells. — Scale bar for A, B, D = 100 µm; for C = 200 µm. Downloaded from Brill.com10/05/2021 07:09:00AM via free access Rajput & Gupta – Successive cambia in Cocculus 403 Figure 2. Transverse view of young stem of Cocculus laurifolius showing initiation of successive cambium (A–C) and structure of mature secondary xylem (D). – See full legend on next page. Downloaded from Brill.com10/05/2021 07:09:00AM via free access 404 IAWA Journal 36 (4), 2015 cell layers remain thin-walled while the rest of the derivative cells differentiate into fibres (Fig. 1C, D). The pericyclic fibre caps are interconnected to form a continuous, grooved cylinder (Fig. 1A, B). On the inner side of the pericyclic fibres, medullary rays separate a ring of 11–13 conjoint collateral vascular bundles. The pith is composed of thin-walled parenchyma cells (Fig. 1A) in the early stages of secondary growth but the cells become thick- walled and sclerenchymatous (Fig. 1C) in later stages of development. As the secondary growth progresses, the pericyclic bundle caps become disconnected at the position of the grooves (Fig. 1C). Origin of successive cambia – As the stem reaches a diameter of 4–5 mm, the vascular cambium ceases to divide and a first ring of successive cambium is initiated from the cortical parenchyma cells situated outside to the perivascular fibre cap (Fig. C1 , D). These parenchyma cells divide repeatedly after de-differentiation and form 4–6 cell layers of radially arranged cells (Fig. 2A). Cells situated on the inner side differenti- ate into conjunctive tissue while cells on the outer side serve as site for initiation of further successive cambia. The cells located in the middle of these layers give rise to the first successive cambium (Fig. 2B). From the newly originated cambium, several small alternate segments of it begin to differentiate into xylem fibres internally and phloem elements externally (Fig. 2C) while cells from the rest of the alternate segments of the cambium undergo radial enlargement and differentiate into rays (Fig. 1D, 2C). Formation of further successive cambia follows a similar pattern. Structure of secondary xylem – Thick stems of C. laurifolius are composed of thick concentric rings of secondary xylem alternating with the