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IAWA Journal, Vol. 29 (2), 2008: 143–152 VARIABILITY IN APICAL ELONGATION OF WOOD FIBRES IN LONCHOCARPUS SERICEUS Joanna Jura-Morawiec1*, Wiesław Włoch2,1, Paweł Kojs2,1 and Muhammad Iqbal3 SUMMARY Morphological variability in wood fibres ofLonchocarpus sericeus (Poir.) Kunth ex DC. (Leguminosae), a tropical hardwood tree with double- storeyed cambium, was examined in thin tangential and transverse sec- tions as well as in macerations of wood tissue. Occurrence of character- istic protrusions (lateral expansions) was detected on the extended part of the main fibre body. Distance between the adjacent protrusions cor- responded to the height of a storey (horizontal tier) of the cambial initials. Rays were shorter in height than the neighbouring fusiform initials and therefore unable to reach the boundary of the storey. This situation facili- tated the lateral expansion of the adjoining fibres during their apical elon- gation by intrusive growth. The presence of the characteristic protrusions on the fibre body thus indicated that the given fibre was associated with a double-storeyed cambium having rays shorter than the length of fusiform initials. The ultimate shape of fibres was thus correlated to the height of storeys and the height and width of rays. Key words: Wood fibre morphology, intrusive cell growth, vascular cam- bium. INTRODUCTION Lonchocarpus sericeus is a leguminous tree, native to Africa and Southern America. Wood anatomy of the genus Lonchocarpus has been described by Metcalfe and Chalk (1950), Wagenführ and Schreiber (1974), Chudnoff (1984), Richter and Dallwitz (2000) and Gasson et al. (2004). Lonchocarpus sericeus possesses diffuse-porous xylem with paratracheal confluent parenchyma. In transverse sections, bands of light-stained axial parenchyma surrounding the vessels alternate with bands of dark-stained fibres. Like many other tree species growing in the canopy and emergent layers of the tropical rainforests, this species is characterized by a double-storeyed arrangement of cambium, where both fusiform cells and rays are arranged in storeyed fashion. The pattern of cambial cell arrangement is also reflected in the arrangement of wood elements (Harris 1) Botanical Garden, Centre for Biological Diversity Conservation of the Polish Academy of Sciences, Prawdziwka 2, 02-973 Warsaw 76, Poland. 2) Department of Biosystematics, University of Opole, Oleska 22, 40-052 Opole, Poland. 3) Department of Botany, Jamia Hamdard (Hamdard University), New Delhi, 110 062, India. *) Corresponding author [E-mail: [email protected]]. Associate Editor: Nigel Chaffey Downloaded from Brill.com09/24/2021 04:43:43PM via free access 144 IAWA Journal, Vol. 29 (2), 2008 Jura-Morawiec et al. — Apical fibre elongation 145 1989; Romberger et al. 1993; Carlquist 2001). In the wood of L. sericeus, which consists of vessel members, fibres and axial parenchyma cells in addition to rays, fibres are the only non-storeyed axial elements. Information on the shape of xylem fibres is scanty. Fibres are known to grow by apical intrusive growth (Fahn 1990; Larson 1994; Evert 2006). The rate and duration of cell expansion and secondary wall formation affect the overall cell size and wall thickness of xylem elements (Ridoutt & Sands 1993, 1994). Final dimensions of wood fibres are also affected by the state of maturation of other xylem elements in the vicinity (Honjo et al. 2006). The arrangement and rearrangement of cambial cells influence the pattern of fibre organization in the grain of the wood (Harris 1973, 1989; Hejnowicz & Zagórska-Marek 1974; Włoch et al. 2002; Kojs et al. 2004). In some species, like Pterocarpus soyauxii Taub., Ceiba pentandra Gaertn., Dalbergia nigra Fr.All., even the fibres reflect the storeyed arrangement of their precursor cambial cells, thus forming a perfect storeyed structure in the wood (Metcalfe & Chalk 1950; Wagenführ & Schreiber 1974; Richter & Dallwitz 2000; Carlquist 2001). However, no information is available in the literature to determine whether the arrangement of cambial cells also influences the shape of the fibres. The present report on the wood fibres of L. sericeus demonstrates their peculiar shapes and examines if the structure of the cambium has a bearing on the shape of wood fibres in this species. MATERIALS AND METHODS Sampling and slide preparation The material used in this study, i.e., a piece of wood of Lonchocarpus sericeus (Poir.) Kunth ex DC., was obtained from the collection of wood samples in the Botanical Gar- den, Centre for Biological Diversity Conservation of the Polish Academy of Sciences, Warsaw. Microscopic study of wood fibres was made both from sections and macerations. Small samples of wood (2 × 0.7 × 0.5 mm), deaerated in boiling water and then fixed in 2.5 % glutaraldehyde, were dehydrated in an acetone series and embedded in Epon (Meek 1976). The embedded samples were cut into 3-μm transverse and tangential sec- tions, using a glass knife and microtome, glued to the slides with Hauptʼs adhesive, stained with PAS and toluidine blue and mounted in Euparal, as described by Włoch and Połap (1994). The samples were examined by light microscope. For maceration, a piece of wood (approximately 10 × 5 × 4 cm) was cut into a few tangential slices of less than 0.5 mm thickness. The slices, taken in test tubes containing the Franklin (1945) mixture (1:1, glacial acetic acid : 30% hydrogen peroxide), were placed in boiling water for a few hours for macerating the tissue. The macerated wood elements were separated by shaking the solution vigorously. The solution was then centrifuged and the liquid decanted. The remaining pellet of macerated wood elements was washed with water, centrifuged again, dehydrated in ethylene and placed in isopropanol. Drops of the resultant suspension were placed on slides, stained with safranin T and mounted in Euparal, as described by Włoch (1976). Downloaded from Brill.com09/24/2021 04:43:43PM via free access 144 IAWA Journal, Vol. 29 (2), 2008 Jura-Morawiec et al. — Apical fibre elongation 145 Measurement of wood elements The length of axial parenchyma cells, height of a storey, and height and width of rays in parenchyma bands were measured from tangential longitudinal sections using the calibrated ocular micrometre scale in the eyepiece of a light microscope. Macerated material was used for measuring fibre length, distance between two adjacent protrusions of fibres, and width of fibre protrusions of various orders. The first order protrusions were those formed at the ends of the main fibre body, around the boundaries of storeys. During apical growth of fibres the second and third order protrusions could be formed if the growing fibre tip reached the next boundaries of the storeys. The length of the fusiform axial parenchyma cells has been used as an indicator of the length of fusiform initials (Süss 1967). For calculating mean fibre length, 200 unbroken and randomly selected fibres were measured. For the rest of the parameters the mean is based on 50 measurements each. For measuring the mean height of a storey, microphotographs of tangential sections were obtained with a microscope. Boundaries of the storeys were marked by horizontal lines in randomly selected areas of parenchyma bands on these photographs and the distance between the lines was measured. OBSERVATIONS The storeyed structure in the wood of Lonchocarpus sericeus, as seen in TLS, involved axial parenchyma cells, rays and vessel members (Fig. 1a). The heights of vessel members, ray bodies and 2–4-celled strands of axial parenchyma were mutually comparable (Fig. 1b–e). Rays were enclosed within the horizontal storeys (Fig. 1a, c). Nonetheless, fibres did not form a storeyed structure. Comparison of sections passing through a parenchyma band (Fig. 2a) and a fibre band (Fig. 2b), revealed that the fibre lumen was bigger at places near the boundaries of the storeys (Fig. 2b, 3a). Normally the tips of rays, as seen in tangential view, were not extended exactly up to the boundary of the storey it belonged to; this became evident also from transverse sections obtained from areas where storeys ended (Fig. 3c, 4b). In transverse sections, ray cells were not visible at the boundary line of the cell storeys (3c); only fibres with a distinctly large lumen diameter were present. In these fibres with larger lumen, pits were visible (Fig. 3c). On the contrary, at the mid-height of storeys, the fibres between rays were distinctly thinner with smaller lumen (Fig. 2b, 3b, 4a). It was interesting to confirm whether the characteristic differences in fibre-lumen diameter, as visible in thin sections, were detectable also in the macerated wood. It was found that the middle part of a macerated fibre cell, corresponding to the length of the cambial fusiform initial from which the given fibre had been derived, formed the main fibre body; this portion of the fibre was usually wider than the rest of the fibre body which was an outcome of subsequent cell elongation by apical intrusive growth (Fig. 5). Apical elongation of the main body of a fibre seemed to have completed after a prolonged course of intrusive growth. It was usual to find more than two protrusions (lateral expansions) on a fibre. The mean distance between two adjacent protrusions, i.e. 182 μm, was comparable with the mean height of a storey (Table 1). The first order Downloaded from Brill.com09/24/2021 04:43:43PM via free access 146 IAWA Journal, Vol. 29 (2), 2008 Jura-Morawiec et al. — Apical fibre elongation 147 Figure 1. Paratracheal parenchyma of Lonchocarpus sericeus wood. – a: Tangential section with visible storeyed structure. – b–e: Macerated cells of a storey; b: fusiform cell and 2-celled paren- chyma strands; c: axial parenchyma cells and rays enclosed within the storey as seen in the radial plane; d: macerated broad vessel member; e: narrow vessel member. — p = axial parenchyma cells; r = rays enclosed within the storey; v = vessel members.— Scale bar = 100 μm.