IAWA Bulletin n.s., Vol. 9 (4),1988: 332-336 XYLEM ANATOMY OF DIEGODENDRON HUMBERTll by William C. Dickison Department of Biology, University of North Carolina, Chapel Hill, N.C. 27599, U.S.A. Summary The first description of the wood of the the family in the order Ochnales, along with monotypic Madagascan genus Diegodendron the Ochnaceae, Lophiraceae, Sauvagesiaceae, is provided. The xylem of D. humbertii is Strasburgeriaceae, Quiinaceae and Scytopeta­ characterised by short, solitary vessel ele­ laceae. Goldberg (1986) classified the Die­ ments with alternate lateral wall pitting and godendraceae between the Strasburgeriaceae simple perforation plates, imperforate trache­ and Sphaerosepalaceae in the Theales. Cron­ ary elements of the libriform fibre type, near­ quist (1981) preferred to merge the genus ly all biseriate, imperfectly storied, homogen­ within a broad concept of the Ochnaceae (also eous rays composed of procumbent cells including the Lophiraceae, Luxemburgiaceae, only, and diffuse and diffuse-in-aggregates Strasburgeriaceae, Sauvagesiaceae, and Wal­ axial parenchyma. The specialised wood anat­ laceaceae). Straka and Albers (1978) con­ omy of Diegodendron supports a close alli­ cluded from a study of pollen morphology ance with both Sphaerosepalaceae and Mal­ that a position near Ochnaceae cannot be vales. negated. Thorne (1983) believed that avail­ Key words : Diegodendron, wood anatomy, able evidence suggested treatment as a mono­ Sphaerosepalaceae, Malvales. typic subfamily Diegodendroideae in Sphae­ rosepalaceae. In commenting on the genus, Introduction Airy Shaw (in Willis 1973) remarked, "Pro­ The genus Diegodendron was initially de­ bably related to Sphaerosepalaceae, differing scribed and recognised as a distinct family from them in the glandular-punctate leaves, by Capuron in 1963, and is represented by a pentamerous flowers, and outwardfacing mi­ single species (D. humbertii R. Capuron). cropyle of the ovules . The conspicuous de­ The monospecific Diegodendraceae are con­ velopment of the disk in Rhopalocarpus con­ fined to Madagascar, where plants are either stitutes another part of difference. The aspect shrubs or small trees, having simple, alter­ of the twigs, leaves, stipules , and stipular nate leaves and intrapetiolar stipules that sur­ scars is strongly reminiscent of Irvingia and round the terminal bud. Bisexual flowers are its immediate allies." borne in terminal panicles and have numerous This study was undertaken in order to stamens, a 2- or 3-carpellate gynoecium with provide another approach toward assessing a gynobasic style, and two ascending ovules the relationships of the genus. in each locule. As described by Capuron (1965), the fruit is composed of 1-3 dry, Materials and Methods coriaceous mericarps. Although the genus is The specimen described here represents a widely regarded as having general ochnalean piece of mature wood of Diegodendron hum­ or thealean affinities, little has been written bertii R. Capuron obtained from the Centre about the taxon and the morphology and ana­ Technique Forestier Tropical (CTFT 14338) tomy is almost totally unknown. The xylem with accompany ing herbarium voucher SF has not been described. 18964. This specimen was collected in the Hutchinson (1973) placed the Diegoden­ Arkarana forest in the vicinity of Diego Sua­ draceae in the Ochnales, near the Strasbur­ rez, Madagascar. The sample was boiled in geriaceae . Takhtajan (1987) also positioned water and cut on a sledge microtome at a Downloaded from Brill.com10/02/2021 09:41:47AM via free access Dickison - Xylem anatomy of Diegodendron humbertii 333 Figs. 1-3. Wood anatomy of Diegodendron humbertii. - 1: TS showing solitary pore distribu­ tion and weakly defined growth ring, x 125. - 2: TS showing diffuse and diffuse-in-aggregates axial parenchyma, x 230. - 3: TLS illustrating imperfectly storied rays, x 125. Downloaded from Brill.com10/02/2021 09:41:47AM via free access 334 IAWA Bulletin n.s., Vol. 9 (4), 1988 thickness of 20 Jim. Resulting sections were of biseriate rays is 13 cells. Ray parenchyma stained wtih saffranin. Data relating to wood is filled with dark-staining deposits . Solitary cell length were obtained by making 50 mea­ prismatic crystals are frequently present in surements from macerations prepared using ray cells. Jeffrey 's fluid. Cell diameters were measured from transverse sections and include walls. Discussion There is wide concensus, based upon Generic wood anatomical description available evidence, that Diegodendron has Wood hard, very dense. Growth rings are theoid affinities, although the question of weakly differentiated by a narrow zone of whether the genus deserves familial status, closely spaced, uniseriate tangential lines of and if not, to which existing family it should axial parenchyma at the boundary of a growth be assigned, is unsettled. Wood anatomy of­ increment (Fig. 1). Vessels are diffuse and fers additional points of comparison. exclu sively solitary, often filled with solid, The wood of Diegodendron, when com­ amorphous deposits (Fig. 1). Average pore pared with thealean families, is seen as struc­ density is 86 per sq.mm. Pores are rounded turally rather specialised. The combination of in outline and tangential pore diameters are short, nearly all biseriate, homogeneous and mostly very small, ranging from 17-45 Jim, imperfectly storied rays, libriforrn fibres, and with an average of 35 Jim. Mean vessel wall mostly short vessel elements with alternate thickness is about 3 Jim. Vessel elements are intervascular pitting and simple perforation mostly short, ranging from 192-293 Jim in plates represent a suite of advanced characters length, with a mean of 237 Jim. Perforations that are exceptional for the Theales. As noted are exclusively simple in slightly inclined to by Carlquist (1988), however, differences in transverse end walls. perforation plate type among theoid taxa may Intervascular pitting is noncrowded and not be significant in assessing relationsh ips, predominantly alternate. Lateral wall pits are as this feature is undoubtedly related to the less than 3 Jimin diameter. Vessel-ray pitting physiology of conductance. By way of com­ is the same. Ground tissue is composed of parison, the monotypic family Strasburgeria­ thick-walled to very thick-walled imperforate ceae from New Caledonia are distinguished tracheary elements with minute pits that are by a decidedly less advanced xylem structure. either simple or provided with inconspicuous Strasburgeria possesses mostly solitary, long borders. Elements are, for all practical and angular vessel elements with a high num­ purposes, of the libriforrn fibre type. Pitting ber of scalariforrn bars. Intervascular pits are is present on both radial and tangential walls. opposite. Ground tissue is compo sed of long Fibres are short to medium-sized, ranging in fibre-tracheids with prominent bordered pits. length from 525-1292 Jim, with an average Ray tissue is distinctly heterogeneous with of 975 Jim. Mean wall thickness is 4.5 Jim. both uniseriate and multiseriate rays present. Axial parenchyma is apotracheal diffuse and Crystals are absent (Dickison 1981). diffuse-in-aggregates, occurring as isolated As broadly circumscribed, the Ochnaceae elements and in short to long, discontinuous are wood anatomically rather diverse, being or continuous, irregularly spaced, wavy ag­ characterised by extremely small to rather gregations from ray to ray (Fig. 2). Paren­ large pores that are solitary, or distributed in chyma lines sometimes interconnect as seen pore clusters, or radial, or tangential pore in cross section and contact the pores. Axial multiples. Pores range from angular to cir­ parenchyma cells are filled with dark staining cular in transectional outline and individual deposits . Rays are nearly all biseriates, with vessel elements range in length between very exclusively uniseriate rays scarce. Rays are short and extremely long. Perforation plates homogeneous and imperfectly storied as seen are simple in most genera, however, both on the tangential surface (Fig. 3). Rays are simple and scalariforrn plates occur in some composed of moderately thick-walled cells taxa. Intervascular pitting is alternate. Imper­ that are exclusively procumbent. Elongate forate tracheary elements are fibre-tracheids, marginal extensions are absent. Mean height often septate. Fibres range in length between Downloaded from Brill.com10/02/2021 09:41:47AM via free access Dickison - Xylem anatomy of Diegodendron humbertii 335 very short and long. Rays are heterogeneous, dendron, and the fact that rhombohedral of either the I or IIA types of Kribs (1935), crystals are present in the axial parenchyma or, infrequently homogeneous. Crystals are of Rhopalocarpus whereas crystals are re­ often present in ray parenchyma. Axial paren­ stricted to the ray parenchyma in Diegoden­ chyma varies from vasicentric and diffuse, dron. diffuse-in-aggregates, or reticulate, some­ The data from wood anatomy lend addi­ times metatraeheal in distribution (Metcalfe & tional support to the interpretation that Diego­ Chalk 1950; Decker 1966). Although many dendron (and Sphaerosepalaceae) are closely xylem features of Diegodendron fall within related and perhaps transitional to the Malva­ the range of variation of Ochnaceae, as a les (Cronquist 1981). The presence of storied member of the family the genus is structurally rays is an unknown feature within the Thea­ somewhat discordant. Homogeneous rays are les, whereas this character state is found in