IAWA Bulletin n.s., Vol. 1 (3), 1980 87 WOOD ANATOMY OF THORNEA, INCLUDING SOME COMPARISONS WITH OTHER HYPERICACEAE by Arthur C. Gibson Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona 85721, U. S. A. Summary The wood anatomy of the new genus Thor­ num virginicum (Holm, 1903) lacked descrip­ nea (Hypericaceae) is described and compared tions of secondary tissues. Nevertheless, with that of other presumed relatives in the sections of hypericaceous stems are available in family. Woods of Thornea are very similar to wood collections, and the goal of this study those of Hypericum and Triadenum but quite was to describe the woods of Thornea and to dissimilar to those of Cratoxylum, with which make comparisons with its putative relatives. Thornea presumably shares common ancestry. Materials and Methods Introduction Wood specimens from mature plants of Breedlove and McClintock (1976) proposed Thornea calcicola (StandI. & Steyerm.) Breedl. the genus Thornea to include two evergreen & McClint. 7 cm and 4 cm in diameter were fruticose species from Chiapas, Mexico and collected in Chiapas and air dried by Dr. adjacent Guatemala that were originally de­ Dennis Breedlove (36973, 37070, OS). These scribed as species of Hypericum. Thornea has specimens were also those used in the generic nine fertile stamens grouped in three fasci­ description. In addition, Breedlove provided cles of three each, three persistent and free small stems of T. matudae (Lundell) Breedl. & styles, tricarpellate ovaries developing into McClint. liquid-preserved in formalin-acetic three nearly separate dehiscent capsules, and acid-alcohol (Breedlove 40408, OS). Wood pink and white petals with parallel stria­ samples of T. calcicola have been deposited in tions, features characteristic of Triadenum. the Madison Wood Collection (MADw). On the other hand, Triadenum are herbaceous Air-dried blocks were sectioned after re­ perennials of wet temperate habitats, quite hydrating, on a sliding microtome and stained unlike the habitus of Thornea. with safranin. Macerations were prepared by Based on the studies by Robson (1956, Jeffrey's macerating fluid and used to ob­ 1972), Wood and Adams (1976) and Breedlove tain 50 measurements of length for elements, and McClintock (1976) suggested that Triade­ whereas other features were obtained from num and Thornea, respectively, are closely sections. Young stems of T. matudae were em­ related to Cratoxylum of tribe Cratoxyleae, bedded in paraffin, sectioned at 15 J.Lm, and subfamily Hypericoideae, having an indepen­ stained with safranin and fast green. These dent origin from Hypericum, which is external­ microslides were examined but not used in the ly similar to Triadenum. Robson (1977) generic diagnosis for older woods. Nonethe­ elaborated on the close relationships of Triade­ less, woods of the two species are qualita­ num and Thornea to Cratoxylum and Eliaea tively very similar. and not to Hypericum, and he defined three Intergeneric comparisons with Hypericoi­ tribes, Hypericeae, Vismieae, and Cratoxyleae, deae were made using wood microslides borrow­ in subfamily Hypericoideae to recognize those ed from the Harvard Wood Collection (from differences in flower colour, staminal composi­ Vestal, 1937) and Rancho Santa Ana Botanic tion, and fruit and seed features. Garden, Claremont, California. Materials exam­ Vegetative anatomy of Hypericoideae is very ined include species of Harungana, Psorosper­ poorly known. Useful surveys on woods of mum, and Vismia of tribe Vismieae; Triadenum Hypericaceae (including Guttiferae and Hyperi­ virginicum and numerous species of Cratoxy­ caceae s.s.) are in Vestal (1937), Record and lum of Cratoxyleae; and over 100 species of Hess (1943), and Metcalfe and Chalk (1950), Hypericum, including the segregate genus and Baas (1970) has provided illustrated Ascyrum (Adams & Robson, 1961), especially descriptions of woods in Cratoxylum and the least derived and most woody sections of Eliaea. The only anatomical study on Triade- the genus (Robson, 1977). Downloaded from Brill.com09/24/2021 10:24:58PM via free access 88 IAWA Bulletin n.s., Vol. 1 (3), 1980 Fig. 1-4. Thornea calcicola. - I: Transverse section, showing diffuse porous wood with faint growth rings; x 50. - 2: Tangential section, in which uniseriate and multiseriate rays can be observed; x 50. - 3: Transverse section; tissue is composed of vessels, libriform fibres, and rays but not axial paren­ chyma; x 540. - 4: Tangential section, in which vessel-element length can be observed; x 540. -­ Fig. 5-6. Triadenum virginicum (Hw 5649). - 5: Transverse section at same magnification as Fig. 3, showing general similarities in these two woods. - 6: Tangential section as in Fig. 4; x 540. Downloaded from Brill.com09/24/2021 10:24:58PM via free access IAWA Bulletin n.s., Vol. 1(3),1980 89 Observations Axial parenchyma absent. Rays exclusively uniseriate, ca. 20/mm. Cells Wood diagnosis for Thomea (Figs. 1-4) exclusively upright and containing resin. Macroscopic description - Wood moderately Secondary xylem nonstoried. light, reddish brown to cinnamon brown in col­ our, no distinctive figure. Heartwood not Secondary xylem of Hypericum (Hw; those clearly differentiated. Rays not conspicuous used in Vestal, 1937; Figs. 7-9) on the end grain. Microscopic description - Growth rings Microscopic description - Growth rings generally not well defined, sometimes observed presen t, weakly defined by strongly lignified in semi-ring porous species, e.g., H. patulum; wood fibres. Pores diffuse, 75-110/mm2, soli­ rarely distinctly marked by fibre dimorphism, tary and in radial pore chains and pore clus­ e.g., H. densiflorum. Pores typically diffuse, ters mostly of 2-4 but chains up to 14. Pores less commonly semi-ring porous, very numer­ irregular in outline; diameters in longest di­ ous, in many greatly exceeding 200/mm2, rection 18-81 j.lm, with means 40-59 j.lm. Walls solitary and in radial pore chains and irregular about 3 j.lm. Vessel-element length medium­ pore clusters. Pores typically irregular in ou t­ sized, 226-712 j.lm, with means 425-507 j.lm. line; maximum diameters in longest direction Perforation plates simple, rarely aberrant multi­ 75 j.lm. Walls usually very thin, mostly 3 j.lm or perforate, mostly oblique and having short to less. Vessel-element length very short, mostly fairly long pitted tails. Intervascular pitting mul­ less than 250 j.lm as measured in tangential sec­ tiseriate alternate and bordered; pits mostly 2- tion. Perforation plates simple, rarely multi­ 4 j.lm across with a slit-like aperture, especially perforate, oblique to near transverse and ap­ elongate on tails. Pits to parenchyma as be­ pearing to have relatively short tails or none tween vessels or more elongate. Tyloses absent. at all. Intervascular pitting alternate and Wood fibres libriform and mostly septate; bordered; pits mostly 4 j.lm or less; some with wall thickness 4-6 j.lm. Maximum fibre diame­ large bordered pits to 6 j.lm, tending to have ter 30 j.lm. Fibres very short, 425-971 j.lm, fewer longitudinal rows; pits mostly circular means 665-719 j.lm. Average fibre-vessel with slanted slit-like apertures. Tertiary helical element length ratio 1.5. thickenings and tyloses present in some species. Axial parenchyma absent. Fibre-tracheids and libriform fibres even in Rays uniseriate and narrow multiseriate (to the same wood, in which very thick-walled ele­ 4), 16-22/mm, often difficult to distinguish ments have simple pits and fairly thin-walled in transection. Uniseriate rays homocellular, ones have prominent circular bordered pits. composed of upright cells; very low, generally Walls of libriform fibres mostly 2-4 times less than 10 cells high. Multiseriate rays hetero­ thicker than vessel walls, but in some both cellular with mostly square cells; 250-1,900 walls are equal; where fibres have thin walls j.lm high. Some ray cells with resin and starch and lack lignification, vessel walls have the grains. thicker walls. Fibre-tracheids usually associated Wood nonstoried. with vessels (Vestal, 1937); these cells, called vascular tracheids (Baas, 1970), are narrower Secondary xylem of Triadenum virgin/cum and longer than vessel elements and have (Hw 5649, sections only; as Hypericum virgin­ pointed end walls. icum; Figs. 5-6) Axial parenchyma absent. Microscopic description - Only one growth Rays uniseriate except in exceptional cir­ layer present. Pores diffuse, ca. 500/mm2, soli­ cumstances where branch traces diverge through tary and in radial pore chains and small pore secondary tissues; rays low to extremely low, clusters. Pores irregular to somewhat angular composed mostly of square cells; rays often in ou tline; diameters in longest direction homocellular with upright cells in stems with 20-50 j.lm. Walls 1-3 j.lm. Vessel-element very little secondary growth. Resin present in length very short, mostly 200-250 j.lm as many species. measured in tangential section. Perforation plates exclusively simple, mostly oblique and Wood diagnosis for Vismieae (Figs. 10-12) appearing to have relatively short tails or none Microscopic description - Growth rings at all. Intervascular pitting multiseriate alternate generally present, faint, marked only by thick­ and bordered; pits mostly 2-4 j.lm across with er fibres. Pores diffuse but often unevenly slanted slit-like apertures. Tyloses absent. distributed in radial or oblique bands; pores Wood fibres libriform nucleate and frequent­ solitary and in small multiples, the latter with ly septate; wall thickness 3-5 j.lm. Maximum 1-4 wide ~Iements and sometimes several ex­ fibre diameter 20 j.lm. Fibres very short. tremely narrow ones. Pores circular to radial- Downloaded from Brill.com09/24/2021 10:24:58PM via free access 90 IAWA Bulletin n.s., Vol. 1 (3), 1980 Downloaded from Brill.com09/24/2021 10:24:58PM via free access IAWA Bulletin n.s., Vol. 1 (3), 19BO 91 ly ovoid in outline; diameters medium sized, wider pores than any other species and tend to maximum observed 240 Ilm in Vismia ferrugi­ have more paratracheal parenchyma, whereas nea and Psorospermum androsaemifolium, banded apotracheal parenchyma and vascular means of outer wood 90-lBO Ilm, less in tracheids are less developed.