A review of the genus Curtisia ( Curtisiaceae )

A review of the monotypic southern African endemic genus Curtisia Aiton is presented. Detailed studies of the fruit and seed structure provided new evidence in support of a close relationship between the family Curtisiaceae and Comaceae. Comparisons with several other members of the Comales revealed carpological similarities to certain species of Comus s.I., sometimes treated as segregate genera Dendrobenthamia Hutch, and Benthamidia Spach. We also provide information on the history of the assegai tree, Curtisia dentata (Burm.f.) C.A.Sm. and its uses, as well as a formal taxonomic revision, including nomenclature, typification, detailed description and geographical distribution.


INTRODUCTION
Curtisia Aiton is a monotypic genus traditionally placed in the family Comaceae.It is o f considerable interest because of the many uses o f its timber and bark-but no recent reviews o f the morphology, taxonomy or anatomy are available.Recent cladistic and molecular systematic studies have revealed new evidence of relationships at fam ily level (Murrell 1993;Xiang et al. 1993;Noshiro & Baas 1998;Xiang & Soltis 1998;Xiang 1999), including support for a separate family, Curtisiaceae, or a close relationship with the family Grubbiaceae, even resulting in a proposal for extending it to also include Grubbia P.J.Bergius (Xiang et al. 2002).As no taxonomic revision o f Curtisia has ever been published since its original description in 1789, we present here a formal taxonomic treatment of this inter esting southern African endemic genus.A further aim was to investigate the structure o f the fruit and seeds to allow comparisons with putative relatives.

MATERIALS AND METHODS
Young branches with leaves and fruits at different stages o f m aturation were fixed in FA A (for a minimum o f 24 hrs).The voucher specimen is Van Wyk & Yembaturova 4218 (JRAU), collected at Diepwalle Forest (3323CC), Knysna District, Western Cape, South Africa.Some Curtisia fruits and seeds were also obtained from the carpological collections o f LE and STU (collec tion o f Felix Hohmann).Suitable portions were treated according to the method o f Feder & O 'Brien (1968) for embedding in glycol methacrylate (GMA).Specimens from the following herbaria were examined; JRAU, NBG, PRE and STU.Herbarium acronyms are given after Holmgren et al. (1990).
A Porter-Blum ultramicrotome was used to cut trans verse sections o f the fruits from 3 to 5 jam thick, and the periodic acid-Schiff/toluidine blue staining method (Feder & O 'Brien 1968) was applied.Suitable sections were photographed.Fruits obtained from carpological collections were rehydrated and then softened by means o f prolonged heating in Strassburger mixture (water, glycerol and 96 % ethyl alcohol in equal proportions), in accordance with traditional anatomical procedures (Prozina 1960) and then sectioned either by hand or sledge microtome.Test-reactions to identify lignification (phloroglucine and concentrated HC1), cutin and fatty substances (Sudan III, IV), starch (I-KJ) and protein (biuret test with 5 % copper sulphate and 50 % KOH) were applied to the sections.The terminology used is illustrated in the fig ures.

Vegetative morphology
The plants are tall shrubs or medium-sized trees 8 to 13 m high (Figure 1A) although specimens o f up to 20 m are not uncommon (Coates Palgrave 2002).Young twigs are densely tomentose; the hairs are rusty brown to dark brown but become sparser and greyish with age.The bark is initially brownish and smooth; in older trees it becomes darker, tinted with grey and rough with square fissures (Figure ID, E).The leaves are opposite, simple, petiolate and lack stipules.They are elliptic to ovate-lanceolate, 130-150 x 50-70 mm, leathery, bright to dark green and glossy above (but sometimes sparsely pubescent, mostly along the midrib, with simple, straight, thin hairs) and markedly reddish tomentose below (tending to become somewhat glabrescent with age).The lamina is entire, dorsiventral, with its apex broadly tapering to rounded and abruptly pointed; the base is mostly broadly tapering, less often cuneate; the margins are usually slightly revo lute and are markedly dentate to serrate or almost sinuate.The venation is pinnate and markedly raised on the abaxial surface (Figure 1C).Stomata are anomocytic.Petioles are up to 30 mm long and rusty tomentose when young but may become sparsely hairy with age.Bud scales are brown or rust-brown and very densely pubescent.

Reproductive morphology' and anatomy
Floral morphology and vasculature was thoroughly investigated by Eyde (1967Eyde ( , 1988)).The small flowers o f Curtisia are arranged in terminal, rather large (up to 120 mm long) panicles (Figure IB).Sometimes second-order inflorescences are formed o f flowers clustered together, giving a somewhat capitate appearance.The peduncles and pedicels are covered with dense yellowish hairs.The opposite primary branches o f the panicles are subtended by pronounced, markedly pubescent bracts (Cannon 1978).As is common for Comaceae, floral parts are in fours (visible in Figure IB).The petals are twice as long as the calyx tube, oblong in shape; the stamens are equal to petals in length.A characteristic centrally located vascular bundle in the four-loculed inferior ovary was described by Eyde (1967) who suggested the removal o f Curtisia from Com aceae on the basis o f this feature.
Curtisia fruits (Figure 1C) are usually referred to as drupes (or 'drupaceous') (e.g.Dyer 1975;Leistner 2000) but this term is imprecise.True drupes are found only in the subfamily Prunoideae o f the family Rosaceae where the fruit develops from a monocarpous gynoecium (Shibakina 1984;Levina 1987).In members o f Cornales, as well as many other taxa, the fruits are syncarpous (with two or more fused carpels) and often pseudomonomerous (with only one o f two or more locules and seeds reaching full development).Therefore, the term 'pyrenarium ', proposed by Z. Artyushenko (Arty ushenko & Fyodorov 1986) for such fruits as those o f Curtisia, will be used.The term 'pyrene' applies to fruits such as those o f Ilex L. and some Araliaceae, where each seed is surrounded by a lignified endocarp (each called a pyrene).However, when the endocarp is fused into a single bony structure around the seeds, the term 'fused pyrenes' or 'pyrenarium ' is preferable.The pyrenaria o f Curtisia are small (10-15 mm in diameter), globose, subglobose or rarely ovoid, snow-white and sometimes with ± persistent calyx teeth (Figure 1C).The outer peri carp tissues are fleshy (Figure 2A) and comprise both hypanthial exocarp and mesocarp, the latter developed from both hypanthial and carpellary tissues.The heav ily lignified endocarp constitutes a four-loculed stone, usually with a fully developed seed in each.Only the very central area, where the vascular bundle is located, is not lignified (Figure 2F, arrow).The seeds are exotestal (see description o f the seed coat later on), with copi ous endosperm and a well-developed, small, centrally located embryo.
The structure o f the fruit wall and the seed was stud ied in detail.The exocarp is formed by a single layer o f radially elongated cells with heavily thickened and cutinized walls (Figure 2B).The mesocarp is primarily parenchymatous.A hypoderm is clearly visible (Fig ure 2B, K) as the outermost zone o f the mesocarp, with two to three layers o f minute cells which are flattened in the tangential direction and sometimes have an almost  There is a very conspicuous intermediate zone between the mesocarp and endocarp, formed by five to seven (or up to nine) layers o f small, tangentially elongated cells filled with brightly coloured tanniniferous substances (dark zone in Figure 2 A, D).These cells possess slightly thickened walls that are somewhat lignified.
The typically woody endocarp comprises 17 to 20 layers o f fairly small sclereids (Figure 2E), mostly ori entated parallel to the longitudinal fruit axis, except for the innermost layer(s), which can be variously orientated to form a 'lining' layer for each locule.Some o f the scle reids appear to contain tannins.
The seed coat o f Curtisia was previously incom pletely described as 'thin and filmy, made o f compressed and partially obliterated cells' (Trifonova 2000).How ever, our study revealed several interesting structural details.Firstly, the exotesta is composed o f fairly large, tangentially elongated cells, which appear oval or almost crescent-shaped in cross section (due to concave outer tangential walls) (Figure 2G).These cells are filled with tanniniferous substances and have walls consisting o f cellulose.In certain parts o f the seed coat, large sclerified vascular bundle derivatives are found (Figure 2H), surrounded by a few layers o f very small, compressed parenchymatous cells.This type o f seed coat can be clas sified as 'exotestal' because o f the pronounced, thickwalled outer cells.
The meso-and endotestal components o f the seed coat largely consist o f compressed and deformed cells o f irregular shape, sometimes only with their walls per sisting.The part o f the endotesta which lies against the endosperm is homogenous and the cell walls are indis tinguishable and cutinized.
The seed has a massive copious endosperm, with pro tein as the main ergastic substance; the minute embryo is located centrally within the endosperm (Figure 21).

Phylogenetic relationships
The relationships o f Curtisia have not yet been clari fied.Its position within the Com aceae had been stable for a long time (Harms 1898;Wangerin 1910;Philipson 1967;Cronquist 1981; Thom e 1992) but its isolated posi tion within the family was noted-hence the subfamily Detailed molecular systematic studies by Xiang and co-authors (Xiang et al. 1993(Xiang et al. , 1998(Xiang et al. , 2002;;Xiang & Soltis 1998;Xiang 1999;Fan & Xiang 2003) showed Curtisia, however, differs from all other taxa in having stones with all four locules similar in size and shape, each containing a fully developed seed.The other taxa usually have only a single fully developed locule and seed.There are also differences in some finer structural details.Alangium and Mastixia, for example, have con spicuous secretory structures in their fleshy mesocarp tissues which are absent in Curtisia.Species o f Nyssa have either no sclereids or far more mesocarp sclereids than Curtisia.Furthermore, Curtisia is the only genus with a centrally located vascular bundle in the ovary; other putative Comaceae relatives are reported to have transseptal bundles (Eyde 1967).
As is well known, the Comaceae have a rich fossil record which includes wood, leaves and especially fruits (Miki 1956;Eyde & Barghoom 1963;Eyde et al. 1969;Mai & Walther 1978;Arbuzova 1988;Eyde 1988;Eyde & Xiang 1990) The greatest degree o f resemblance to Curtisia in pericarp structure (Figure 2J, L, with a portion o f Cur tisia pericarp given in Figure 2K for comparison) is shown by two representatives o f the genus Comus s.I., currently segregated into the genera Dendrobenthamia (Asian dogwoods with compound fruits) and Benthamidia (American dogwoods with fruits clustered together without fusing).These three taxa all have radially elon gated exocarp cells with heavily cutinized walls, solitary or clustered sclereids scattered throughout the parenchy matous mesocarp and a homogeneous woody endocarp.It is noteworthy that scattered sclereids were previously treated as an adaptation to synzoochorous dispersal in the relatively large, compound fruits o f Dendrobenthamia, believed to be dispersed by monkeys (Eyde 1985).Cur tisia fruits, despite their bitter taste, are also dispersed by animals such as birds (loeries, rameron pigeons), wild pigs, monkeys and less often baboons (Watt & Breyer-Brandwijk 1962).It seems likely that Curtisia fruits may be oil-yielding, as many white-fruited dogwoods from Cornus s.I. [e.g.Swida alba (L.) Opiz, S. sericea (L.) Holub and other shrubby species] have fruits rich in oil (Sozonova 1992).These fruits look white because o f the oil droplets that reflect light.Thus, fruit anatomy has hereby provided evidence o f a possible Curtisia-Cornus affinity and therefore supports Takhtajan's (1987) treat ment o f Curtisia as a monotypic family Curtisiaceae within the order Comales, close to the core family Cornaceae.The latter is currently treated as consisting only o f Cornus s.I. but it has often been divided into several segregates (Takhtajan 1987(Takhtajan , 1997)).Note: the generic name Curtisia Aiton was conserved against the earlier homonym Curtisia Schreber, Apr.1789 (family Rutaceae) and is listed in the International Code of Botanical Nomenclature (McNeill et al. 2006).Both these generic names were given in honour of William Curtis, the founder and for a long time the proprietor of the famous Curtiss Botanical Magazine, published since 1786 and known as Botanical Magazine (Burtt Davy 1932;Marais 1985).

The species o f Curtisia
There is only one extant species in the genus (see generic description given above).

History and uses
The most well-known name for Curtisia is assegai, assegai-tree (Afrikaans: assegaaiboom) or assegaiwood (assegaaihout).The records for this name date back to the earliest colonial days, recorded, according to Smith (1966), by Hermann in 1672 and Burmann in 1692.These plant names apparently were the first cases o f modem spelling o f the name.Early writers used such versions as hazegaiejn boomen hout, assgaaij, assegaje and hassagay-hout.As far as the origin o f the name assegai/assagay is concerned, several versions exist.One o f them is that the name is derived through Portuguese from the Latin word hasta, meaning a spear (Palmer & Pitman 1972).Another idea is that it refers to the Arabic word Azzaghayah, adopted by the Portuguese with vari ous spelling modifications and taken over by early Dutch writers (Smith 1966), evolving to its present-day form with time but always referring to weapons (spears and bows) used by the native African people.The ethnobotanical importance o f the tree is reflected in the large number o f vernacular names recorded in southern Africa.In Table 2, all these are listed exactly as they were published.
The wood o f Curtisia is iron-hard but elastic, strong and very durable-traditional javelins and spears, with 1.8 m-long tapering shafts, were made from this timber.Thunberg, in his descriptions o f his travels in the Cape o f Good Hope between 1772 and 1775 (Forbes 1986), repeatedly mentioned Curtisia (as 'assagay w ood') and its practical use for 'assagays' (javelins that Khoikhoi people carried with them on their journeys, with which they 'defended themselves against their enemies and wild beasts, and were able to kill them, buffaloes and other anim als').Later on, many other uses were found.The fact that assegai wood shrinks less than most other woods, made it very useful for the early settlers.It was the preferred timber for the axles and poles for wagons as well as the spokes, naves and felloes o f the wheels.The highly decorative qualities o f Curtisia tim ber-resembling mahogany due to developing a reddish brown, cinnamon tint with age-were appreciated by craftsmen producing tool handles (hammer handles in particular), implements o f husbandry, furniture and floor covering (Watt & Breyer-Brandwijk 1962;Smith 1966;Palmer & Pitman 1972;Van Wyk et al. 2000).The bark was used for tanning, yielding 2.98 to 14.05 % o f tan nin; up to 4 % o f tannin was yielded by the leaves and twigs o f the plant (Watt & Breyer-Brandwijk 1962;Van Wyk et al. 1997).Curtisia is also an attractive ornamen tal tree, especially for moist gardens.
Curtisia dentata is important in traditional medicine (Arnold et al. 2002).Zulu people use the bark to treat diarrhoea and stomach ailments.It serves as an aphro disiac (a 'love charm ' to make a man attractive) and as a 'blood purifier' or 'strengthener' (Cunningham 1988;Hutchings et al. 1996;Van Wyk et al. 1997;Ngwenya et al. 2003).Since the tannin-rich bark is used for medici nal purposes, the debarking and ringbarking o f trees caused significant damage in some localities.The bark (Figure IE) used to be sold by traditional Zulu herb gatherers for R30 per 50 kg bag but after becoming scarce due to overexploitation, it is only included in spe cial bark mixtures, such as 'Special K hubalo' (Cunning ham 1988).Rapid coppicing and the apparent capability o f the bark to regrow, played a very important role in the survival o f this forest tree.Despite a broad phytochemi cal survey o f the Comaceae by Bate-Smith et al. (1975), the chemical rationale for the traditional uses is still poorly understood.It may be assumed, however, that the high tannin content contributes to the efficacy in treating diarrhoea and stomach ailments.

Geographic distribution
Curtisia dentata is restricted to southern Africa, pri marily along the southern and the eastern coasts of South Africa and extends into Swaziland, Mozambique and eastern Zimbabwe (Figure 4).The genus is confined to montane forests (mainly found on southern and south western slopes), at altitudes from sea level to 2 300 m.

that
Curtisia is sister to another southern African endemic genus Grubbia, and that the combined clade is again sister to the rest o f the Comales.A suggestion by Xiang et al. (2002) to expand the Grubbiaceae to include Curtisia (citing similarities such as the geographical distribution, woody habit, leathery simple leaves with revolute margins, opposite phyllotaxy, lack o f stipules, hermaphroditic flowers, inferior ovary and copious endosperm) was not followed in later treatments (e.g.Angiosperm Phylogeny Group II 2003; Heywood et al. 2007) where the two monotypic families were retained.According to Xiang et al. (2002), 'no apparent morpho logical synapomorphies can be found for the two genera at present' and there are indeed important differences between them.The distribution o f Curtisia extends along the southern and eastern coast o f South Africa north wards as far as Mozambique (coastal zone-Da Silva et al. 2004) and eastern Zimbabwe (Baker 1911-1912; Eyles 1917), whereas Grubbia is restricted to fynbos.The large, leathery leaves o f Curtisia are coarsely den tate or sometimes sinuate and only slightly revolute, whereas the small, almost ericoid leaves o f Grubbia spe cies have entire margins which are distinctly revolute.There are also important morphological differences in reproductive structures-inflorescences, flowers and especially the fruits.Xiang et al. (2002) ascribed these differences to ancient divergence.It is possible that a more detailed investigation o f the anatomy o f Grub bia fruits and seeds could shed some light on the Curtisia-Grubbia relationship, but hitherto this appears to be based only on molecular evidence.Carpological research o f Comales and allies byYembaturova (2001Yembaturova ( , 2002)), the comparative results o f which are summarized in Table1, showed a number o f basic structural traits that appeared to be o f diagnostic value.Curtisia resembles Cornus L. s.I., Nyssa L., Alangium Lam. and Corokia A.Cunn.most closely, and Davidia and Mastixia to a lesser extent.All o f these taxa have epigynous flowers (however, this is still question able for Davidia), typical pyrenaria, thick, succulent exomesocarps and well-developed woody endocarps.
. Most o f the fossil structures have been identified as belonging to the genera Davidia, Mastixia, Diplopanax Hand.-Mazz., Nyssa, Alangium and Cornus.Recently, a fossil species o f the Eocene London Clay flora, originally referred to as Leucopogon quadrilocularis Reid & Chandler or Cornus quadrilocularis (Reid & Chandler) Chandler was shown to belong to Curtisia (M anchester et al. 2007) on the basis o f the endocarp structure.The fossil endocarps o f Cornus quadrilocula-ris are composed o f slightly elongated sclereids and pos sess four seed-bearing locules and a vascular bundle in the centre, as in extant Curtisia.