The genus Ehretia ( Boraginaceae : Ehretioideae ) in southern Africa

The genus Ehretia P.Browne in southern Africa is revised. Six species and five subspecies of Ehretia are currently rec­ ognized in southern Africa, of which E. alba Retief & A.E.van Wyk, E. namibiensis Retief & A.E.van Wyk subsp. namibi­ ensis, E. namibiensis subsp. kaokoensis Retief & A.E.van Wyk, E. rigida subsp. silvatica Retief & A.E.van Wyk and E. rigida subsp. nervifolia Retief & A.E.van Wyk are newly described. The genus is widely distributed in the region and occurs in a variety of habitats, ranging from the forests of the Eastern Cape to the hot, arid, semidesert parts of Namibia. Members of Ehretia in southern Africa are predominantly multistemmed shrubs or small trees. Characters of the leaf, the trichome com­ plement, inflorescence and corolla are used to distinguish between the different species. A key to the species, distribution maps and illustrations of various microand macromorphological as well as palynological features are provided.


INTRODUCTION
The genus Ehretia was described by Browne (1756).In 1759 Linnaeus validated the generic name with the binominal E. tinifolia.Ehretia, with about 33 species in the tropics of both the Old and New World, is found par ticularly in Africa and Asia, with a few species in tropi cal America and the West Indies.Lebrun & Stork (1997) recognized 12 species in tropical Africa, three of which extend to southern Africa (Herman 1993).The genus is widely distributed in southern Africa, occurring in a vari ety of habitats ranging from lush forests of the Eastern Cape to hot, arid parts of Namibia.Members of Ehretia in southern Africa are predominantly multistemmed shrubs or small trees, with rigid, arching or drooping, often entangled branches.The inflorescences are cymose, the individual cymules usually scorpioid, termi nal on young shoots and/or lateral at apices of abbreviat ed branchlets.The style is terminal and bifid, whereas the fleshy fruit is subglobose with four 1-locular pyrenes.The genus belongs to the family Boraginaceae s.l. of mainly herbs, characterized by inflorescences of scorpi oid or helicoid cymes, terminal or gynobasic styles and fruit usually consisting of four nutlets.
The purpose of this paper is to present a taxonomic revision of the genus Ehretia in southern Africa.Diagnostic characters, an identification key, full descrip tions of existing and new taxa, illustrations and distribu tion maps are provided.The generic description of Ehretia and also species descriptions arc based on occur rence in southern Africa and adjacent Flora zambesiaca regions.This paper forms a part of a revision of the Boraginaceae for the Flora o f southern Africa which is currently in progress.

MATERIALS AND METHODS
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Herbarium specimens of Ehretia in BLFU.BM, BOL, COI, E, GRA.J, K, KNP.NBG, NH.NMB.NU, PRE, PRU, SAM.UNIN and WIND were used to gather data on morphological characters, flowering time and distribution.Extensive fieldwork was done to study live plants in their natural habitat.Both untreated and acetolysed pollen grains were studied.Pollen was acetolysed according to the standard method of Erdtman (1960).For scanning electron microscopy studies samples were coated with gold and studied with an ISI-SX-25 SEM.Although acetolysis is the basic technique used by virtually all palynologists, untreated pollen grains were also studied because certain characters are destroyed by this treatment.

HISTORICAL OUTLINE
After Browne established the genus Ehretia in 1756, De Candolle (1845) enlarged the concept of the genus significantly by publishing about 58 species within the genus.Many of these species are now treated as segre gate genera, for example Bourreria, Carmona, Rochefortia and Rotula (Thulin 1987;Miller 1989).In De Candolle's revision only Ehretia hottentotica, a species described by Burchell (1824), is mentioned for South Africa.De Candolle recognized four sections within Ehretia and placed E. hottentotica in the section Bourreria, a taxon characterized by four pyrenes.Thunberg, the 'father of Cape botany' (Gunn & Codd 1981), collected a specimen in the Cape which he named Capraria rigida in his Prodromus plantae capensis of 1800.To him this specimen showed similarity to two species of Capraria, a genus now placed under Freylinia (Scrophulariaceae).Certain specimens collected by Drege (1843) were regarded as belonging to Grumilea, a genus subsequently sunk under Psychotria (Rubiaceae).Drege (1847) applied the name Ehretia zeyheriana, a nomen nudum of Buek (1796Buek ( -1878)), to specimens col lected by Ecklon, Zeyher and himself.In 1859 Harvey also used this name when describing a new species dif fering from 'E. hottentotica' in having larger, less obo vate and thinner leaves.He stated that differences 'may be owing to a freer growth in better soil, and eventually the two varieties will perhaps be united'.Harvey men tioned that the taxon E. eckloniana (another manuscript name of Buek) is also closely related to E. hottentotica.Wright (1904) published a revision of Ehretia in South Africa in which he used the name E. hottentotica of Burchell, although he gave Thunberg's Capraria rigi da as a synonym.Druce (1917) had to arrange and exam ine a large number of African and Australian plants and consulted rather critically the Floras of these regions.He published the name E. rigida (Thunb.)Druce for this taxon.The method of nomenclature of the Hookerian school differed from the continental plan in not insisting upon the permanence of the specific epithet (trivial name) when transferred to a different genus.Wright (1904) thus did not use Thunberg's epithet, but followed the so-called Kew Rule.He also published another species for the region, E. amoena Klotzsch, with Galpin 1242 as the only cited specimen.
In a revision of the Boraginaceae of South West Africa (the present Namibia), Friedrich-Holzhammer (1967) recognized three microfamilies, namely Heliotropiaceae, Boraginaceae and Wellstediaceae.Ehretia, represented by E. amoena and E. rigida, was placed in Heliotropiaceae together with Heliotropium and Cordia.Martins (1990Martins ( , 1993) ) revised Boraginaceae for Flora zamhesiaca (FZ) and Flora de Mozambique.He record ed E. amoena, E. obtusifolia Hochst.ex DC. and E. rigi da for the FZ region.These three species names were also listed by Herman (1993).The present revision, the first since Wright (1904)

Habit
Species of Ehretia are predominantly shrubs, occa sionally small or straggling trees.However, suffrutices no taller than up to 0.4 m are known to occur in E. obtusifolia and E. rigida.We support the observation by Burtt Davy (1922) that the gap between trees and herbs is bridged by forms of every gradation in size, from those that may be classed as either trees or shrubs to those sometimes defined as woody herbs, suffrutices or 'obscurely shrubby' plants.The 'suffrutex type' of habit, occurring in the species mentioned, can be regarded as transitional to the many herbaceous growth forms in the family.However, shrubs occur in Lobostemon, a genus more or less endemic to the fynbos of South Africa and also a member of the mainly herbaceous subfamily Boraginoideae.The occurrence of suffrutices supports the view that there is no clear division of Boraginaceae into two families, Ehretiaceae and Boraginaceae, on the basis of habit.Various other growth forms (ecotypes) are found in E. rigida.Plants of subsp.rigida are usually stunted in habit, whereas those of subsp.silvatica and subsp.nerx'ifolia are more lush, with branches often straggling, entangled and arching at the top.E. rigida subsp.nerx'ifolia, a large, deciduous or evergreen, multi stemmed shrub, is sometimes characterized by long, drooping, precocious branches with pale mauve flowers appearing early in spring together with the new leaves.

Leaf
Various leaf characters are important in distinguishing between taxa.Leaves of southern African species of Ehretia are usually obovate to broadly obovate or ellip tic, displaying quite some variety in size and venation (Figure 1).The size of the blade varies from 6-95 x 3-56 mm, the largest ones found in E. amoena (Figure IE, F) in the relatively moist eastern regions of the Northern Province, Mpumalanga and Swaziland, to the smallest in E. alha (Figure 1K), a species of the drier central regions in southern Africa.Reduction in leaf size is regarded as an adaptation for reducing transpiration under stress con ditions.The venation in all species is brochidodromus and is usually prominent on the lower surface (especial ly on dried specimens) in most species, but in E. alha and E. rigida subsp.nervifolia, occurring under more arid climatic conditions, the tertiary veins are obscured on the lower surface.
Three trichome types are found on leaves and other parts of Ehretia: (a) 2-or 3-cel led short hairs, (b) setae with a 1-layered, multicellular base, and (c) simple, multicellular, capitate glandular trichomes (Figure 2A-C).The indumentum varies from sparse to dense (Figure 2D-F).The blade is sometimes glabrous except for setae occurring along the margin, a condition characteristic of various boraginaceous species (Figure 2D).These setae are often bent and usually appressed.Leaves of E. amoe na, for example, are often described as rough (like sand paper) owing to short, stiff setae orientated at an angle to the upper surface.E. alba which occurs under relatively arid climatic conditions, has numerous stomata sunken in a prominently ribbed epidermal surface (Figure 2G), in contrast to the smoother surface of E. rigida from a moist habitat (Figure 2H).Domatia are sometimes present in the axils of principal lateral veins on the lower surface (Figure 21), but may be present or absent on leaves of the same plant.Domatia are not specific for Ehretia in Boraginaceae; they also occur in Cordia, for example.The pit domatia (which are usually hairy) are believed to provide shelter to beneficial mites.

Inflorescence
Inflorescences of the Boraginaceae are characteristical ly scorpioid or helicoid cymes, often coiled at the apex, uncoiling and elongating at maturity.Southern African species of Ehretia have inflorescences with scorpioid cymules, borne apically on young shoots and/or at the apex of abbreviated branchlets (brachyblasts) (Figure 3A,  D).On the basis of the composition of the inflorescence, local members of Ehretia can be divided into different  D).The peduncle of the inflorescence is sometimes very short, with flowers clustered at the end of an abbreviated branchlet, a condition found in E. obtusifolia.An inflores cence often develops at the end of a new branch, whereupon new growth of the branch continues sympodially.
Trichome complements of the inflorescences (and calyces) differ and can be used to distinguish between species.E. amoena, E. rigida and E. alba usually do not have simple, multicellular, capitate glandular trichomes, whereas E. coerulea, E. obtusifolia and E. namibiensis are characterized by this type of trichome, especially on the inflorescence, calyx and midrib of the leaf.Hybridization between species is suspected; the presence of unexpected glandular trichomes may signify this phenomenon.elliptic or rectangular in equatorial view.Apertures are long, broad or rather narrow in diameter and mesocolpium centres ( 'pseudocolpi') are present.The tectum is reticulate to rugulose.Orbicules are present, covering the surface of the thecae.A grain of E. obtusifolia was found to have four apertures instead of the normal three (Figure 5C).Two pollen types are often recognized in Ehretia.
Species ± 33, in the tropics of both Old and New World, particularly in Africa and Asia, a few in tropical America and West Indies; six in southern Africa, wide spread (Figure 7).Shrub or small tree, single or multistemmed, up to 5 m high, branches arching at top.Leaf blade occasional ly elliptic, 45-95(-114) x 23-56(-75) mm.apex obtuse, cuspidate, margin entire, sometimes with one or more teeth, widely spaced, on either side of apex, veins promi nently raised on lower surface, lower surface of leaf blade prominently veined, densely to sparsely hairy, setae appressed, setae on veins conspicuously orientated at right angles to veins, upper surface less hairy, setae often short and stiff at an angle to surface of leaf blade, giving a rough texture to blade; petiole 4-10 mm long, length of petiole : length of midrib = ± 1 : 1 4 .Inflores cence a branched cymose panicle, terminal on a young shoot or terminal on a short young shoot at apex of an abbreviated branchlet; axes and calyx clothed with setae and multicellular hairs.Calyx lobes narrowly triangular.2-3 x 1 mm.apex acute.Corolla white; tube widely fun nel-shaped, as long as calyx; lobes reflexed, usually as long as calyx or longer, apex obtuse, margin not ciliate.Fruit glabrous; pyrenes 3^4-mm long.Flowering time: October to December.

E h retia obtusifolia
Ehretia obtusifolia is closely related to E. amoena and E. rigida subsp.nervifolia.Usually they are readily distin guished, but in areas where the species are sympatric, spec imens with intermediate features occur (Martins 1990).
Distinguishing characters: corolla lavender-blue or pale mauve, tube cylindric; leaf blade sparsely hairy with setae and scattered glandular trichomes on midrib only.Distribution: occurs over a large area of Namibia and Botswana (Figure 10); according to Giess (1971) it is known from mopane, thombush and highland savanna in Namibia.Distribution: endemic to Namibia in the Kaokoland Centre of Plant Endemism (Van Wyk & Van Wyk 1997, Anderson & Van Wyk 1999), characterized by desert grassland and shrubland, arid savanna and desert annuals; probably also present in adjacent southwestern Angola (Figure 10).Habitat: sandy banks of watercourses.
TYPE.-Namibia, 2218 (Gobabis): Breitenberg, Gobabis Dist., (-DC).Merxmiiller 1071 (WIND, holo.; PRE, iso.).Shrub up to 4 m high.Branches rigid.Leaf blade 6-25 x 3-13 mm, apex acute or obtuse; glabrous except for appressed setae along margin, thin or thick in texture, drying reddish brown when pressed or blade with lateral veins obscure, drying the same colour as midrib; midrib and secondary veins on lower surface prominent; petiole up to 3 mm long, length of petiole : length of midrib = 1 : 10-15.Inflorescence usually single at apex of abbre viated branchlet; axes and calyces with multicellular hairs and scattered setae; peduncle short.Flowers sometimes fragrant.Calyx lobes narrowly triangular, up to 3 mm long; apex acute to acuminate.Corolla white, fading to cream-coloured: tube 3 mm long; lobes ± 3 mm long, as long as tube or slightly shorter.Style white, sometimes tinged purple or lilac.Flowering time: August to October.
Much-branched shrub or multistemmed tree, up to 12 m high.L eaf blade 20-60 x 9-45 mm, apex slightly retuse, obtuse to acute, occasionally cuspidate, venation on lower surface distinct, lower surface glabrous or hairy with setae and multicellular hairs along veins, upper sur face glabrous or hairy with setae or multicellular hairs scattered along veins, coriaceous or thin; petiole 2-10 mm long, length of petiole : length of midrib = 1 : 3-11.Inflorescence single or more than one terminal on a prominent new shoot and/or terminal on an abbreviated branchlet, densely clothed with multicellular hairs and scattered setae; peduncle varies in thickness.Calyx lobes ovate to triangular, 2.0-2.5 mm long, apex obtuse to acute.Corolla tube cylindric, 7-10 mm long, usually white; lobes pale mauve, lilac or purple, shorter or as long as tube.Fruit glabrous; pyrenes ± 3 mm long.Shrub or tree, often stunted, up to 4 m high.Leaf blade 20-35 x 12-23 mm, apex acute or rounded, glabrous or main veins with setae and others with scat tered setae, coriaceous or thin; petioles 3-8 mm, length of petiole : length of midrib = 1 : 4-7.Inflorescence ter minal, one or several in a 'head' on young shoots, manyflowered, peduncles and pedicels thick, up to 1.5 mm diam., multicellular hairs or long, thin setae present.Calyx lobes triangular, ± 2.0-2.5 mm long, apex obtuse to acute.Corolla cylindric, 8-10 mm long; tube usually white; lobes lilac, blue, bluish purple or purple, shorter than tube.Fruit ± 3 mm long.Flowering time: October to March.
Distinguishing characters: corolla lobes pale mauve; branches arching prominently or conspicuously drooping w hen plants are growing in a more arid environment; leaf blade usually glabrous except for appressed setae along margin or pubescent with setae along midrib and veins; multicellular hairs present on inflorescence and calyx; tertiary veining not prominent on lower surface but sec ondary veins so, usually three on either side of midrib, raised.Distribution: North-West, Northern Province, Mpumalanga, Swaziland and KwaZulu-Natal (Figure 12); extends into Mozambique and Zimbabwe; sympatric with E. amoena and E. obtusifolia.Habitat: shrub of savanna, dry thornbush.coastal scrub.Common names: puzzle bush; mordbe (Tswana); deurmekaarbos (Afrikaans)-according to Smith (1966) Pollen grains of southern African Ehretia species (Figure5A-F), are tricolporate, isopolar, oblate-spher oidal with P = 20-31 pm, E = 17-29 |jm (immature pollen grains are prolate, P = 13-22 pm, E = 17-29 jam).The grains are ± circular in polar view and hexagonal.