S. Afr. 1. Bot. 1998, 64(2) 12 1

Micromorphological structure of the fruit and seed of Smodingium argutum (Anacardiaceae), as an adaptation to its natural habitat

I. von Teichman Department of Botany, University of Pretoria, Pretoria, 0002 Republic of South Africa

e-mail: Teichman@scientia.up.ac.za

Received 26 Seplember 1997: revised.J November /997

Smodingium argutum E. Mey. ex Sand. is very closely aWed to the genus Rhus l., but easily distinguished from it by

the inflorescence which is a thyrse: its pendulous ovule; an 'apical-lateral' style; winged fruit; a sap (noxious secretion

from all organs) which tums black on exposure to air; indumentum on the pedicel, bracteoles and ca lyx, as well as an

unique, papery pericarp. The latter comprises a uniseriate exocarp, a mesocarp with large secretory ducts and a

mostly twO-layered, weakly developed endocarp. An anatropous, bilegmic, crassinucelJale ovule develops into a small.

partially pachychalazal, exalbuminous seed, also characterized by an endotegmen and lipidic nutrient reserves. The

following characters are obviously of crucia l importance for the survival of this species: efficient fruit dispersal, aided by

the samaroid drupe; easy germination; the well~developed cuticles on the calyx, ovary wall and exocarp; and the thick

cuticular layer delimiting the mature integumentary seed coat, which, together with cell wall impregnations in the

physiologically mature hypostase cells, protects the embryo against dehydration .

Keywords: Callose, hypostase, lipids, pericarp. samara, seed coat.

Introduction According to Mabberley ( 1987) the Anacardiaccae, which is mainly tropical and subtro pical , comprises 73 genera. In south­ern Africa this family is represented by ten indigenous genera, five of which are monotypic (Arnold & De Wet 1993). One of these is the endemic SmodmgillJ11 E.Mey. ex Sond. which was placed in the tribe Rhoeae by Engler (1892). The only species, SmO(/;ngillnl argUlllnl E.Mey. ex Sand., is closely allied to and resembles the genus Rlll/s L.. but was previously easi ly distin­guished by its winged fruit , a pendulous ovule, 'lateral' style and the sap which turns black on exposure to a ir. Characte ri sti cs of the fruit and seed of S. argutlll1l will be compared with those of the genus Rhus. However, for the sake of brevity, only those characteristi cs of Rhus that differ significantly from those of Sm odillg i1l11l. are specifica lly mentioned. Ovary. pe ri carp and seed coat anatomy of several species of RllIfs has been studied previously (e.g. Pienaar & Von Teichman 1998; Von Teichman 1991a; Von Teichman & Robbertse 1986; Von Teichman & Van Wyk 1991; Wan nan & Quinn 1990, 1991). Watt and Breyer­Brandwijk (1962) mention contact dermatitis in susceptible indi­viduals caused by the poisonous sap occurring in all parts of 5 argutlllll. Findlay el al. (1974) found a close chemical relation­ship between the allergens from . amongst others, the American Poison Ivy (Toxicodendron radicans (L.) Kuntze) and S. argll­fum (henceforth referred to as Smodingium). Two heptadecyl cat~ echols constitute the main components of their noxious secretions. These authors also refer to the treatment of Smodill­gium dermatitis. S modillgium is the only known indigenous spe­cies of the Anaca rdiaceae causing dermatiti s, i. e. severe erythema with oedema and bl istering. Most American research~

ers in this field cons ider Toxicodendron Mi ll. (comprising the notorious species) to be separate from Rims (Mitchell 1990; Reveal 1990). On the other hand, authors from the United King­dom mostly do 11 0t segregate Toxicodelldron from Rims (e.g. MabberJey 1987).

The common names of Smodil1gium , African Po ison Oak. Pai n Bush, inTovane and Rainbow Leaf, reflect its poisonous nature and beautiful autumn foliage (e.g. Pooley 1993). The frost~res is­tant shrubs or small trees (up to six m high) are characterized by their drooping habit and occur mostly in stony soil. They are

found in Acacia-dominated and ri verine thorn bush and on forest margins. The natural distribution of the species extends from the mid lands of the Easte rn Cape and KwaZulu-Natal , into the east­ern Free State and the lowveld of Mpumalanga. A distribution map is suppl ied by Wells ( 1988).

The importance of data 011 seed structure for taxonomic revi­sion has been stressed repeatedly (e.g. Boesewinkel & Bouman 1984; Corner 1976; Groenendijk el af. 1996). T hese authors also suggested that seed studies began with the ovule and ended with germination, and that a correct interpretation of mature seed coat structure necess itated ontogenetic studies. In a recent encyclo­paedic work, Werker (1997) followed the holistic approach by relating seed anatomy to var ious physiological processes. In the present paper, the contribution of the micromorphological struc­ture of the fruit and seed to the adaptation of Snlor/ingillll1 to its natural habitat is discussed. Informat ion on the position of the sty les , structure of the ovu le and notes on germination are also supplied fo r the first time. This study is part of a comprehensive survey of the fruit and seed structure of the Anacardiaceae. It aims to supply new d ata requ ired for a revised infra-familial clas­sification of thi s family as envisaged by Wan nan and Quinn (1990, 199 1),

Materials and Methods Floral buds. flowers and frllit of Smodinglllll1. in various stages of deve lopmen t. wt! re collected from trees ill the Pretoria Nmional Botnnicai Garden (voucher spccilll~ns: " Oll Teichman 536. 5./6 & 548 in PRU). Materia l ror anatomical studies was fixed in 2_5% cat .... feinated. sodium phosphate-buITert:d glutaraldehyde. Dehydration. infiltrat ion and t!mbcdding in glycolmethacrylnte (GMA) were done according to Feder and O'Brien (196R).

Composi tion of the monomer mixture. preparation and stain ing of tht: 3 I--un ~ thick GMA sections. as wel l as the histochemical test for cUl in. arc described by Von Teichman (1987). How\!ver. in th~

pres!.!llt study thl.! sections were cut on a Reichert-Jung Autocut 2040 ultramicrotome. Histochemical sLa ining for proteins was done with amido black lOB according to Bullock el (fl. ( 1980). For the detec­tion of callose. semi-thin st:ctions Wl.!rc mounted in 0.1 % watcr~solu~

ble aniline blue in 0.1 M KJPO,J buffe r at pH 12.4. and examined

wi th a Nikon Optiphot light microscope equipped with

122 s. Atr. 1. Bot. 1998. 64 (2)

B A

Figure I The ovary of Smodingium in the flower at an lhesis . A. one of the longitud inal sections (I s), B, a reconstruction from a series of Is. Abbreviations: d vb. dorsa l vascular bundle: e, embryo sac; f, fun ick: h. hypostasc; i. inner integument : 11 , nuce ll us; 0, ollter integument ; p, peta l: r. raphe vasculnr bund!!:; s, sepal; sd. secretory duct: st. part of rccurved style with stigma: v, vent ral vascular bundle; wm, willg meris­tem. Scale bur: A = 200 ~m. B = 300 ~m.

cpitluorcscencc optics (450-490 nm = B-2A ~xcitalion filter and 520 11m barrier til ter) . For the micropho tography. CF N PLAN APO objectives were used with the Nikon microscope. Crystals and starch grains were observed under crossed polarized filters.

Results and Discussion Morphology of flowers, fruit and seed Smodingilll1l is dioecious. According to the interpretation of Weberling ( 1992), the inflorescence is a many-flowered, ax illary and lerminal, determ inate (doub le) thyrse. Barfod ( 1988:8) states that the ' infl orescences of the tr ibe Rhoeae are typ ically thyr­soid'. This applies to a number of North American species of RIms, but Toxicodendron is an exception, hav ing a panicle. Female flowe rs of Smodillg illl11 are fun ctionally unisexual, very small , ca. 2 mOl in diameter, and whitish to cream-coloured . Pedicels are articulated near the apex. The pedicel, the two minute bracteoJes and the sepals have numerous, straight or var i· ously bent, st iff, lignified, mostly uniceitular hairs. In Rhus lan­cea LJ. , the perianth is glabrous. In Smodingilll11 the calyx and corolla are pentamerous . The abaxial epidermis of the perianth has a very well-developed, serrated cutic le. Five well-developed anti sepalous staminodes , situated at the deeper notches of the di sc, alternate with the re fl exed petals. An orange, annular, 10-crenate intras taminal di sc is present (Figure 1). It is characterized

by a well -developed cuticle on the non-papillate epidermal cells. Abundant, mostly open stomata occur within the epidermis . The hypodermal parenchyma cells seem to be metabolically active, judged by the large nucle i and abundant cytoplasm. Whether these cells function as a secretory ti ssue, is not known.

The tricarpellary gynoecium is characterized by a unilocular, ellipsoidal, laterally compressed ovary (ca. 1.5 mm high and 1 mm in diameter). In flowers at anthes is, the ovary wa ll (Figures 1-3) comprises the following: a) an outer epidermis (fu ture exocarp sensu stricto) with slight ly radially e longated cel ls and well-developed cuticle; b) a parenchymatous ground ti ssue with crystalliferous cells con­ta ining calcium oxalate; on the lateral s ides of the ovary the outer part of the ground ti ssue represents a ' wing meristem ', cons isting of small , metabolically active parenchyma cell s lying in distinct radial rows; the ground ti ssue is furt hermore characterized by one row of very large secretory ducts surrounded by cytoplas­mic-rich epithelial cells and two or three ce ll layers of small, thin-walled parenchyma cell s forming a di stinct sheath, as well as by the vascular tissue; and c) the innermost epidermis, which has formed two distinctly meristematic cell layers by means of periclinal divisions. In some places even three cells may li e in radia l rows after a further peric linal division has taken place.

This vascular ti ssue in the ovary of Smodingium compri ses

S. Ali'. 1. Bot. 1998,64(2)

Figure 2 Transverse section of the oval)' at anthcsis. Abbn;via~ lions as in figure 1. Scak bar = 200 ).1n1 .

one large dorsal, two di stinct ventral bundles (Figures 1-3) and lateral procambiul11 strands. mostly alternating with the secretory ducts or distinguishable as a few xylem elements lying near the secretory duct sheath. This vasculature in the ovary and stylar base is of significance, as already in 1883 Engler described Smodingillnl with lateral styles. However, he did not state whether they were dorsally or ventrally positioned. Walll1an and Quinn (1991) have already described and am ply illustrated very similar dorsal and ventral vascular bundles in various taxa of the Anacardiaceae, amongst others, in the genera Blepharocarya F.Muell. and Alang{lera L. Comparing the present Figure 1 with that of iHang[fera indica L. by Wannan and Quinn (1991, amongst others, Figures 2IK & 27), the style base in Snwdin­gium may be described as 'apical-lateral'. since it is not as pro­nouncedly ' lateral' as that of Afang[fera indica. The larger, dorsal vascular bundle does not bend over the ovule to the same great extent as in A1ang{fera. Again the concept of a continuum applies, as the style in Smodingiunl is an intermediate form between di stinctly apical and distinctly lateral. In S'modingillnl the styles are situated on the ventral side of the ovary (Figure 18 ). The three styles are free (fused only basally) and recurved. Three functional and capitate stigmata , with a non-papillate

Figure 3 Transverse section (ts) of a small part of the ovary wall at higher magnification, illustrating, amongst others. the futurc endocarp, white arrow heads; fu ture exocarp, exo: outer epidermis of ovule in the chalaza I region, oe; wing meristem. wm; and xylem elements.t, close to the secretory duct (sd) bet\vcen the two ventral vascular bundles (v). Scale bar = 50 ,.uu.

Figure ~ Morphology of the fruiL seed and embryo of SlI1odm­gil/m. Abbreviations: line at a, indicates the position of the ts in Figure 5; Co persistent calyx: cst'. chalazal seed coat: em. t:mhryo: fs. fruit stalk; i se. ill lt:gulllentary sct:d coat : s. seed (one sho\\ing the impressions of secretory duets in the mesocarp): sl, remnants of persistent style and stigma: \\. wing. Se,lie bar = 4 min.

epidermis, terminate the sty les. The single pendulous ovule is very small (ca . 0.7 mm long),

bitegmic, anatropous, crassinucellar, with a ventral raphe (F igure I). As in most ovules/seeds of the Anacard iaceae studied previ­ously, the raphe has an amphicribral vascular bundle. A hypostase sensu /alO, consisting of a group of metabolically active parenchyma cells, is present. Although this hypostase sensu lata is not as sharply demarcated as in O::oroa Oclile (Von Teichman 1993: Figure 4), it characterizes the chalazal-nucellar tissue. The distinction between a hypostase sensli lata and sensu stricto in the Anacard iaceae is exp la ined by Von Teichman (1988). It is only the hypostase sensli lalo which takes part in the formation of the seed coat. In ,)"modingium the outer integumen t (0) is proximally four or five cell layers thick, while the inner integument (i) consists proximally of only two epidermal layers, with the inner epidermis (ie ii) . i.e. the future endotegmen, easily distinguishable due to its meristematic character.

Fruitset can be so high that great parts of the plant are draped with light brown infructescences. Drupes are laterally com­pressed, samaroid, subcircular (ca. 11 x 9 mm), light brown when ripe, indehiscent, with a conspicuous circular, papery wing which developed from the ' wing meristem', secretory ducts and persistent styles and calyx (Figure 4). In contrast to the observa­tions of Engler (1892) , who most probably used dry herbarium material , it was found that the seed coat is not attached to the pericarp. The partially pachychalazal seeds are kidney-shaped and show the impressions of the secretory ducts occurring in the mesocarp (Figures 4 and 5). The seed coat is light brown (integu­mentary part) with a dark brown, saddle-shaped part (the cha­lazal seed coat) - the dark colour is due to the presence of the tannini ferous cel1s in the hypostase senSll lalo. This seed coat is very simi lar to that of Rhus lanexa (Von Teichman 199Ia), as well as those of the other species of Rhus studied previously.

Anatomy of the peri carp

In order to be useful in phylogenetic considerations, i.e. compar­ing homologous parts in different taxa, the peri carp in

124 S. Mr. J. BoL 1998,64(2)

.cot·

w

Figure 5 Ts or mature pericnrp and seed. Abbreviations: cot, cotyledon; d vb. large dorsal vascu lar bundle; end. endocarp; aste risk, cellular remnants of the endosperm; ct, cndotcgmen in integumentary seed coat; exo, exocarp; h. hypostase in chalazal seed coat; tn, mcsocarp: n. nuccllar remnants; QC, outer epidcnnis of seed coat: sd, secretory duct: sh, parenchyma sheath: v, ventra l vascular bundle; w, wing; x, xylem d ements within vascular tissue. Scale bar ;:: 300 ~tn.

Smodillgilll1l is interpreted sensu stricto. My reasons have been published previollsly in a paper on the reinterpretation of the peri carp of Rhus lancea (Von Teichman \989). In Smodillgillln the exocarp (Figures 5- 7) with its we ll -developed cu ticle com­prises relative ly thin-walled parenchyma. (t originated from the outer epidermis (oe) of the ovary wal l. The mesocarp s. sir. is characterized by large secretory ducts (F igures 5-7), the vascula­ture, thin-walled parenchyma and crystalliferous cells. This vas­culature comprises a large dorsal and two ventral vascular bundles, as well as vascular tissue which usually, together with the epithelial cells and parenchymatous sheath cells, surrounds the secretory ducts. The wing is formed by the exocarp and the outer parenchyma of the mesocarp. The thin , s light ly leathery endocarp s, sIr, consists of one or mostly two layers of relatively thin-walled brachysclereids (Figures 5-7). Parenchyma cells adjacent to the ventral vascular bundles (Figure 6) have formed thin-wal led brachysclereids.

In the pericarp of Rhus lancea, the exocarp is very s imilar to lhat of Smodingiul1I . However, the outermost layer of the meso­carp of Rhus lancea forms a sclerenchymatous hypodermis, fol­lowed by parenchyma and pronounced inner sclerenchymatous ridges, associated with the secretory ducts and vascular bundles (VOIl Teichmall & Robbertse 1986). In R. ioncea and II. prohlem­a/odes Merxm . & Roessler (Von Teichman & Van Wyk 1991), as well as in Lithl'aea brasiliensis Marchand, which should rather be class ified under the genus Rhus (Pienaar & Von Teichman in press), the thick, stony endocarp sensu stricto is three- or four­layered (stratified), consis ting of an innermost layer of palisade­like macrosclereids, a layer of osteosclereids, followed by a laye r of brachysclereids. A dist inct fourth crystalli ferous layer, wi th cell walls also slightly thickened, occurs in R. lancea and L bra­siliells;s. Such a distinctly layered (stratified) eodocarp senSll

stricto as in R. lancea represents the Anacardium-type endocarp sensu Wannan and Quinn ( J 990). Under the Anacardium-type, these authors recognize three endocarpal sub-types (or taxon groups) in the tribe Rhoeae. An endocarp of four discrete layers as in Rhus, Ozo1'o(l, Schil1lls L. and seven other genera, forms Group A. A two- layered endocarp, as was found in Smodingiurn during the present study, which is also present in Pr%rhus iongifolia (Bernh.) Eng l. (VOIl Teichman 199 1b) and Heeria argelllea (Thunb.) Meisll . (Von Teichman & Van Wyk 1996) does not fit any of the sub-types or Groups of Wannan and Quinn (1990). Therefore, as proposed by Von Teichman and Van Wyk (1996) for H. argemea, th is type of endocarp in Smodingium, represents a fourth Rhoeae endocarpal sub-type (Group D). The latter is also derived from the Group A-type.

Anatomy of the seed The seed of SmodingiulIJ, which is very similar to that of the spe­cies of Rhl/s mentioned above, is partially pachychalazal, the seed coat being of dual origin, since the secondarily extended chalaza takes part in the development of the mature seed coat. Anatom ically the chalazal seed coat comprises the outer epider­mis, a few layers of hypodermal, thin-walled, tanniniferolls and slightly compressed parenchyma and the hypostase. The latter conform s to the Anacardiaceae-type descri bed for all the previ­ously studied taxa (e.g. Von Teichman 1991 a,c; Von Teichman & van Wyk 1996). lis cells are contiguous, thill-walled, dis­tinctly tannin iferous and form a zone which is easily distinguish­able (Figures 5 and 6). Cell walls of the hypostase in the mature, phys iologically ripe seed remained unstained in periodic acid­Schiff s reagent and in toluidine blue (Figure 6). However, in the tests for lipids, namely cutin, and also callose, these cell walls

S. Afi·. J. Bot. 1998,64(2 )

Figu re 6 t\ parl llftht: ts shown In Figun .. ' 5, at higher magnilica­tion. Abbreviat ions as in Figure S. Scale bar = 150 ~U11.

reacted posi ti vely. The periphery of the hypostase, covered only by the ou ter epi­

derm is, merges wi th the integumentary seed coat. A well-devel­oped cuticula r layer (Figure 7), comprisi ng the cuti cles of the inner epidermis of the inner integument as well as the nuce lla r epidermis, fo rms the innermost part of the integumentary seed coat (Figure 7). At maturity the latter consists of the endotegmen, namely the diffe rentiated inner epidermis of the inner integu­ment, and very compressed, minute remnants of the outer epider­mis of the inner integument as we ll as remnants of the who le outer integument. Radial and tangential cell walls of the endoteg­men are secondarily thickened, lignified , and abundant ly pitted (F igure 7). The well-developed cuticular layer, which merges into the hypostase, as well as the cutinized ce ll walls of the hypostase, fo rms a continuous lip idic layer protecting the who le, phys iologically mature embryo against dehydration.

At the anti-raphal side the endotegmen abuts directly on the cellula r endos perm remnants ( to the inside) and the endocarp (on the outs ide) (F igure 5). In Smoding illlll, in contrast to other taxa of the tri be Rhoeae. the extent of the endosperm remnants is more sign ifi cant , si nce at the anti-raphal side o f the seed they are practi ca ll y absent in e.g. Rhus, 0 201"00, Prolorhus Eng!. and Heeria Meisn. (Pienaar & Von Teichman 1998; Von Teichman 1991a & c, 1993, 1994; Von Teichman & Van Wyk 1996). According to Boesewinkel and Bouman ( 1984 ), seeds are called exalbum inous or exendospermoLis if the endosperm is (almost) completely used up by the developing embryo. In Smodingiulfl the remnants a re more extensive and the seeds may therefore poss ib ly be called semi-exalbuminous. Th is surely represents a continuum, as the presence o f endosperm in Smodingilllll is the intermediate form . Functionally the lipids with in these endosperm remnants poss ibly also serve to prevent the.

125

dehydration of the cmbryo. The white. bent embryo is laterally com pressed. Its cotyledons

consist of parenchymatolls epidermal ce ll s which are distinctl y smaller than the sto rage parcnchyma ce ll s. In the centre of this storage tissue is a row of abOllt five secretory ducts. Protein bod­ies. which fill al l these ce ll s, and also fats const itute the food reserves in this semi- cxalbuminous seed (Figures 6 and 7). This small, part ially pachychalazal seed, with lipids as main food reserve, is also typ ical of several other indigenous or even endemic species of the Anacardiaceae from savanna or extremely arid desert regions, where plants are periodically sub­j ec ted to drought. These species, in contrast to those with large, pachychalazal, recalcitrant (a lso exalbumillous) seeds which mainly store starch , are considered more derived . Large seeds characterize, amongst others, Heeria argen/ea, Prolorlllls long;­folia and Mangifel"a indica. T he results obta ined in this study therefore support the hypothesis of Von Teichman and Van Wyk ( 1994) that pachychalazy with associated greater seed size and the recalcit rant seed viabi lity behav iour may be regarded as ancestra l character states in dicoty ledons.

Notes on seed germination

When whole fruit were planted in sand, the seeds started to ger­minate after seven days. As shown in Figure 8, germination in 5'modillg iul11 is epigea l. The same app lies to Rhus lancea. Loxo­slyh.') alala Sprcng.f. ex Rchb. and Lalll"ophy llus capel1sis Thunb . (both also dispersed by wind) , S'c lerocwya birrea (A. Rich.) Hochs!. subsp . carfia (Sond.) Kokwaro (the manila), Harpephyl­lum cc{(frum Bernh . ex Krauss and O=oro(/ prmiclIlosa (Sand.) R. & A. Fern. On the other hand, Ifeeria argentea and Prolorhus

Figure 7 Another part of the ts shown in Figure 5, at higher magnificat ion. Abbreviations as in Figure 5, wi th the white arrow head points to the cu ticular layer. Scale bar = 50 j..l tn.

126

Figure 8 Seedling of Smodingillln grown in a plastic bag with a sandy so il mixture~ with the cotyledons distinctly above lhe surfl!.:e oj' the so il. Scale bar = 10 mm.

/()l1g~(o/ia (with large, pachychalazal and recalcitrant seeds) and Lmmea discolor (with small , partially pachycha lazal seed) ger­minate hypogea lly (Von Teichman & Logischen 1988). The small seeds of Smodingium, Rhus and O=oroa, store mainly lip­ids, comprisi ng a higher energy reserve. SmodiJlgilllll usually produces abundant seeds which germinate eas il y, and since the fruits are very effecti vely dispersed by wind, the su rvival of the species is ensured. It has been reported that small , light, wind­dispersed fruits are often associated w ith epigeal germination. T his is indeed the case in Smodingiwll, Loxosry/is a/ara and Lau­rophyfflls capensis. On the other hand, the larger, heavier, ani­mal-distributed seeds usually exh ibit hypogeal germination (P.

langiro/fa and Ii. argenlea). However, the an imal -dispersed L. (/;scofor, the manila and H. cafrrum do not fit into this general­ized scheme (Von Teichman & Logischen 1988).

Conclusions Smodingillm is not only easily distinguished from Rhus species (especia lly R. leplodiclya Diels) by its pendulous ovule, 'apical ­lateral ' styles, winged fruit with persistent sty les and calyx, and sap which turns black on exposure to air - bu t al so by the follow­ing: an inflorescence which is a determi nate (double) thyrse, not thyrso id ; hairs on the pedice l, bracteoles and ca lyx , which may possibly di ssipate heat by scattering/reflecting the light; shape of the ovary; and a unique pericarp structure. Fruit and seed charac­ters are relevant ecologica ll y since they represent adapt ive fea­tures which cont ribute to the survival of indigenous/endemic species in their natural habitat. Adaptation can be circumscribed as any heritable character or a set of such associated characters, which increase the probability that the specific individual can survive or rep roduce more successfully under its speci fic envi­ronmental condit ions (N iklas 1997). Obviously of crucial impor­tance for the survival of Smoding;um are the following characters: fruit dispersal aided by the structure of the samaro id drupe; easy germination; the mentioned ind umentum ; the well­deve loped cut ic les on the calyx, ovary wall and later on the exo­carp, and the cuticular laye r de limi ting the mature integumentary seed coat as well as the impregnation of the cell walls of the hypostase cells with cutin and callose in the physio logica ll y matu re seed.

S. AIr. J. But. J'!9X. 64(2)

Acknowledgements

Financial support from the Universi ty o f Pretoria is gratefu lly acknowledged. Sincere thanks arc due to the staff o f the Pretor ia National Botan ical Garden for permiss ion to collect the plant mater ial ; to Petro Pienaar and Rone li Visser for technical ;tss is­lance; to Emsie dll Plessis for c ri tica ll y reading and improving the manuscript.

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