Notes on the genus Frithia ( Mesembryanthemaceae ) and the descrip tion of a new species , E humilis , in South Africa

Frithia N.E.Br (Mesembryanthemaceae), formerly thought to be a monotypic genus, has been found to comprise two species. Populations from the eastern parts of the distribution range of Frithia pulchra N.E.Br. are recognised as a distinct species, Frithia humilis PM Burgoyne The genus has a limited distribution, although present in three provinces o f South Africa, namely Gauteng, North-West and Mpumalanga. The two window-leaved species are allopatnc and morphological differences between the roots, leaves, flowers, pollen, capsules and seeds are discussed. A formal description of the new species, an identification key and a distribution map of the two species are provided. INTRODUCTION AND HISTORICAL OUTLINE The genus Frithia (N.E.Br.) was first mentioned in a key by Brown (1925). At that time no species were assigned to the genus and only later was a full descrip­ tion of Frithia pulchra given (Brown 1926). The genus was named after Frank Frith (1872-1954), a railway ser­ vices gardener stationed at Park Station, Johannesburg. He was responsible for decorating railway platforms from 1900 until his retirement in 1932 (Kroon 1997). In 1906, Olive Nation sent a specimen of a plant she dis­ covered near Rustenburg to Brown at Kew for identifi­ cation. The live specimen did not survive the journey, but the remains were seen by Brown who regarded it as a distinct entity. After Miss Nation died, a search for more material proved unsuccessful. Some time later, a Mrs Dobie of Rustenburg sent plants to Frank Frith, who took the specimens to Brown at Kew while on a visit to London, to create the African garden at the Wembley Exhibition. Dobie’s specimen allowed Brown to finally describe the genus. De Boer (1968) published the name Frithia pulchra var. minor in the Dutch journal Succulenta, but as no type material was cited the name was invalid. Plants of this variety are generally smaller than those included in var. pulchra and arc restricted to the eastern parts of the distribution range of the genus. These differences were again alluded to by Hardy & Fabian (1992). Zimmermann (1996) confirmed the different characters of var. minor, but gave no formal description or type validating the varietal epithet. Although a number of short articles have been written on Frithia (Brink 1985; Germishuizen 1975; Steffens 1988; Venter 1979, 1983), no in-depth study has been done on the genus. This paper reports on the taxonomic status of the genus, and specimens from the eastern parts of its distri­ bution range are formally described as a new species, Frithia humilis. * National Botanical Institute. Private Bag X 10 1, 0001 Pretoria ** Department of Botany, University of Pretoria, 0002 Pretoria MS received 1999-03-15 TAXONOMY To prevent possible confusion with the illegitimate name Frithia pulchra N.E.Br. var. minor de Boer, the varietal epithet minor is not used at the specific rank. The name of the new species is derived from the Latin humilis, which means ‘smaller than others of its kind’.

Perennial, dw arf succulent with fleshy roots branch ing laterally; plants retracting into sandy soil by means of contractile leaves shrinking lengthways during dry win ter months, leaving holes marking their presence.Stems single, short, up to 10 mm long.Leaves arranged spiral ly, dull green to brown-green with a purple tinge in dry winter months, highly succulent, up to 15 mm long, cylindrical, covered by waxy idioblasts arranged in dis tinct rows, tips windowed, with concave centre and con spicuous crenulate markings along perimeter.Flowers single, 15-20 mm diam., white or very pale pink, with yellow centre, opening during mid-morning to mid-afternoon.Pollinated flowers turn yellow or salmon-orange.Sepals 5, unequal, resembling leaves, united to form a short tube.Petals 20-30 per flower, arranged in several whorls, tips mostly acuminate, sometimes rounded.Staminodes petaloid to filiform, surrounding several whorls of stamens.Hypanthium formed by fused bases of petals, staminodes and stamens.Nectaries 5, free, dark green, crenulate.Ovary slightly conical above; stigmas 5 or 6 , very short.Fruit a capsule, 5-or 6-locular, barrel shaped, very fragile, breaking up when ripe and then dis persing seeds; valves opening to an upright position, valve wings absent, valve margins recurved when fully open; expanding keels light yellow-brown, parallel, with diverging tips and torn margins; covering membranes reduced to a ledge; closing bodies absent.Seeds redbrown, small, covered by small tubercles.Flowering time: D ecem ber-February (summ er in the southern hemisphere).
As the formerly monotypic genus Frithia now has two species, the type species of the genus is Frithia pulchra N.E.Br.

SPECIMENS EXAMINED
All specimens held at PRE.

DISTRIBUTION AND HABITAT
Frithia is one of the few genera in the Mesembryanthemaceae exclusive to the summer rainfall region of South Africa.Other mesemb genera with a distinctly summer rainfall distribution include Neohenricia, Mossia and Khadia, while Delosperma, Hereroa, Lithops, Chasmatophyllum, Nananthus and Stomatium may occur in summer rainfall areas but also have wider distribu tions.
Previously thought to be a monotypic genus and a Magaliesberg endemic, an enlarged Frithia still has a restricted distribution.Populations of these miniature window plants have been found in two disjunct regions, in the North-West between Rustenburg and the Hartebeespoort Dam in the west, and in an area between Bronkhorstspruit (Gauteng) and Witbank (Mpumalanga) in the east (Figure 1).The two areas are roughly 150 km apart, and so far, no specimens of either species have been collected in the intervening area.
Both species of Frithia grow in very shallow soils derived from coarse sedim ents: quartzites of the Magaliesberg Formation of the Pretoria Group of the Transvaal Supergroup in the case of Frithia pulchra and sandstones of the Irrigasie Formation of the Ecca Group of the Karoo Supergroup for F. humilis.Rocks in both areas are very rough, porous and weather to form a very coarse gravel.
Frithia pulchra mostly grows exposed on rock plates, the roots anchored in cracks between the coarse quartzites.This substrate reaches very high temperatures in summer.Plants are also found in coarse gravel and are not restricted to rocky outcrops.F. humilis grows pre dominantly in shallow sand along the rims of large, flat, rock plates.Temperatures of the substrate are probably lower as more organic matter is present, insulating the plant bodies against heat and dessication.
Both species grow at altitudes ranging from 1 368 m to 1 616 m, and rainfall varies between 700 and 800 mm per annum.Winters are cold and dry and severe frost occurs in the areas where the plants grow.Other species often associated with both species of Frithia are the fern ally Selaginella dregei and the legume Indigofera melanadenia.Species sometimes found associated with either species of Frithia are the succulents Anacam pseros subnuda subsp.subnuda, Crassula lanceolata subsp.transvaalensis, C. setulosa var.setulosa and Mossia intervallaris.Monocots like Microchloa kunthii, Anthericum calyptocarpum together with an extremely minute and monophyllous species of Drimia, are also found in these habitats.The habitat of Frithia pulchra tends to be drier than that of F. humilis, the soils where the latter grow, having a higher organic content, sometimes resembling peat, and thus retaining moisture better.

FRITHIA IN HORTICULTURE
Frithia pulchra and F. humilis differ widely in their horticultural history.F. humilis was introduced to the Dutch seed trade by de Boer thirty years ago and a few of the plants dating from that introduction are still alive.This species is obviously quite tolerant and it responds to water more eagerly than does F. pulchra.The latter has been in continuous cultivation since the late 1920's but the plants are usually not long-lived, easily succumbing to rot.Both species can mature in a few months from seed under favourable conditions, and in this respect they are typical of a Delosperma alliance.The two species readily hybridize (S.Hammer pers.comm.)producing fertile offspring, with a variety of flower colours includ ing orange and bright pink.Many attempts made by Hammer to hybridize Frithia with other genera (Delo sperm a, Drosanthemum, Dorotheanthus, Lithops and Fenestraria) have always failed, not even producing the 'dum m y' (empty) fruits which often result from such dis junct liaisons.

Habit
Both species are dwarf perennials with thickened roots.The stems are much reduced and during periods of drought the plants retract into the sandy soil.This has been ascribed to contractile roots, but no such roots are present in either species.However, in Frithia the cells of the leaves are arranged in columnar, axial rows and when moisture is lost and the cells shrink, the tangential walls contract.Artificially induced dessication using silica gel, indicated that a leaf may contract to up to one third of its length (Figure 2B).This causes the plants to retract into the soil, a mechanism which renders protection to the plants during times of drought (Figure 3C).Retraction into the ground is thus achieved by means of 'contractile leaves', not contractile roots.

Roots
Roots of Frithia pulchra differ from those of F. humilis in being more fibrous, possibly because of the drier conditions prevailing in its habitat and the strategy to insulate the plants against the heat of surrounding rocks in summer.

Leaves
Borne spirally, the leaves of both species are cylindri cal with windowed tips and are covered by an epidermal layer of waxy idioblasts arranged in distinct rows (Figure 2A).Leaves in adult plants of F. pulchra are longer (15-25 mm) than in F. humilis (shorter than 15 mm ).Leaf colour also varies slightly between the two species, those of F. pulchra having a bluish tinge, whereas those of F. humilis are tinged brown or purple.The windowed tips of the leaves, however, differ conspicuously between the two species.Windows of F. pulchra are convex when turgid, slightly concave when flaccid (Figure 4A) and those of F. humilis are concave even when turgid, with crenulate markings (Figure 4B) along the margins.These leaf differences were also noted by Zimmermann (1996).

Flowers
Flowers in both species are borne singly and on very short stalks or are stalkless.They are subtended by five unequal sepals closely resembling the cylindrical leaves.Flowers of F. pulchra are bright magenta with a white or light yellow centre and are 25-35 mm in diam.Those of F. humilis are white with a yellow centre, sometimes tipped with pale pink, and are generally smaller (15-20 mm diam.).The petals number between 30 and 45 in F. pulchra and tend to have blunt, rounded tips, whereas those of F. humilis number between 20 and 30 and usu ally have acuminate tips (Figure 3A, B). attached to the funicle (hilar end) has a sharp point, whereas in F. humilis it is more rounded.The length of the micropylar regions appears to be similar in the two species.The surface sculpturing is irregular in both species of Frithia (Figure 7B, E) and although there are no microbaculae present, the surface of the epidermal cells differs markedly between the two species (Figure 7C, F), that of F. pulchra being rough-textured whereas the cell surface of F. humilis is smoother.

Pollen
Pollen in both species of Frithia is yellow.The grains are tricolpate and simplicolumellate in F. pulchra (Punt et al. 1994) with a perforate surface and lumens of dif ferent sizes.Pollen in F. humilis has a perforate surface, and is pluricolumellate, with lumens of more or less equal size (Figure 5).

Fruit
Fruits are hygrochastic capsules, the shape resem bling a barrel.Thick tissue surrounds the capsules of F. pulchra, whereas that of F. humilis is more fragile (Figure 6C).However, this character is not constant for the two species and seems to vary with environmental conditions.Capsules of both species tend to break up shortly after ripening.In both species, there are five or six locules, no valve wings and no closing bodies.Expanding keels are parallel with divergent tips (Figure 6 ), and are dark brown in F. pulchra and lighter brown in F. humilis.Covering membranes are reduced to a ledge in both cases.

Seeds
From Figure 7 it can be seen that the seed of F. pul chra is quite different from that of F. humilis.In F. p u lchra (Figure 7A) the end where the seed has been

NEAREST RELATIVES
The position of Frithia within the Stomatium Group proposed by Hartmann (1998) has always been tentative because of the outlier geographical distribution range and unique leaf characters displayed by the genus.Perhaps the most unusual feature is the spirally arranged leaves, a unique character within the subfamily Ruschioideae Schwantes in Ihlenf., Schwantes & Straka (1962) emend.Bittrich & H.E.K.Hartmann.Superficially, plants of Frithia resemble the genus Fenestraria N.E.Br., also with windowed leaf tips.However, Fenestraria occurs in northern Namaqualand and Namibia and no other char acters are shared by the two genera.
As stated by Hammer (1998), Frithia has characters in common with Delosperma, but major differences still separate the two genera.One of the similarities is the leaf epiderm is which is covered by opaque idioblasts.Idioblasts of Delosperma deilanthoides S.A. Hammer (1998)   coarse lithology) to those of Frithia, but the Steenkampsberg receives a higher rainfall (± 1 200 mm per annum).Flowers of both genera belong to the white/pink colour range and open from mid-morning to mid-after noon (Smith et al. 1998).However both flower colour and the presence or absence of epidermal idioblasts are considered to be pleisiomorphic and are not suitable to indicate relationships.
The spiral leaf arrangement, not opposite as in other mesembs, gives rise to the interpretation that Frithia may have retained this primitive character (spirally arranged leaves are regarded as primitive) while developing advanced states in other characters.A more likely expla nation may be that the spiral leaf arrangement found in Frithia is a derived feature, since it is not present-in any genus holding a more basal position within the M e sembryanthemaceae.Based on this data it could be assumed that Frithia may be considered a highly spe cialised 'Delosperma The leaves of Neohenricia sibbettii (L.Bolus) L.Bolus most closely resemble those of Frithia (Figure 2C) in shape, but the leaf surfaces of these two genera differ markedly.Leaves of Neohenricia are covered by wart like crustose epiderm al outgrow ths, with opaque idioblasts scattered among them.Leaves of Neohenricia are opposite, whereas those of Frithia are arranged spi rally.Capsules of the two genera are similar except that those of Neohenricia have four to six locules (four locules being the norm) and are shallow; those of Frithia are five-or six-locular and barrel-shaped.Moreover, the capsules of Neohenricia are borne on a remarkably thin pedicel and stand above the mass of leaves, whereas those of Frithia are buried within the leaves on a short pedicel and tend to be expelled via leaf pressure when ripe.However, flower colour and morphology differ con siderably.Flowers of Neohenricia are pale yellow, borne on long pedicels and have thin spiky petals, opening in the mid-afternoon to evening.Nectaries are in the form of a glandular ring in Neohenricia, whereas those of Frithia are free.Flowers of both genera have very short stigmas with the staminodes and stamens that are fused, almost forming a hypanthium.The distribution ranges of these two genera do not overlap.
Further investigation is being done to examine the relationship (if any) between Conophytum limpidum S.A. Hammer and Frithia (Burgoyne in prep.) as they have some characters in common: a hypanthium is pre sent; the petals, petaloid staminodes and anthers are comparable; windowed leaf tips present; fruit capsules can be compared as closing bodies; covering membranes are absent.Both are summer rainfall mesembs.

CONSERVATION STATUS
Use of the IUCN red list of categories (IUCN Species Survival Commision 1994), indicate that both species of Frithia should be regarded as Vulnerable, as the total area that they occupy is less than 100 km2.Although the areas where these species grow are not in any immediate danger of being destroyed because they are too rocky, the limited distribution poses the risk that human activity could wipe out a large part of the populations should their habitat be used and transformed in the future.One locality of F. humilis is situated at the edge of an infor mal housing development, but the habitat is so unsuitable for any utilisation by man that it has remained largely undisturbed except for littering.The conservation status of F. pulchra is more secure, as a large part of the popu lation is situated in the Rustenburg Nature Reserve.All other areas where populations of both species of Frithia grow are in the hands of private land owners, many not even aware of the presence of these tiny plants.
Unscrupulous succulent collectors may pose the greatest threat to populations of Frithia.Further population stud ies of both species of Frithia will be undertaken and their new conservation status will be determined (Burgoyne, Krynauw & Smith in prep.).

FIGURE 4 .
FIGURE 4.-Surface o f leaf tips o f Frithia: A, F. pulchra showing convex tips and no markings; B, F. humilis with concave cen tre and crenulate markings on margins.A, Burgoyne 6699c; B, Burgoyne 6694b.Scale bars: 1 mm.

Burgoyne 6692, 6693, 6694, 6694b, 6696, 6698. 6699 (2), 6699b. 6699c ( 1).
most closely resemble those of Frithia and are also arranged in rows.Capsules in both genera lack cov ering membranes and closing bodies.Distributions of these two genera overlap.The distribution of Delo sperma deilanthoides is centred in the Steenkampsberg, Mpumalanga.It has similar habitat requirements (sandy, well-drained soil with a high organic content and porous.