A review of generic concepts in the Stilbaceae

The generic concepts in the Cape endemic family Stilbaceae (± 14 species), are reviewed. Proposals by various authors to include Retzia capensis Thunb. in the Stilbaceae are supported. Xeroplana Briq. and Eurylobium Hochst. are shown to be congeneric with Stilbe L.. resulting in one new combination and one new name in Stilbe. Seven currendy recognised species o f Stilbe are enumerated. A key to the genera of the Stilbaceae, as presently understood, is provided. A new genus Kogelbergia is described to accommodate two species previously assigned to Stilbe sect. Amphistilbe and two new combi­ nations in Kogelbergia are proposed


INTRODUCTION
The Stilbaceae Kunth, a small family in the Lamiales.is endemic to the southern Western Cape.It is variously regarded as a subfamily within the Verbenaceae by some authors (Cronquist 1981) or as a family by others (Dyer 1975;Dahlgren 1980).The entire family consists of about twelve currently recognised species arranged in six genera.
Four of these (Eury lobium Hochst., Campylostachys Kunth.Euthystachys A.DC. and Thesmophora Rourke) are monotypic.While Campy lostachys and Thesmophora are defined on sound morphological characters, Eurylo bium and Xeroplana Briq.(two species) are here consid ered to be congeneric with Stilbe L. On present evidence Euthystachys seems sufficiently distinct to be upheld as a monotypic genus but further studies of fresh material from a range of populations are required before an ade quate assessment can be made.Over and above these genera there is material of at least one and possibly two further undescribed taxa represented in local herbaria so it is unlikely that the final species count for the family will exceed fourteen.

INCLUSION OF RETLIA IN THE STILBACEAE
Before discussing these generic concepts any further, the position of Retzia Thunb.must also be considered in any review of the Stilbaceae and its relationships.
Retzia capensis Thunb., a monospecific endemic Cape genus, has been assigned to a wide range of families since it was described by Thunberg in 1776 (see Dahl gren et al. 1979, for a detailed review of its past taxonomic history).It has also been treated as a monospecific fami ly (Dyer 1975).Similarly, Cronquist (1981) upheld the Retziaceae as a monospecific family, dismissing any relationship with the Stilbaceae largely on account of the placentation and 'organisation of the gynoecium' in Retzia.In a similar view.Takhtajan (1997), while acknowledging that Stilbaceae and Retziaceae are close ly related, continued to uphold both as separate families.However, modern data from several different sources now indicate conclusively that Retzia is most closely allied to the Stilbaceae.
In a comparative study of iridoid glucosides in Stilbe ericoides L. and Retzia capensis, biogenetically similar compounds were found in both species (Dahlgren et al. 1979).These authors further pointed out that morpho logical and anatomical evidence also suggested a close relationship between the two families.An extensive investigation of the wood anatomy of several genera in the Stilbaceae has further highlighted the similarities between Retzia and the Stilbaceae (Carlquist 1986).More recently, rbcL sequences for Euthystachys (Stilbaceae) and Retzia were found to be very similar, 'differing by only eight substitutions', (Bremer et al. 1994), again confirming the close relationship between the two families.Thus morphological, anatomical, phy tochemical and molecular evidence supports the inclu sion of Retzia in the Stilbaceae.
In an updated phylogenetic classification of the flow ering plants.Thome (1992) incorporated Retzia into the Stilbaceae, dividing the Stilbaceae in two subfamilies, Retzioideae and Stilboideae.This view is followed in the present treatment and is, similarly, the current opinion of the Angiosperm Phylogeny Group (1998).

STILBACEAE SUBFAMILY STILBOIDEAE
All members of the the Stilbaceae subfamily Stilboideae are small, inconspicuous ericoid shrublets with very small white or pinkish flowers.Although typical components of the Cape fynbos they are never dominant and tend to be easily overlooked.Several taxa are rare localised endemics, and some are possibly palaeoendemics.In almost all cases the flowers are extremely small (shorter than 10 mm) which has made the dissect ing and interpretation of herbarium material very diffi cult, often leading to serious misinterpretations of the flo ral structures (Rourke 1977).During the course of revis ing the family I have been able to examine a wide range of fresh living material collected in the field which has greatly clarified an understanding of the floral structures.This review of the generic characters within the fami ly has led to an expanded concept of Stilbe which will be discussed here and also the exclusion from

CURRENT GENERIC POSITION
The problem of finding a rational generic classifica tion of the Stilbaceae has been known for some time (Dyer 1975;Rourke 1977).In this review an interim solution is provided to satisfy the immediate needs of two generic treatments of the family for 'Seed plants o f southern Africa: fam ilies and genera' and K. Kubitski's 'The fam ilies and genera o f vascular pla n ts', currently in progress.A major taxonomic difficulty concerns the pre sent circumscription of Stilbe, Xeroplana, Eurylobium and Euthystachys.Stilbe L.
Two distinct groups can be recognised within Stilbe based on the symmetry of the corolla and the presence or absence of pubescence on the corolla lobes.
Stilbe verticillata and S. phylicoides A.DC. have actinomorphic corollas and densely pubescent corolla lobes of equal size [Stilbe sect.Amphistilbe (Pearson 1901)1.The ovary in both species is single-chambered (but has mar ginal septum remnants), with two basal ovules.
The remaining species are characterised by promi nently bilabiate flowers with two broad, erect posterior corolla lobes and three narrow anteriorly deflected corol la lobes, all of which are quite glabrous [Stilbe sect.Eustilbe (Pearson 1901)].The sepals are fused to form a tubular, usually glabrous calyx.The ovary is two-cham bered with a single ovule in each chamber.At anthesis one of the ovules and its locule aborts, leaving a single ovule in a single chamber.
It is proposed that the present broad circumscription of Stilbe be emended to encompass merely Stilbe sect.Eustilbe and that the two species currently placed in Stilbe sect.Amphistilbe be removed to a new genus, herein described as Kogelbergia (Figure 1).
In Xeroplana the corolla is strongly bilabiate with two broad, erect posterior petals, and three narrow anterior petals.The corolla lobes are glabrous.Initially the ovary is two-chambered with a single ovule in each chamber, but the abaxial chamber and ovule soon abort, leaving a single adaxial chamber with a solitary basal ovule.These similarities with Stilbe sect.Eustilbe have already been noted; moreover, it has previously been pointed out that the generic status of Xeroplana is questionable (Rourke 1977).On the basis of gross morphological evidence Xeroplana and Stilbe sect.Eustilbe are congeneric.Consequently, as no rational justification can be found to uphold Xeroplana, it is proposed that it be merged with Stilbe sect.Eustilbe under a modified concept of Stilbe, after having removed the two species in Stilbe sect.Amphistilbe to a new genus, Kogelbergia.

Eurylobium HochsL
Vegetative and floral characters in Eurylobium are the same as in Stilbe sect.Eustilbe, namely, a tubular corol la, strongly bilabiate with two large, erect posterior lobes and three narrower, anteriorly deflected lobes.The corol la lobes are glabrous.The sepals are fused for V4 of their length to form a glabrous, actinomorphic calyx tube.Initially the ovary is bilocular with an ovule in each chamber but at an early stage one of the ovules aborts, leaving the ovary to become unilocular with a single, functional, basally attached ovule.As these characters are the same as in Stilbe sect.Eustilbe, there is no justi fication for upholding Eury lobium as a monotypic genus.
The single species in this monotypic genus, E. abbreviata A.DC. is distinguished by the following characters: soft, pubescent sepals fused for about '/4 of their length at the base; an actinomorphic corolla of five, glabrous, equal corolla lobes and a bilocular ovary with a single functional ovule in each chamber.
These characters, though slender, provide sufficient justification for maintaining Euthystachys as a mono typic genus at this stage.Nevertheless, there are some differences in the degree of fusion of the sepals in certain collections.Further field work and the study of a range of fresh material is required before the generic status of Euthystachys can be adequately assessed.
Under the revised generic concept of Stilbe discussed above, two genera in the family, Xeroplana Briq.and Eurylobium Hochst.fall clearly within the emended cir cumscription of Stilbe and must be reduced to synonymy as indicated below.

KOGELBERGIA. A NEW GENUS IN THE STILBACEAE
As previously mentioned, it is proposed that the two species in Stilbe sect.Amphistilbe be removed to a new genus, Kogelbergia.Its distinguishing characters are dis cussed here.

Inflorescence structure
Superficially the inflorescence appears to be a sessile, globose capitulum .However, careful dissection of K. verticillata indicates that it is paniculate in structure with several highly condensed axillary branches bearing 3 or 4 flowers.The inflorescence structure in all other Stilbaceae subfamily Stilboideae is spicate.It seems probable that although much reduced and condensed, the paniculate inflorescence of K. verticillata represents a less specialised condition than the racemose pattern in the rest of the Stilbaceae.In K. phylicoides the inflores cence is a condensed spike with no trace of a paniculate structure.

Corolla
Although the corolla is actinomorphic at anthesis con sisting of 5 equal, linear corolla lobes, as the corolla opens, two lobes briefly assume an erect (posterior) posi tion with the remaining three assuming a patent (anteri or) position.This condition can only be briefly detected by observing developmental stages in living material and is soon lost as the flower matures.It seems probable that actinomorphy in the corolla is a plesiomorphic condition with bilateral symmetry and a bilabiate corolla repre senting the derived condition.
The presence of dense villous pubescence on the lin ear corolla lobes of Kogelbergia is a character highly unusual in the family.All other Stilbaceae except Thesmophora and Retzia, have completely glabrous corolla lobes.This pubescence is formed by the presence of 1-2 mm long white trichomes on the inner surface, apex, and outer surface of each corolla lobe.Like almost all other Stilbaceae, Kogelbergia has a distinctive ring of pubescence in the throat of the corolla.Only three other species in the entire family lack this feature.The dense pubescence on the apices of the corolla lobes appears to be unrelated to the throat pubescence and is regarded here as a completely new character.

Ovary
Typically the ovary in Stilbaceae subfamily Stilboideae is bilocular with a single ovule in each cham ber, or initially bilocular but with one abortive ovule in an atrophied or abortive second locule leaving a single functional ovule to develop.In Kogelbergia the ovary is unilocular with two basal ovules.However, serial sec tioning of the ovary shows clear remnants of a septum in the position where this would normally be expected to be (i.e. at right angles to the axis).The septum breaks down very early in the development of the ovary leaving a unilocular condition which is clearly derived (Figure 2).

Conclusion
Fundamental morphological differences exist be tween the two species previously known as Stilbe verti cillata and Stilbe phylicoides and other members of the genus Stilbe (Table 1).These are considered sufficient to justify separate generic status.Accordingly, a new genus Kogelbergia is described to accommodate these species.Part of the distribution area of one of these species falls within the Kogelberg Nature Reserve, one of the centres of the highest endemism in the Cape Floristic Kingdom.
The genus Kogelbergia is distinguished from all other genera by its dense, globose, sometimes inconspicuously branched (apparently paniculate) or spicate inflores cences, the actinomorphic corolla, 5 equal petals densely pubescent at their apices, and the single-chambered ovary with 2 basal ovules.Two species are currently recog nised.

Distribution, habitat and biology
Endemic to the Langeberg Range in the southern Western Cape, Kogelbergia phylicoides occurs sporadi cally on the upper south slopes in mesic mountain fynbos between the Clock Peaks at Swellendam and the Robin son Pass near Mossel Bay (Figure 4).Populations are mostly small, usually consisting of less than 12 individ uals, and are generally found at elevations between 425 and 1 100 m.Flowering tim e: October and November.E W H 1 3 3 2 .

FIGURE 1 .
FIGURE 1.-Diagramatic representation of the differences in corolla structure and pubescence between A, Stilbe and B, K ogel bergia.

FIGURE 2 .
FIGURE 2.-Kogelbergia verticillata; cross section through base of ovary showing single chambered ovary with two ovules, x 77.Note remnant of ovary septum placed at right angles to inflor escence axis.Two anterior sepals at base o f calyx tube slightly larger than posterior sepals.
Stilbe of two species previously known as Stilbe mucronata N.E.Br.[more recently as Stilbe verticillata (Eckl.& Zeyh.)Moldenke] and Stilbe phylicoides A.DC.It is proposed that they be placed in a new genus, Kogelbergia Rourke.

TABLE 1 .-Summary o f major morphological differences between Stilbe and Kogelbergia Kogelbergia Stilbe Corolla actinomorphic at anthesis, o f 5 equal petals Corolla lobes densely pubescent Inflorescence a condensed globose panicle or spike Ovary single-chambered with two basal ovules Pollen exine striato-reticulate (Raj 1983) Corolla strongly zygomorphic, bilabiate with 2 erect posterior petals and 3 patent anterior petal limbs. Corolla lobes not equal Corolla lobes glabrous Inflorescence a spike Ovary two-chambered with a single ovule in each chamber, one ovule/chamber often abort ing Pollen exine rugulose, tectate (Raj 1983) 1983
).Instead of having a rugulose exine (as in Campylostachys) or tectate-perforate exine as in the remain ing species in the family, the exine of Kogelbergia (the 'Stilbe mucronata type') has a striato-reticulate exine, set ting it apart from all its congeners (Raj 1983).