Taxonomic Groups

Agavaceae

Wendy Hodgson

The Agave family (Agavaceae Endlicher) is a group of economically important succulent plants with a natural distribution in the drier regions of the tropics and subtropics. The botanical limits of the family are undecided, but for the purposes of this Action Plan the Agavaceae is considered to comprise 18 genera and approximately 625 species. Many of these species remain poorly known in the wild. Herbaria and field surveys are urgently needed to determine conservation status for the species. Despite lack of detailed field information, progress has been made towards developing a conservation programme, particularly for the American species, for example through the SSC Agavaceae Action Plan Workshop held at the Desert Botanical Garden, Phoenix, in 1992.

Systematic treatment

The Agavaceae has undergone many changes since it was proposed by Endlicher in 1841. Cronquist (1981) maintains the Agavaceae as a family distinct from the Liliaceae and Amaryllidaceae based on the specialised growth habit. Agavaceae are characterised by being stout, simple or branched shrubs or trees, or herbaceous plants arising from a caudex, often forming succulent rosettes as opposed to Liliaceae and Amaryllidaceae which are herbaceous perennials, usually dying back to the ground. Yucca and Agave share the karyotype of 20 small and 5 large chromosomes. This trait was considered unique at one time and provided the incentive to remove Yucca from the Amaryllidaceae to the Agavaceae. However, the presence of few large and many small chromosomes is more common throughout the Liliaceae than once thought. Hosta, a genus with a very different habit and included within the Liliaceae, resembles yuccas and agaves in this respect while other genera now considered in Agavaceae (Nolina and Dracaena) do not. Cronquist (1981) considered the Aloaceae and Agavaceac as parallel derivatives from the Liliaceae with only slight differences.

McVaugh (1989) points out that unless some basis other than plant habit can be found, it is impossible to distinguish between the Agavaceae and Liliaceae in the Nueva Galicia flora of western Mexico. In Manfreda, Polianthes, and Prochynanthes, genera generally considered to be closely related to Agave, plants are herbaceous, commonly have fleshy roots developed from a short rhizome, and have soft, thin, slightly succulent leaves which in most species die back annually; marginal teeth, if present, are soft. McVaugh (1989) suggests one solution might be to include all herbaceous genera in the Liliaceae, and only the more woody groups in Agavaceae. However, the problem is more complicated in that there are obvious similarities (and presumed relationships) between Agave and Manfreda. In addition, Manfreda is closely related to Polianthes and Prochynanthes. He then justifiably asks that if Agave and Manfreda, and Manfreda and Polianthes, are so closely related that they could even be considered congeneric, then can Agave and Polianthes be assigned to different families?

Various subdivisions of the family have been made. Hutchinson (1934) divided Agavaceae into six tribes: Yucceae (Yucca and Hesperaloe), Dracaeneae (Cordyline, Cohnia, Dracaena, and Sansevieria), Phormieae (Phormium), Nolineae (Nolina, Calibanus, Beaucarnea, and Dasylirion), and Polyantheae (Polianthes, Prochynanthes, Bravoa, and Manfreda). Cronquist (198 1) also recognised 18 genera but conceded that so treated the Agavaceae may not form a natural group. Phormium and Doryanthes, usually included in this family, may not properly belong with other genera. Serologically Agave and Yucca seem to stand apart from a group consisting of

Box 1 .1 Botanical characteristics of Agavaceae

The Agave Family is characterised by stout, simple or sparingly branched, arborescent shrubs (or sometimes trees), or short-stemmed, somewhat herbaceous plants with a short rhizome or erect caudex; leaves simple, alternate, sessile, tending to be crowded in dense rosettes at ends of stems or branches or at ground-level on a short stem, generally thickened, leathery or firm-succulent (in contrast to soft-succulent as in Aloaceae), often prickly on margins and spine-tipped; flowers in dense racemes or panicles or heads terminating the stem (plants are monocarpic, as in Agave), or axillary and subterminal (plants are polycarpic, as in Yucca); perfect, sometimes unisexual, perianth consisting of tepals arranged in 2 whorls of 3, petaloid, often thick and fleshy, distinct or fused below to form a tube, stamens 6, filaments distinct, fused to tepals or base of tube, ovary superior or inferior, 3-carpeled, usually with nectaries style usually terminal with 3 stigmas, ovules l-many, fruit a loculicidal capsule or berry, seeds flattened; chromosome counts vary from x=16-30+.

Dracaena, Nolina, Sansevieria, and Cordyline. Dahlgren et al. (1985) recognise Agavaceae with eight genera subdivided into two subfamilies: Yuccoideae (Yucca and Hesperaloe) and Agavoideae (Agave, Manfreda, Polianthes, Prochynanthes, Beschorneria, and Furcraea), based on cytological, anatomical, and embryological studies.

Recent studies at the University of Texas, Austin, support recognising the Nolinaceae as a monophyletic group including, among others, Beaucarnea, Nolina, Dasylirion, and Calibanus (see Hernandez and Simpson 1992). Recent studies on the molecular and morphological phylogeny of the Agavaceae by David Bogler at the University of Texas support the treatment of Hutchinson and Dahlgren and the findings of Hernandez and Simpson. Preliminary work, which included chloroplast DNA restriction site analysis, suggests that two distinct major lineages occur (D. Bogler, pers. comm., 1994). The first group includes Hosta, Camassia, and Agavaceae. Within the Agavaceae are the seemingly closely related Agave, Manfreda, Polianthes, and Prochynanthes; Beschorneria and Furcraea appear more closely related to the more primitive Agave dasylirioides and A. striata, while Yucca appears to be more closely related to Camassia than to Agave. The second lineage includes the Convallariaceae, Dracaenaceae and Nolinaceae, the latter a monophyletic group including Nolina, Dasylirion, Calihanus, and Beaucarnea. In this group, Calibanus appears most closely related to Beaucarnea.

In this account, the Agavaceae is recognised sensu lato, following Cronquist's more inclusive, albeit unnatural treatment. Most scientists, at least in the USA, recognise this treatment and the Flora of North America includes Dasylirion in the Agavaceae. As defined here, the Agavaceae includes 18 genera and approximately 625 species.

Distribution and diversity

Naturally distributed through the American and Caribbean arid and tropical climes, agaves and their relatives have been widely dispersed since 1492 to other continents, where they are viewed as having considerable economic and horticultural importance. The origins of these genera and species appear to be related to the evolution of the Neotropical flora in the cordilleras of Mexico and Central America. Best known is the considerable adaptive radiation of the genus Agave that has occurred in the Sierra Madre Occidental of Mexico (Gentry 1972, 1982), undoubtedly associated with the development of the Madro-Tertiary flora (Axelrod 1958).

Through secondary adaptive radiation, members of the Agave family have adapted to habitats ranging from the temperate woodlands and prairies of eastern North America to the wet tropics and subalpine habitats of South America. Nevertheless, most species in the family occur presently in arid, semi-arid, and dry subtropical vegetation zones in both coastal plains and adjacent montane landforms. The predominance of CAM metabolism and succulence among these species allows them to remain dormant during extended seasonal droughts and periods of high temperatures. The extremely high water use efficiencies that have been recorded among agave cultivars (Nobel 1988) are also characteristic of the wild species and not simply a product of domestication.

Of the 18 genera in the Agavaceae, species richness is highest in the genus Agave (200+ species) and then descends in roughly the following order: Pleomele (140, Old World tropics and subtropics, if treated as distinct from Dracaena), Dracaena (50, Old World tropics), Sansevieria (50-60, South Africa, Madagascar, and Arabia), Yucca (35-40, south USA, Mexico, Guatemala, and Cuba), Nolina (25-30, south-west USA to central

Photography Madagascar Old

Collection of centre of Agave plant for fibres, Penita, Tamaulipas, Mexico.

Mexico), Manfreda (25, south and south-east USA to Honduras), Furcraea (c. 20, central Mexico to South America), Dasylirion (14-17, south-west USA to Oaxaca), Cordyline( 10-15, Old World tropics), Polianthes (including Bravoa 20, Mexico), Beschorneria (10, Mexico and north Guatemala), Beaucarnea (9, Mexico to South America), Hesperaloe (4, central Texas and north Mexico), Doryanthes (3, Australia), Phormium (2, New Zealand), Prochynanthes (1, Mexico), and Calibanus (1, Mexico).

The following centres of Agavaceae diversity have been identified by Gentry (1982), Reichenbacher (1985), and Garcia-Mendoza (1987, 1989): the Apachean woodlands and grasslands of the Deming Bridge between the Sonoran and Chihuahuan Deserts; the subtropical Cape region of Baja California; the northern and central Sierra Madre Occidental, including temperate and semiarid woodlands and dry subtropical thornscrub; the ecotone between the semi-arid highlands of central Mexico and the wetter Sierra Madre Oriental; the more tropical montane belt of ranges in Michoacan and adjacent states; the Chiapan and Oaxacan highlands of south-western Mexico and adjacent Guatemala, including semi-arid valleys in the rain-shadow of the Sierras, and adjacent humid uplands. Oaxaca has the greatest diversity of Agavaceae in the Mexican Republic with 42 taxa.

The species list of Agavaceae in Annex 1 indicates species which are currently considered to be narrow endemics, inhabiting zones of less than 100 km in length and width (10,000 km2). Here high levels of endemism in areas of high species diversity are expected. Indeed, of the 42 taxa in the State of Oaxaca, Mexico, 32 are endemic to the Republic of Mexico and six are endemic to the state. Hybridisation, polyploidy, and vegetative reproduction are important processes in species formation in Agave (Pinkava and Baker 1985), and probably other members of the family. For example, Yucca campestris, a sand-dune endemic of south-western Texas, may have originated from hybridisation between Y. elata and Y. constricta (Powell 1988). Peninsular floras such as that of Baja California also contain high levels of endemism. Of the 25 taxa of Agave that occur in Baja California, 20 are endemic (Gentry 1972).

Species richness is also affected by the local diversity and abundance of their pollinators; for example, Oaxaca has nine species of nectar-feeding bats specialising on Agave (A. Garcia-Mendoza and H. Arita, pers. comm.). The interactions between certain members of the Agavaceae, e.g. Agave, Manfreda, and Yucca, and their pollinators Leptonycteris and Tegeticula, have been widely discussed as classic examples of pollination syndromes between mutualistic species which generate and maintain certain aspects of the structure of their biotic communities (Webber 1953; Howell 1974; Arita and Martinez del Rio 1990). Several genera of nectar-feeding bats, bees, hummingbirds, wasps, moths, and hawkmoths may have co-evolved mutualistic relationships with some agaves; yucca moth co-evolution with Yucca species has also been documented in intricate detail. The population reduction or local extirpation of these pollinators may lead to reduced fertility and seed set in individual plants (Equiarte and Burquez 1988), but there is as yet no evidence that populations of long-lived polycarpic perennials have been severely reduced (Nabhan and Fleming 1992). However, reintroduction and translocation efforts of species in the Agavaceae are likely to fail if pollinators are no longer present in sufficient abundance to sustain populations.

Local uses and commercialisation

The Agavaceae is of considerable economic importance. Fibrous leaves provide cordage and are used in making mats, baskets, hats, thatches, paper, fans, sacks (Nolina, Dasylirion, Beaucarnea, Agave, Hesperaloe funifera, Phormium, Furcraea, Sansevieria, Yucca), flower arrangements (Agave), brooms (Dasylirion), and pack saddles (Furcraea). Trunks, stems, and flower stalks

Cut Agave lechuguilla, San Luis Potisi, Mexico.

provide food for humans and cattle {Agave, Dasylirion, Nolina) and alcoholic beverages (Agave, Dasylirion including D. cedrosanum, Yucca). Flowers of Yucca elephantipes are eaten by people in south-east Mexico. Fruits provide food for humans (the baccate-fruited species of Yucca, including the more restricted Y. endlichiana, Y. arizonica, Y. grandiflora, and Y. madrensis). Trunks provide fuel (Dasylirion), while trunks and old stalks are used for the construction of temporary shelters, posts, porches, and corrals (Agave, Dasylirion, Nolina, Yucca). Some taxa are grown as living fence rows (Agave and Yucca). Rhizomes (usually erroneously referred to as roots) and, to a lesser extent, leaves, were or still are used to produce soaps, shampoos and detergents (Yucca including Y. madrensis, Manfreda, Prochynanthes, Polianthes, Furcraea longaeva, and Nolina palmer-i, a Baja California endemic), as insect repellents (Prochynanthes), and as medicines (Aga ve, Yucca, Manfreda, Prochynanthes). Flowers are used in perfumery (Polianthes tuberosa, known only from cultivation). Flowers of Polianthes tuberosa are used as cut and garden flowers while other species of Pohanthes are sometimes sold as cut flowers in Mexican markets. Many taxa are now grown as ornamentals: Agave, Calibanus, Dasylirion, Doryanthes, arborescent Nolina, Hesperaloe, Yucca including the Joshua tree Y. brevifolia and Y. rostrata collected from the wild for landscaping, Manfreda, Pleomele, Polianthes, and Beaucarnea.

Rare and threatened taxa

It is difficult to determine whether specific taxa within the Agavaceae are threatened, or if so, to what degree. Despite recent field work, taxa are still poorly known or collected, particularly in south-central Mexico. In addition, interspecific hybridisation is a relatively common phenomenon within the family and there occurs much variability within and intergradation between

Machine preparing Agave fibres, Ecuador.

populations. As a result, defining taxa becomes more problematic and adds to the difficulty in determining rarity.

In the Republic of Mexico the native habitat for seven percent of Agave species is unknown, while 15 percent are known from three or fewer herbarium collections at the time of Gentry's (1982) monograph (Reichenbacher 1985). Approximately 50 percent of the entire genus is either poorly known, potentially rare, or occur as fairly restricted endemics (Annex 1), and therefore of considerable conservation concern. Of the 25 known species of Manfreda, 14 have limited ranges or few collections. Of these 14 species, six are known from six or fewer collections. Of the 13 species of Pohanthes at least nine have limited ranges within which populations are scattered (S. Verhoek, pers. comm. 1993). Until there is a revision of Furcraea identification of all species (with the exception of F. bedinghausii) is doubtful. Authors point

out that some Furcraea species have limited ranges, with some native to only a few Caribbean Islands. At the time of Sfandley's (1920) text, of the 22 or so species of Nolina, five were known only from their type localities and seven were endemic to a state or small part of a state. Likewise, of the approximately 17 species of Dasylirion, five were known only from their type localities and an additional six were endemic to one state. Subsequent studies of Dasylirion will result in taxonomic changes and range extensions for Mexican species; in addition, it appears that none of the (approximately 10) well defined species are rare (D. Bogler, pers comm. 1993), although they may still be endemic to one state. All members of the genus Hesperaloe are considered to have limited ranges, with the exception of H. funifera. Hernandez (1993) considers the whole genus Beaucarnea as threatened, with five species being particularly sensitive.

Regionally, Agavaceae taxa are often poorly known or limited in distribution. For example, over 30 percent of the agaves in Oaxaca are represented by only a few specimens. Little or no information is available for seven Agave species endemic to the Bahamas and they may be very rare. Within Guatemala, eight species of Agave, one species of Beaucarnea, and three species of Furcraea are considered to have limited distributions.

Recognised threats

Threats to agave family members can be classified as global (climate change, air contamination), habitat-specific (overgrazing, land conversion, competition with exotics), taxon-specific (vulnerability to introduced pests and diseases, loss of pollinators, economic overexploitation), and population-specific (collection by hobbyists, depletion for local, traditional uses). Because many species in the agave family are characterised either by few populations, low numbers of individuals per population, or both of these factors, they are vulnerable to environmental and demographic stochasticities. In addition, little information is available on the majority of species within the family thereby reducing the chances of appropriate management of their populations.

By far, the three most common pressures on Agavaceae taxa are: 1) land clearance and conversion of native vegetation for agriculture; 2) direct and indirect effects of overgrazing by livestock; and 3) overcollection for ornamental purposes, and to a lesser extent, fibre and alcohol. For example, the very beautiful Agave wercklei is endemic to the Rio Grande region on the Pacific slope of Costa Rica. At present, their populations are being adversely impacted by the development of coffee plantations (A. Mendoza, pers. comm. 1993).

Livestock, particularly goats, eat young plants and emerging flowering stalks which limits sexual reproduction and reduces genetic variation. Agave peacockii is known from only a small area in the vicinity of Tehuacan, Puebla, where Gentry (1982) observed only 4050 plants in the mid-1960s. Local people believe it has the finest, strongest fibre of any of the numerous agaves growing in the region, and use it for making rope and nets. Its use may account for its scarcity, but Gentry observed several flowering stalks bitten off by cattle. A similar situation exists for the fibre-producing A. potrerana in central Chihuahua.

Fortunately, with the availability of commercial alcoholic beverages, the desire and need to produce 'bootleg' alcohol from Agave and Dasylirion have become less important. There have, however, been dramatic local declines in Agave abundance in parts of Sonora where Agave angustifolia, A. palmeri, and A. shrevei have been intensively harvested for illicit mescal production (G. P. Nabhan,pers. comm. 1993).

Overcollection for ornamental purposes continues to pose serious threats to various genera within the Agavaceae. The fact that all species of Beaucarnea are considered threatened, mainly by overcollection, raises serious concerns. Illegal collectors have made it profitable for campesinos to collect material which is said to go to either the United States or Germany (Hernandez 1993). Hernandez (1993) points out that overcollection of seeds and seedlings are affecting natural repopulation of the species. As members of the genus are dioecious, overcollection of young and adult plants is changing the sex ratios and reducing the production of seeds, thereby lowering the size and genetic variability of the populations. The need to regulate the export of these and other overcollected plants is critical. The interest and demand for particular genera, including Agave, Hesperaloe, Yucca, and Beaucarnea, in the horticultural trade has increased dramatically over the last 10 years and is expected to continue. However, only five out of over 180 potentially rare or restricted taxa within the Agavaceae sensu Zato are listed in the Appendices of CITES.

Actions for conserving genetic diversity

There are four current activities which directly or indirectly conserve the genetic diversity found within this family: 1) efforts to resolve the causes of threats to habitat; 2) efforts to protect selected habitats where rare or otherwise threatened taxa occur through establishing biosphere reserves, sanctuaries, parks, or other protected areas where their populations can be maintained, or enhanced; 3) efforts to establish and enforce laws limiting the extraction from the wild and transport and commerce of rare, threatened, or vulnerable taxa; and 4) efforts to establish ex situ samples of the genetic variation of each species for propagation in botanical gardens, in vitro laboratories, or for storage as seeds or tissue cultured germplasm in gene banks. Only the Desert Botanical Garden, Phoenix, has developed a gene bank specialising in the Agavaceae, although over 40 botanical gardens contain CITES-listed endangered species from this family. The principal ex situ collections of the agave family occur at the Desert Botanical Garden

Box 1.2 Ex situ conservation of Agavaceae at Desert Botanical Garden, Phoenix

During 1991 and 1992, a systematic review of the Agavaceae collection was carried out at the Desert Botanical Garden as part of an Institute of Museum Services grant (Ecker and Burgess 1992). The review and subsequent report addressed I) the determination of the scientific value of the Agavaceae collection and verification of the identity of as many species as possible, 2) identification of rare species in the collection which were noted in the database, 3) creation of a priority list of species to be propagated, and 4) development of a Long Range Conservation Plan which identified species requiring propagation and/or acquisition. Strategies for propagation and collection of plants were developed on a species-by-species basis. A conservation plan outlining the status of each taxon in the Agavaceae collection and a plan for the long term conservation of each taxa were developed. Genetically representative seed collections have been established from several populations of Arizona taxa. Pollen is collected and stored from live plant material for controlled pollination. Seed produced from controlled cross-pollination is made available to other institutions via the Index Seminum program. Although still improving, the Desert Botanical Gardens ex situ program for Agavaceae can provide a template for other institutions considering the development of such a program.

(Arizona), the Jardín Botánico of the Universidad Autónoma de Mexico (UNAM) (Mexico City), and huntmgton Botanica'i Garden (California).

With regard to significant in situ conservation efforts which include rare or threatened populations of Agavaceae, the following UNESCO-recognised biosphere reserves contain several species: Rancho El Cielo, Tamaulipas; Bolsón de Mapimi, Coahuila; Big Bend, Texas; El Pinacate, Sonora; Isla Tiburón, Sonora; Desierto de Vizcaino, Baja California; La Michilia, Durango; Sierra de Manantlan, Jalisco; and Selva Lacandona, Chiapas. None of these protected areas, as far as is known, has developed action plans specific to managing and conserving local members of the Agavaceae.

A conservation strategy for the Agavaceae of central Mexico to northern South America was presented at the SSC Agavaceae Action Plan Workshop held at the Desert Botanical Garden, Phoenix in 1992 (Mendoza and Bye 1992). The strategy is incorporated into the Action Proposals in Chapter 4 of this Action Plan.

The author would like to acknowledge the following people who contributed to the preparation of this account for the Action Plan: Dr Edward F. Anderson, Alfonso V. Banuet, Miquel Chazaro Basenez, David Bogler, Dr Tony Burgess, Joe Clements, Liz Slausen (Ecker), Dr Richard Felger, Gary Lyons, Dr Bruce MacBryde, Enrique Martinez, Nora Martinez, Abisai Garcia Mendoza, Esther S. Monarque, Dr Gary Nabhan, Peggy Olwell, Sue Rutman, Dr Luis Hernandez Sandoval, Dr Susan Verhoek.

Aizoaceae

Steve Hammer

The Aizoaceae Rudolfi, the Mesembryanthema, is the second largest family of succulent plants after Cactaceae. The family is currently divided into five subfamilies, of which two, Mesembryanthemoideae and Ruschioideae, comprise the bulk of the succulent species within the family as a whole. The Mesembryanthemoideae and Ruschioideae form a clade known informally as Mesembryanthema sensu H.E.K. Hartmann. Only the plants of this group, known colloquially as 'mesembs', are considered here. The remaining three subfamilies have few, if any, true succulents and have thus escaped the collector's zeal, although they do include plants of agricultural importance.

For two centuries Mesembryanthema or 'mesembs' have been intensely studied both by professionals and amateurs, resulting in 'taxonomic chaos'. Now, through work at the University of Hamburg, some accord is being reached. As presently understood the clade (the former Mesembryanthemaceae Fenzl) comprises some 120 genera informally arranged into 15 groups (Hartmann 1991) based on fruits, flower, and vegetative characters.

Among the succulent subfamilies the two basic divisions, Mesembryanthemoideae and Ruschioideae, are separated by nectary form, by the axile or basal-parietal placentation of their gynoecia, and by different types of expanding structures in their fruits. Mesembryan-themoideae comprises perhaps 100 species in about 10 genera. Only a few of its species are suited to or are attractive under cultivation. In the wild, thev do have some importance for grazing.

Box 1.3 Botanical characteristics of Mesembryanthema

Mesembryanthema are characterised by a high degree of leaf succulence, and especially by their hydrochastic capsules (more or less persistent dried fruits, remarkable in structure and highly responsive to moisture). Other characteristics are the (frequent) possession of bladder idioblasts, bright petals (petaloid staminodes) of androecial origin meronectaria, well-developed valve wings in the fruits and a basic chromosome number of x=9. The monophyllum comprises mainly perennials, some biennials, and a few annuals. In their growth form, species range from tiny geophytes which produce only small tufts of barely visible leaves annually, to long-lived shrubs 3m tall.

Ruschioideae, by far the larger group, comprises about 110 genera with perhaps 1200 species. Revisions of the larger genera are not complete. Many more species have been described, especially in the 1920s and 1930s; tautologies resulted from the independence and conflicting concepts of distant authors. As a general rule, the described species will probably be reduced to one-third of their former number. On the other hand, many of the less attractive species in both the subfamilies are undercollected, overlooked, and undescribed even though many of them are biologically fascinating. It is probable that many geophytes and very narrowly endemic dwarf species are still unknown. Only a few extinctions, however, have been suspected.

The most compact species in the Ruschioidaeae, members of the so-called highly-specialised or 'stemless' genera, have attained a degree of diversity and adaptation (such as dimorphic and or fused leaves, fenestration, riddling patterns, hair coverings) unknown in their long-stemmed leafier allies. It is these bizarre forms which have attracted so much attention from collectors. Only a few species in Mesembryanthemoideae, chiefly both species of Dactylopsis have attracted as much notice, though some geophytic 'mesemboids' might do so once their ornamental potential is better understood.

Centres of diversity

The main centres of diversity occur within South Africa and, to a lesser extent, Namibia. The occurrence of Mesembryanthema in northern Africa and the Mediterranean rim involves a few species of natural origin; the majority have, however, been introduced into this area.

Cheiridopsis derenbergiana, Karoo Botanical Garden, South Africa.

Dactylopsis digitata, an unusual member of the Aizoaceae; its finger-like appearance gives rise to its local name of 'bobbejaans vingers' or 'baboon fingers'.

Within South Africa, the highest concentration of genera occurs in the so-called 'Vanrhynsdorp centre' as defined by Nordenstam (1969) with 31 genera within one quarter-degree square. The 'Gariep centre' has 30 genera, and the 'Little Karoo centre' has 27 (Hartmann 1991). The first two centres constitute southern and northern extremes of the area long known as Namaaualand. All the centres are extremely rich in ende mic species, and it should be emphasised that many endemics occupy very small habitats. This is particularly true for the Gariep centre, complex and rough in its geology. Many species are restricted to single hilltops or hill groups and this, of course, has strong conservation implications.

In many areas mesembs form the dominant flora; this is particularly the case in the so-called Knersvlakte (the Vanrhynsdorp centre) and in some similar areas by eroded, quartz-covered shales. In such areas the soils are poor in humic content and often high in salts. Many habitats are extremely sensitive and Unfortunately, many of the more peculiar habitats also harbour valuable mineral deposits such as copper, nickel, titanium, diamonds and gold, and mining has led to the eradication of several plant formations.

Trade

Within the Mesembryanthema, the ornamental species are the most important economically. Creepy or shrubby mesembs excel as ground cover plants in temperate areas; some, such as Delosperma spp., are exceptionally hardy, and some species are wonderfully colourful bedding plants. The highly succulent species in Lithops and are valued as pot plants, though world sales are small compared with figures for Cactaceae.

Illicit trade in field-collected plants is hard to measure, but it seems that the current market is small. This might be partly due to a virtuous green consciousness among sellers and collectors, but it can also be attributed to the fact that mesembs are not currently popular or fashionable with succulent collectors. The strongest factor is that almost all ornamental mesembs are now cheaply available as seeds, seedlings, or cuttings. The price of Conophytumburgeri, a peculiar endemic, fell from US$300 per wild-collected plant in 1978 to US$3.50 for a five-year old nursery seedling in 1992. It is the intention of certain specialist nurseries to widely propagate them, in effect driving prices down, thus eliminating most of the market for wild-collected plants.

Threats

The mountainous areas of Namaqualand are not amenable to cultivation, but they are grazed by goats which have a damaging impact on the succulent plant flora. The fertile and comparatively moist southern Cape, home to some of the more tender mesembs, including the fynbos species, has experienced great destruction from agricultural activities. This has had the greatest effect on Aloaceae, but mesembs have not entirely escaped, which might account for the apparent extinction of Circandra. The greatest threats to the mesembs are certainly those which are hardest to deflect: farming, mining, and urbanisation.

The threat of collecting is generally adequately controlled. In many respects the most attractive plants are fortunate: all can be easily and quickly propagated from seed, and almost all are already in cultivation in the USA and the UK. Moreover, most plants grow rapidly, so the appeal of the big, old wild plant is absent. The activities of the few private field collectors are, however, notoriously difficult to control. The nature conservation laws in South Africa are very stringent, but easily circumvented by anyone with the requisite boldness, as the country is too large for adequate policing. The best hope is to convince collectors that "digging them up" damages the plants that they claim to love. If specialists are to glamorise and popularise the plants by publishing attractive articles and tempting descriptions, they must simultaneously provide a legal quick and convenient method for distributing seeds and seedlings from material already in horticulture.

Protection

CITES protection is probably not appropriate for rare species of mesembs such as Muiria hortenseae. This particular species is threatened in habitat, occupying a tiny but conspicuous roadside site of about one hectare. Trade controls on Muiria could arguably increase the notoriety and market value of a species which is already overvalued. Furthermore, a well-grown Muiria seedling would be impossible to distinguish from a wild plant. Purchase and preservation of the most sensitive tracts of land may be the only option to protect mesembs in the Knersvlakte region.

Aloaceae

Ernst J. Van Jaarsveld and Gideon F. Smith

Although distributed through a large part of the Old World, the Aloe family, a fairly small taxonomic group of monocots, has its major centre both in evolutionary depth and in total number of species and genera in southern Africa. Of the approximately 436 species in about seven genera in the family, there are some 233 species in six genera in the area covered by the Flora of southern Africa (FSA) project. This project aims to provide a taxonomic account of all known indigenous and naturalised plant of the subcontinent south of, but excluding, Angola, Zambia, Zimbabwe, and Mozambique. In terms of number of species, the Aloaceae is more or less eighteenth largest family in the subcontinent, and the sixth largest monocot family (cf. Table 4 in Goldblatt 1978, and assuming fragmentation of the Liliaceae into several smaller families). However, on a world scale the Aloaceae is the largest monocotyledonous family of succulent plants and the second largest succulent plant family in southern Africa (cf. Asphodelaceae in Tables 1

Aloe helenae, listed on CITES Appendix I.

and 2 in Smith etal. 1993, excluding Bulbine and Bulbinella, but including Lomatophyllum). The family has both horticultural and medicinal value. Pharmaceutical products derived from especially Aloe vera and A. ferox are widely used (Kent 1980; Bloomfield 1985).

Systematic treatment

From a modest beginning in 1753 as a single lillioid genus, Aloe (1753), this taxon rapidly gained more genera and species. In recent years the Liliaceae, where the alooid genera have been traditionally classified, has been subjected to significant taxonomic reassessment in terms of its constituent infrafamilial taxa. The different interpretations of the circumscription of the Liliaceae by various taxonomists have resulted in, amongst others, the tribe Aloineae sensu Hutchinson (1959) being removed from the family (Dahlgren and Clifford 1982; Dahlgren et al. 1985). However, the circumscription of the Aloineae, one of 28 tribes recognised by Hutchinson (1959) in the Liliaceae, has undergone comparatively little change. The major controversies surrounding the taxonomy of the Aloineae have rather centred on genus and species concepts. In line with the classificatory interpretation of Brummitt (1992) of this natural group, we regard the alooid genera as warranting segregate familial status in the Aloaceae. The family includes the following seven genera: Aloe (333 species; Reynolds 1966, 1982), Haworthia (68 species; Bayer 1982), Gasteria (16 species; Van Jaarsveld 1991), Lomatophyllum (12 species; Jacobsen 1986), Astroloba (7 species; Roberts Reinecke

1965), and two monotypic genera, Chortolirion (Smith 1991b) and Poellnitzia (Smith and Van Wyk 1992a). The generic status of the two monotypes has been disputed from time to time and they are sometimes included in Haworthia. Although the boundaries of many of the alooid genera and some of the species are still being debated, general agreement exists over the question of familial monophyly. A number of synapomorphies can be listed for this natural entity (Smith and Van Wyk 1991), thus confirming their common origin. It is unlikely that the combination of the distinctive alooid karyotype (x=7, four long and three short chromosomes), and the characteristic leaf morphology have arisen more than once. Furthermore, the vascular bundles of leaves of Aloaceae typically have a well developed cap of thin-walled parenchyma cells at the phloem pole. These are often referred to as aloin cells (Smith and Van Wyk 1992b).

General characteristics

All species of Aloaceae are slow-growing, petaloid, rosulate or distichous perennials. They differ in size from miniatures barely 10 mm high (Haworthia parksiana) to trees of massive bulk up to 20 m tall (Aloe barberae). Almost all species are long-lived succulents that have above-ground storage organs (leaves, stems), but a few have subterranean storage and perennating organs (e.g. Chortolirion). Very few are deciduous in the sense that they die down and become dormant for part of the year (usually the cold, dry season). Floral morphology is variable and some basic forms occur repeatedly in different genera, which adds another dimension to the interpretation of the patterns of variation in reproductive morphology. In general, representatives of the family Aloaceae are easily recognised by their rosettes of usually spiny or tuberculate leaves that with age wither from below. The adventitious root system is usually shallow and utilises the upper leaf litter layer of the soil. In some species the roots may be fusiform and deeper. The racemose or paniculate inflorescence appears to be axillary but is in fact borne apically, bearing fleshy tubular flowers. The fruit is a three-angled, oblong, loculicidal woody capsule. In this regard Lomatophyllum with its fleshy berries is an exception.

The aloes often form a conspicuous feature of the southern African landscape and are popular garden subjects with their tough, thorny leathery leaves. Most species of Aloe have large, colourful, tubular flowers which are pollinated mainly by birds. Flowering is usually in the dry season and the seeds ripen just before the rainy season. Representatives of the other alooid genera are usually cryptic dwarf plants which occur in the shade of nurse plant species. Haworthia and Astroloba have small insect pollinated flowers, whilst bird pollination predominates Gasteria and Poellnitzia.

The genera of Aloaceae differ from the closely related Asphodelaceae in their conspicuous succulent leaf consistency, crescentiform or cymbiform leaf outline in cross-section, and the markedly bimodal karyotype consisting of 2n=14 chromosomes (Smith 1991a). In this regard Bulbine and Kniphofia appear to be problematic, particularly since some of the species of Bulbine have karyotypes and morphologies similar to that of certain taxa of Aloaceae (Rowley 1954). However, Bulbine can be easily distinguished from genera of the Aloaceae on the basis of its open, yellow (only very rarely white or orange) flowers, free perianth-segments, bearded filaments, lack of nectar production and the annual nature of some of its species (for example B.alata). Furthermore, Bulbine has an African-Australian distribution whereas the Alooideae is absent from Australia. Mainly for these reasons Bulbine was not considered to be a constituent of the Aloaceae. In contrast to representatives of Bulbine and the Aloaceae, leaf succulence is absent in Kniphofia, the leaf outline (cross-section) is V-shaped and it has a chromosome base number of six.

Aloe occurs over much of sub-Saharan Africa, ranging from the southern tip of Africa to the Arabian Peninsula. It is also found on Madagascar and Socotra. The genus is ecologically heterogeneous within its range of distribution and has diversified into almost every possible habitat, ranging from deserts, grassland and savanna to comparatively high rainfall coastal forest types. The fleshy-fruited Lomatophyllum is restricted to the Mascarene Islands. The general distribution patterns of Astroloba, Gasteria, and Haworthia closely resemble one another. These three genera are endemic to southern Africa and are more or less restricted to the summer-dry, semi-arid coastal regions below the inland escarpment of the subcontinent. Gasteria and Haworthia have outliers in the arid river valleys of Natal, Swaziland, and the eastern Transvaal, with a single species of Haworthia (H.venosa ssp. tessellata) occurring in the climatically severe central-southern Africa. The distribution of Astroloba is more restricted than those of Gasteria and Haworthia and it is usually found in slightly more arid environments of the Fynbos and Succulent Karoo Biomes of southern Africa. These three genera and Aloe have relatively large numbers of species indigenous and endemic to the arid subtropical transitional thickets of the eastern Cape where they show signs of active speciation. Of the genera of Alooideae, the monotypic Poellnitz'm has the most restricted distribution. This genus is found only in the Robertson and Bonnievale districts of the south-western Cape Province. In contrast, the other monotype, is widely distributed in the summer rainfall grasslands of southern Africa. The genus does, however, enter the winter rainfall region in southern Namibia.

Most species of Aloaceae use chemical (bitter leaf sap) or mechanical (pungent, acuminate leaf tips and prickles) strategies to prevent predation (Van Jaarsveld 1987). Many taxa occur in grassland and savanna habitats where natural fires are common. In the case of arborescent species fire adaptation is usually through the retention of a continuous cover of persistent, dead leaves around the stems, which acts as an insulator. Smaller species are geophytic or semi-geophytic with subterranean stolons. A few species have true bulbs. Many species of Aloe of the dry Mediterranean Fynbos regions are also subject to regular fires and often have resprouting abilities (e.g. the shrubby A. eommixta group which have lignotubers). The arborescent A. plicatillis has a corky bark. A. pearsonii of the dry, winter rainfall Richtersveld and Namib regions has functional persistent leaves for most of the height of the plant. These become turgid during a good rainy season. Species of Gasteria occur mostly in dense vegetation, and with their green, mottled leaves are well camouflaged. When browsed they will resprout from broken leaf fragments. Species of Haworthia are usually small and frequently occur in habitats.

The southern African alooid genera

Aloe L. The genus Aloe has been thoroughly treated by Reynolds (1966, 1982). A revision of the South African species has recently been undertaken by Dr. H. F. Glen and Mr. D. S. Hardy. Aloes have tremendous horticultural appeal in South Africa. The bright flowers and interesting leaf ornamentation, and the horticultural hardiness of most species of A loe make them attractive for cultivation even under less favourable hot and dry conditions. Though the majority of species are well adapted to arid environments, few can survive low temperatures and frost, and severely cold spells will occasionally lead to the demise even of individuals of species that occur naturally above the climatically severe inland escarpment (e.g. Aloe greatheadii var. davyana; pers. obs. by G. F. Smith). To many South Africans aloes are what roses and rhododendrons are to Europeans, and these rosulate succulents will always be horticulturally important in that country. An Aloe craze was sparked off in South Africa in the 1960s and 1970s by the seminal works of Groenewald (1941) and Reynolds (1966, 1982), resulting in the publication of numerous popular works on the genus. These include the works of Beyleveld (1973), Bornman and Hardy (1971), Jankowitz (1975), Jeppe (1974, 1977), Judd (1972), and West (1974). In addition, the Aloe Breeders Association is an active group of amateurs that continuously register new cultivars. Interest in the genus eventually resulted in legislation brought forward to protect species in their habitats, especially to prevent indiscriminate collecting. Nowadays, as a result of their horticultural appeal, most African states impose strict legislation to protect species of Aloe (see section on Conservation Measures, Southern Africa, Chapter 3).

Unfortunately, some Aloe habitats have been destroyed by agricultural activities and urban expansion, and to a lesser extent by collectors and for ethnic uses. Fortunately, some species have restricted habitats not suited for agricultural purposes, and species have been successfully introduced into cultivation and seeds are freely available. Rarest and most threatened of all is A. polyphylla, the species having novelty and ornamental appeal. Its numbers have drastically been reduced by collectors. However, the probability of it becoming extinct even in habitat appears to be slight. Unfortunately, few specimens of A. polyphylla survive ex situ due to its specialised habitat requirements. Smith (1989) showed the destruction of the natural habitat of A. bowiea and A. myriacantha in the south-eastern Cape, and emphasised the importance of the acquisition of land for its immediate protection. A. ferox is of economic importance and is one of the most common of all species. It is easily cultivated and readily establishes from seed. Commercial plantations of A. ferox have been started at Albertinia, South Africa. Concern has been expressed on the effect that the removal of the leaves has on survival of the species in the wild, since the dried leaves protect it against fires. The local Aloe sap tappers industry was worth R2.5 million in 1992 (Newton 1993).

Haworthia Duval. The genus has been taxonomically treated by Bayer (1982) and Scott (1985). The treatment of Bayer (1982) reflects a more natural dispensation.

Haworthia consists mostly of dwarf rosulate succulents, often proliferating from the base and forming dense clusters with smooth to tuberculate (rarely pubescent) leaves. The lax racemose inflorescences bear insignificant, bilabiate, whitish flowers. Of the alooid genera, Haworthia is under most threat due to habitat destruction. These usually cryptic plants are often confined to Renosterveld (sclerophyllous shrublands), Succulent Karoo, and arid savanna regions. The Renosterveld regions are of agricultural importance in the southern Cape and most have been transformed to wheat fields, with the result that many populations have been drastically reduced and a few species could become extinct if drastic measures are not implemented. Smith

Haworthia springbokvlakensis, a highly sought after and very scarce member of the genus.

(1991c) drew attention to the reduction in size of populations of H. fasciata in its south-eastern Cape habitat. Most species have tremendous horticultural value and because of their small size and relative ease of cultivation are popular container plants and highly prized, especially in Japan, Europe, and the USA. Some collectors have also played a role in reducing numbers. This is, however, minimal in comparison to agricultural damage to their habitats. Under the leadership of Mr. Kobus Venter, two Haworthia localities (H. magnifica var. maraisii and H. mirabilis var. badia) within the Napier village municipal region have been declared national heritage sites. Furthermore, many species of Haworthia are being multiplied by seeds and leaf cuttings by interes ted members of the Succulent Society of South Africa. H. limifolia is used medicinally by various African tribes and its numbers have been reduced considerably in the wild. However, this species is cultivated at the Silverglen Nursery in Durban and plants are supplied at affordable prices to traditional healers. Other threatened species in the southern Cape include H. emelyae var. multifolia, H. magnifica var. atrofusca, H. serrata, and H. poellnitziana.

Gasteria Duval. The genus has been revised (Van Jaarsveld 1991) and a more complete account is found in Van Jaarsveld (1994). It is a well defined, monophyletic group with brittle, mottled, keeled leaves which are often tuberculate and usually without spines. It has pendulous pedicels and gasteriform, trichromatic, curved flowers. The black, winged seeds are wind dispersed. The genus has centres of endemism in arid savanna regions and to a lesser extent in the Succulent Karoo of the south-eastern Cape. All species have horticultural potential and they are popular container plants. A number of species are confined to the fertile inter-montane valleys of the eastern Cape which is overgrown with subtropical thicket vegetation or grassland or, to a lesser extent, renosterveld. This region is of agricultural importance and large areas have been cleared for crop farming. This region is the habitat of G. bicolor var. bicolor, G. nitida var. nitida, G. nitida var. armstrongii, G. pulehra, and G. excelsa. Of these, G. nitida var. armstrongii is endemic to the lower Gamtoos River Valley and is under threat. Fortunately the species is represented in a nearby nature reserve. G. bicolor, although still common, has been reduced considerably by agricultural development. G. excelsa occurs on forested, well-drained rocky banks, but due to overgrazing and firewood collecting has become much rarer. G. baylissiana is another very rare species from the summit of the Zuurberg. The reason for its in situ decline is unknown but the numbers of the species have recently been increased by reintroductions to its habitat. Stocks of the species have been increased ex situ by seed and various clones originally distributed by Col. R. D. Bayliss to botanical gardens were used in the project. G. croueheri from Natal has became rare due to medicinal and ethnic uses in that region. Seed of this Gasteria have been made available to Silverglen Nursery, Durban, whence plants are provided to traditional doctors. Fortunately, the remaining species occur in habitats not under immediate threat. All species of Gasteria are being increased from seed as well as vegetatively at the Kirstenbosch Botanical Garden, especially the rare endemics, and seeds are made available on an annual basis.

Astroloba Mitewaal. The genus is closely related to Haworthia, but for some reason has never appealed to collectors and species of this genus are not under immediate threat.

Poellnitzia Mitewaal. At present urbanisation does not pose a threat to populations of Poellnitzia rubriflora. In the entire Succulent Karoo Biome it occurs in only two medium-sized towns (Worcester and Oudtshoorn) and various smaller centres, for example Robertson and Bonnievale. Cultivation of especially wine grapes is limited to fairly narrow strips alongside water courses, such as the Bree River. The colonies of P. rubriflora are mostly confined to rocky hillocks and appear to be in no immediate danger of agricultural activities. However, due to its small world population and its confinement to a restricted geographical area, Poellnitzia has been removed from the Indeterminate (I) category designated to it by Hall etal. (1980) (under the name Haworthia rubriflora in Hall and Veldhuis (1985) and reclassified as Rare (R) in Hilton-Taylor (1996b) (see Annex 16 for definitions of the Red List categories).

Chortolirion A. Berger. The genus has a very effective mechanism in that, in its natural grassland habitat, casual succulent plant collectors are likely to mistake its linear leaves with those of a grass species. The inflorescence and flowers of Chortolirion are also very inconspicuous. In pristine and well-managed pastures grazing pressure is minimal since the abundance of palatable grass species associated with it will certainly be utilised preferentially. Therefore, at least at present, Chortolirion is Not threatened. Where possible, populations should be monitored for decline or expansion, especially since Chortolirion occurs in some of the most densely populated areas of South Africa.

Asclepiadaceae

Focke Albers and Ulrich Meve*

The family Asclepiadaceae R.Br, is characterised by its unique complicated flower morphology and highly evolved pollination system. The family comprises various lifeforms including herbs, shrubs, twiners, succulents, and trees. The species mainly occur in the subtropics and tropics around the world, and only occasionally in temperate regions.

* Manuscript submitted in 1993

Economically the family is of little value. Some genera are of horticultural interest, but most species are not of significant value in terms of horticultural production. Asclepias, Hoyas and related species are grown as outdoor or pot plants, Dischidias, stapeliads, and Ceropegia species can be found all over the world in special hobbyist collections. Other uses of the family are few. The hairs of Asclepias and Calotropis seeds were sometimes used as a substitute for down, and in India ropes are produced from the stem fibres. In Africa and India some species are used medicinally by local people. Species of Caralluma and Ceropegia are eaten in India (see Annex 10) and, in extreme dry areas of southern Africa, succulents are eaten by Nama people and Bushmen.

Systematic treatment

Brummit (1992) compiled 315 genera in this large family. Today the number of species is estimated at more than 2000. In an intensive study of the family, Liede and Albers (1994) determined that about 500 correctly described genera exist. These are arranged in taxonomic order and discussed below. The family is divided into three subfamilies (Periplocoideae, Secamonoideae, Asclepiadoideae), the latter one comprises four tribes (Marsdenieae, Asclepiadeae, Gonolobeae, Stapelieae) according to Bruyns and Forster (1991). A fifth tribe Fockeae is added to this subfamily by Kunze, Meve and Liede (1994).

The subfamily Periplocoideae is a well-defined taxon, which is sometimes excluded from the Asclepiadaceae and treated as a family of its own (Periplocaceae) (Schlechter 1905; Bullock 1956). Most of the species are shrubs and twiners occurring in the Old World. Only two of the described genera, Raphionacme and Petopentia, have fleshy root stocks. Raphionacme is becoming of increasing interest for the hobbyist growers. Venter and Verhoeven (Bloemfontein, RSA) have contributed several valuable papers on this taxon since 1983 and Goyder (Kew) is also dealing with this group.

The subfamily Secamonoideae (five genera) is dominated by the great palaeotropic genus Secamone (Australia, Forster and Harold 1989; Africa, Goyder 1992; Madagascar region, Klackenberg 1992). Many species are known only from one or a few specimens, and 84 per cent are endemic to Madagascar (Klackenberg 1992). The twiners and herbs are not important in cultivation.

The largest subfamily is the Asclepiadoideae. Within this grouping, the tribe Fockeae contains three genera which are excluded from the tribe Marsdenieae. The small African genus Fockea consists of well-known caudiciform succulents. Research on this genus has been carried out by Court (Grahamstown, RSA). One species, Fockea sinuata, is considered to be Endangered. In general, the Fockeae species are of no importance for amateur growers.

The tribe Marsdenieae contains attractive leaf succulents. Many of these are epiphytes of East Asia, Australia, and Oceania: Dischidia, Dischidiopsis, and Some common species of these genera are grown as house plants, but most of the species in collections are cuttings of plants of natural origin, which are traded by a few professional horticulturists. Rain forest species in the centres of diversity in Malaysia, Indonesia, Philippines, and New Guinea are extremely threatened by forest destruction. Research is being carried out by de Koning (Leiden) and H. and R. Donkelaar (Werkendam), both from the Netherlands, and by Forster and Liddle, from Australia.

The largest tribe, Asclepiadeae, has a world-wide distribution in temperate, subtropical, and tropical regions. Lifeforms represented include herbs, shrubs, twiners, and trees, with succulents relatively uncommon. Large genera of this tribe are Asclepias, Cynanchum I Vincetoxicum, and Oxypetalum. The first genus is better known (e.g. Woodson 1954), whereas the others have been neglected for a long time. Cynanchum and related groups are being studied now by Liede (Ulm, Germany) and Nicolas (South Africa). The species of the genera which occur in the New World are quite different from those living in the Old World.

Succulents in the Asclepiadeae include species of Cynanchum with fleshy leaves, and species of Asclepias, Aspidonepsis, Cynanchum, Sta tmostelma, and Stenostelma with succulent subterranean parts. Stem succulents include species of Sarcostemma, Cynanchum, Folotsia, Karimbolea, and Platykeleba, all of which, with the exception of Sarcostemma, exist in the semi-arid parts of south-west Madagascar. Some of these are extremely localised or rare. Liede (pers. comm.) listed 48 species of Cynanchum, five of Folotsia, two of Karimbolea, and Platykeleba insigne as Rare and Endangered succulent Malagasy species. Several of them have apparently declined rapidly in numbers as their habitat is gradually being destroyed. The stem-succulents are of interest to hobbyists, but other species are not considered desirable and therefore they have not been additionally endangered by professional collectors.

The species of the tribe Gonolobeae are restricted to the warmer parts of Central and South America. The herbaceous species often belong to monotypic genera which are poorly known in the wild and in herbarium collections. Several species are represented by single herbarium sheets and some species have never been recollected since they were first described. Stevens (Missouri Botanic Garden) is working on this very difficult and poorly known group.

The Stapelieae is by far the best-known tribe and the most important one for hobbyist growers. All species are of palaeotropical origin, and many have evolved lifeforms with fleshy subterranean parts or stem succulence. The tribe Stapelieae includes the genus Ceropegia and a group of 30 other genera which collectively are commonly known as stapeliads.

The distribution area of Ceropegia covers Africa, Madagascar, Arabia, India, Southeast Asia, the Indonesian Archipelago, and Northern Australia. Although the genus is so widespread, most of the approximately 200 species are endemics with restricted distributions. Due to this fact, and the horticultural appeal of Ceropegia spp., the genus as a whole is listed in Appendix II of CITES. Within the Stapelieae, this genus has the highest rate of insufficiently known species (20 per cent). Since Huber's revision of the genus in 1957, several new species have been described which are highly attractive to hobbyists. Most, except for the geophytcs, can be easily propagated by cuttings, and have been in cultivation for many years.

The species of the genus Brachystelma (about 120), as well as those of Riocreuxia and Tenaris, all have developed geophytic forms and are well-adapted to grassland. The species of Brachystelma have a similar distribution area as those of Ceropegia to which they are closely related, but they avoid wetter areas. The habitats of Bnrchystelma are under extreme grazing pressure. The species do not grow well on their tubers in cultivation, and can bc propagated only by seed. According to the species listed in the "Checklist of Brachystelma, Ceropegia, Riocreuxia, and the Stapelieae" (Boele, Kroesen, and Noltee 1987) 72 per cent of the approximately 120 species are Endangered. Six species are already Extinct or have not been found for a long time. These are B. gemmeum, B. glenese, B. gracillium, B. longifolium, B. na tale use, and B. occiden tale. Valuable information is given by Dyer (1983). The genus is insufficiently known.

The thirty genera of stapeliads contain approximately 400 species. As stem succulents, with often large and attractive flowers, they are very well known to hobbyist growers and can be found in collections all over the world. In the wild, most of the species are confined to the more arid parts of Africa, but there are also several species occurring in Arabia, India, and Myanmar. Two European species are closely related to those of North Africa. The

Orbeopsis gerstneri ssp. gerstneri, a South African asclepiad.

species mainly form small, widely scattered or sometimes solitary populations. It is becoming increasingly apparent that stapeliads are a comparatively old plant group, in which plate tectonics and changes of climate have led to several migratory processes. The present distribution pattern comprises not only various centres of diversity, but also very restricted areas of retreat (Albers and Meve, in prep.). This perspective allows for the introduction of a relatively large number of genera in the stapeliads, a view supported by most of the recent studies. Research work so far, however, has mainly been limited to morphological characters and unfortunately has led to divergent concepts of species and even genera. Experts are far from an acceptable agreement like the one reached by the IOS consensus group for the Cactaceae.

Northeast Africa is assumed to be the centre of evolution for the stapeliads, but most of the species occur in southern Africa today. This secondary centre (Albers 1983) has evolved its own genera: Hoodia, Huerniopsis, Lavrania, Notechidnopsis, Ophionella, Orbeanthus, Orbeopsis, Pectinaria, Quaqua, Stapelia, Stapeliopsis, Tavaresia, Trichocaulon, Tridentea, and Tromotriche. The genus Stapelianthus is endemic to Madagascar. Only a few genera, like Huernia and Duvalia, are known on both sides of the equator, the vast majority of the North African species belongs to the genus Caralluma. The species from India and Myanmar are included here. Despite intensive research which has been done by Gilbert (1990) the genus still needs attention. With the aid of modern methods it seems to be possible to elucidate this complex (Albers and co-workers, in prep.).

Most of the other large genera of the stapeliads have been revised recently or revisions are in preparation (Duvalia, 14 spp., Meve, in press; Echidnopsis, 31 spp., Bruyns 1988; Hoodia, 13 spp., Bruyns 1993; Huernia, 69 spp., Leach 1988; Lavrania, 5 spp., Bruyns 1993; Orbea, 21 spp., Leach 1978; Orbeopsis, 11 spp., Leach 1978; Pachycymbium sensu Gilbert, 31 spp., Gilbert 1990; Piaranthus 7 spp., Meve 1994; Quaqua 24 ssp., Bruyns 1983; Stapelia, 45 spp., Leach 1985; Tridentea, 20 ssp., Leach 1980). Valuable information on the distribution and ecology of the species is included in most of the revisions.

The Hoodia/Trichocaulon complex, which has also been studied by Plowes (1992), still needs a satisfactory nomenclature. Lavranos has contributed to the knowledge of most of the taxa in a valuable way by describing new species and introducing new genera which he had found and collected on field trips in Africa and Arabia (mainly published in the Cactus and Succulent Journal (US).

Little is known about the population behaviour of the stapeliads. Observations on Hoodia spp. in Namibia indicated that some specimens have a lifespan of approximately 60 years (Jurgens ex Erb, pers. comm.). Albers and Meve (1991) demonstrated, in contrast, that some karroid species with creeping lifeform behave like ephemerals, e.g. within nine years a large Duvalia caespitosa population was s

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