Stapelieae

The Tribe Stapelieae consists entirely of stem succulents with thick, often soft and juicy branches covered in low ribs or tubercles, often tipped with a scaly bristle which is all that remains of foliage. A superficial resemblance to cacti is to be found in several. One species from India, Caralluma frerei( 19.6), retains flat, semi-succulent leaves, and is regarded as a pointer to the ancestral prototype from which the Tribe evolved. The main difference from the Ceropegieae is in the form of the flower, which is more open, with a shorter tube and free lobes not united at the tips. However, recent discoveries such as Pseudopeclinaria have broken down these distinctions and some botanists combine both Tribes in one.

Flower structure and pollination. The flowers of the Stapelieae are structurally the most complicated in all succulents, and show a high level of evolution comparable to that of the orchids, with which this group shows certain evolutionary parallels. In explaining the structure of the commonest species, Orbea (Stapelia) variegala (19.7), it will help to have in view the diagram of a simple flower of Crassula (2.18) in order to understand the ways in which the consecutive whorls of floral parts have been transformed.

Beginning at the outside, the least altered organ is the calyx, made up of five small green sepals. Next comes the corolla, also of five petals, which are more or less united basally to form a saucer-like or cup-like tube. The Stapelieae come nearest to qualifying as plants with succulent flowers, for the texture is thick and leathery, and they are able to stay expanded in full sun for two days or more without unduly draining the plant's reserves of water. The corolla surface is smooth, sometimes glossy as if varnished, or covered in ridges, pimples or tiny bristles. The margins and even the surface may be covered in fine hairs, which wave to and fro in the slightest breeze and. combined with the purplish brown mark ings and rank odour, give an uncanny resemblance to fresh carrion, and this attracts the necessary agents of pollination (4.7). Some Stapelieae have an outgrowth of the corolla called an annulus ("little ring"), which surrounds the essential organs at the centre. In the genus Huernia it gives the popular name "lifebuoy huernias" to those species where it is prominently developed (19.9).

Instead of separate stamens, tlfe Stapelieae have them fused into a hollow cylindrical body (column) that surrounds the two, free carpels. Viewed from above this column is seen to produce three different kinds of sterile outgrowths. First, lying close within the cup of the annulus are five tongue-like lobes known collectively as the outer corona—corona mea ning 'crown'. Over a nd above them is the inner corona, which in O. variegala terminates in five inner horns and five

Right (19.5): A close up ol the I.lower ol Orbea (Stapelia) maculata Stapelieae have some ol the most complicated and beautilul /lowers, no two species being alike

Below (19 6) Caralluma trerei (lelt). the missing link " otthe Stapelieae from tropical India art the only speoes with leaves Right, its hybrid with the leetiess C eutopaea

outer horns, as shown in the diagram.

In a flower such as Crassula. each anther is two-lobed and splits vertically to shed a cloud of dust-like pollen on either side. This is not so in Stapelieae. The contents of each anther lobe remain stuck together, and each pair of pollen bodies is linked by a yoke bearing two wing-likeappendagesorcarriers. Wecall such an organ a pollinium, and it has a counterpart in the orchid Family.

Finally we reach the centre of the flower where there are two separate (free) carpels, each containing numerous minute ovules. However, above the carpels is a solid disc of tissue that unites the tips of them to thecolumnandimmersedanthers.

How does pollination occur in Stapelieae? The full story has yet to be told—there are so many variations in flower form within the group—but a good introduction is given by Reese (1973). The strangely formed outgrowths of the two coronas are functionally guide barriers, which ensure that an insect randomly exploring the flower is eventually drawn to face the column at the centre. Unwanted visitors are excluded. There is a narrow vertical groove or cavity between the inner corona lobes. This groove—there are five to each flower — is the site of the receptive surface where the pollen mass has to arrive. The hairs on a leg or proboscis readily remove the pollinium by its yoke, and a similar exploring by the insect of another flower of the same species results in the pollinium falling or being dragged off into the appropriate cavity, where pollen tubes grow inward towards the carpels. The mechanism is very specific, something like a key fitting into a lock, and is responsible for the rarity of hybrids even when different species grow side by side in the wild.

Following fertilization, the two free carpels grow out into a pair of slender, cylindrical fruits, which may be up to 20cm (8in) long. Each splits lengthwise (a follicle, in botanical parlance) to release a cloud of seeds, each with its own white parachute of hairs (19.8).

Classification. In the past, characters of the corona have been much stressed in dividing up the Family into genera. With more material now available, it is evident that considerable variation can occur, even within one species, and the classification of the Stapelieae is in the melting pot. The listing followed here is that of Larry Leach, who has devoted much study in field and herbarium to elucidating this difficult group. For identification of species, flowers are essential, although some genera may be recognized by their vegetative form. While hunting Stapelieae in South and South West Africa I realized the impossibility of deciding the species, and even often the genus, unless blooms were to be found. One can only take a small sample of each large clump, grow them on and then examine blooms. Often what looks like a single population turns out to be made up of two or more sepa ra te species. This accounts for the disappearance and subsequent rediscovery of "Masson's lost Stapelieae" (page 96^

Overall there are from 400 to 450 species, of which about half belong to the two largest and most widespread genera, Stapelia and Caralluma. Slapelia (19.11, 12) is South African, with a thinning out northwards into tropical Africa; Caralluma reaches North Africa. Arabia and India and even ventures into Europe, the only native European stapeliad (Caralluma europaea) being found in southern Spain. The flower size ranges from only a few millimetres in some species to 30cm (12in) or more in Slapelia gigantea (19.11), among the largest of all flowers in the plant kingdom.

Right (19.8): Splitting follicles 10cm (4in) long ol Stapelia shed the numerous seeds, each with a parachute ot hairs. The seeds germinate rapidly but soon lose viability.

Below(19.9}: Huernia zebrina. 4cm (ffyin) across the bloom. The glossy "lilebuoy'rim and the small secondary tooth between each pairol corolla lobes is characteristic ol the genus

Orbea (19.5) is very close lo Stapelia and includes the longest and best-known of all this group, O. variegata( 19.7). This species is also the most adaptable to European cultivation, coming from the Cape Town area, where the annual rainfall is quite high. There is a robust cristate cultivar, and many varieties differing in flower patterning are on record, as well as hybrids.

Duvalia. Huernia and Piaranthus are neat, compact-growing stapeliads that take up little room in collections and are worth having for their diversity of flowers. Those of Huernia have the distinguishing character of a tiny secondary lobe between each pair of the corolla lobes (19.9).

Sfapelianthus comes from Madagascar and, not surprisingly, likes extra warmth. Edilhcolea (19.10) is a real treasure from East Africa and Socotra. It has arguably the loveliest of all stapeliad flowers. I have seen flourishing clumps growing and blooming in California, but in Europe it is the despair of cultivators.

Hoodia (5.9, 19.13), Tavaresia and Trichocaulon belong to a distinct group with rather tough greyish or purplish glaucous stems with irregularly scattered tubercles, each often crowned with a bristle, and rarely arranged in ribs. They are intense xerophytes and extra sensitive to overwatering in cultivation. Trichocaulon has minute flowers, but those of Hoodia (flat and disc-like) and Tavaresia (deeply bell-shaped) are striking and unusual.

Somewhat similar in body form are Pseudolithos and Whitesloanea, rarities from East Africa (7.2), where the ill-fated flora has been so depleted by destruction of habitats. Whitesloanea is presumed extinct and Pseudolithos cubiformis is not known alive anywhere at the time of writing. P. migiurtinus lives on precariously in cultivatioa although grafting on Ceropegia tubers is usually necessary to preserve it. These are most curious and fascinating plants: Whitesloanea is reminiscent of Astrophytum myriostigma (16.31). The world of succulents would be the poorer for their loss.

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