The phylogenetic relationships between the species and the populations within species are the subject of intense molecular systematic and population genetic research at the moment. This research is aimed at understanding the evolution of species and their conservation status and is ongoing (for initial publications of this work, see 3). This chapter does not focus on this work, but reviews the useful but relatively informal classification first outlined in detail by Hilliard and Burtt (1971).
As discussed in Growth Forms, the genus Streptocarpus can be divided into the subgenera Streptocarpus and Streptocarpella. Within these taxonomic entities, the customary subdivision into sections has not been attempted. Within subgenus Streptocarpus a number of relatively informal groupings has been suggested (Hilliard and Burtt, 1971) and these are elaborated below. The groupings are convenient for keying purposes, but also seem to have some taxomonic significance.
As will be evident from the discussion below, there are too many species in this genus to allow a discussion of each one. I will, however, highlight certain species, in particular, those of horticultural interest. In the species listings I have indicated in which country they are found by abbreviation (Ang, Angola; Bur, Burundi; Cam, Cameroon; Con, Congo; Eth, Ethiopia; Ken, Kenya; Mad, Madagascar; Mal, Malawi; Moz, Mozambique; SA, South Africa; Swa, Swaziland; Tan, Tanzania; Zam, Zambia; Zim, Zimbabwe).
Subgenus Streptocarpus, Group A:
These are usually monocarpic unifoliates, rarely plurifoliate perenials, of medium to large size; corolla tube usually wide open (tube-shaped), blue,, violet, whote, cream, with or without yellow markings, and colour markings on lower lip, but never spotted or clearly striped; stigma usually stomatotrophic; fruit usually exceeding 50 mm (abbreviated from Hilliard and Burtt, 1971).
This group consists of a group of about twenty species including Streptocarpus porphyrostachys (SA), S. wendlandii (SA), S. molweniensis (SA), S. saundersii (SA), S. cooperi (SA), S. grandis (SA), S. michelmorei (Mal, Moz, Zam, Zim), S. solenanthus (Mal, Tan, Zam, Zim), S. goetzei (Mal, Moz, Tan), S. compressus (Tan), S. monophyllus (Ang), S. eylesii (Mal,Tan, Zam, Zim) S. wittei (Con, Mal, Zam), S. arcuatus (Mal), S. vandeleurii (SA), S. kungwensis (Tan), S. trabeculatus (SA), S. cooksoni (SA), S. galpinii (SA), S. lokohensis (Mad) and S. itremensis (Mad). Some, such as S. vandeleurii and S. wittei, have a scent which is referred to as mixture of creosote and honey which is a distinctive feature. Many of these species are in cultivation and many have beautiful large inflorescences such as S. porphyrostachys, S. cooperi, and S. grandis.
Subgenus Streptocarpus, Group B also referred to as the S. meyeri alliance:
These are very variable in habit, monocarpic unifoliates or plurifoliate, often with an aerial stem-structure (often a petiolode) in association with flowering; leaf margins often jagged or toothed; corolla variable in form, white, pinkish, red or violet, often with heavier spots in lower lip and throat or with stripes in corolla tube; stigma stomatotrophic or unequally two-lipped; capsule less than 50 mm; unicellular hairs in corolla, conspicuous lateral staminodes and a style longer than the ovary often occur (abbreviated from Hilliard and Burtt, 1971).
This group, which consists of about 52 species, includes S. denticulatus (SA), S. pole-evansii (SA), S. dunnii (SA), S. myoporoides (Moz), S. pogonites (SA), S. montanus (Tan, Ken), S. hirtinervis (Mal), S. nimbicola (Mal), S. brachynema (Moc), S. umtaliensis (Zim), S. micranthus (SA), S. bullatus (Tan), S. decipiens (SA), S. brevistamineus (Mad), S. mangindranensis (Mad), S. capuronii (Mad), S. burundianus (Bur), S. masisiensis (Con), S. fanniniae (SA), S. candidus (SA), S. wilmsii (SA), S. davyi (Swa), S. pusillus (SA), S. rimicola (SA), S. makabengensis (SA), S. occultus (SA), S. stenosepalus (Mad), S. exsertus (Ken), S. phaeotrichus (Eth), S. erubescens (Mal), S. cyanandrus (Zim), S. pumilus (Zim), S. hirticapsa (Zim), S. cordifolius (Mad), S. sambiranensis (Mad), S. stellulifer (Mad), S. velutinus (Mad), S. bolusii (SA), S. latens (SA), S. leptopus (Mal), S. rhodesianus (Ang, Zam, Con), S. suborbicularis (Mad), S. ibityensis (Mad), S. revivescens (Mad), S. boinensis (Mad), S. polyphyllus (Mad), S. variabilis (Mad), S. perrieri (Mad), S. hildebrandtii (Mad), and the S. meyeri alliance: S. meyeri (SA), S. kentaniensis (SA) and S. modestus (SA).
In particular, mention must be made here of S. dunnii which is one of the few species (along with S. myoporoides) which has red-coloured flowers and which has been used to develop the red hybrids. S. candidus is a particularly floriforous species which is often cultivated and also has the honey scent. S. pusillus, S. rimicola, S. makabengensis, S. occultus are all dwarf species often found on isolated mountain outcrops in Kwa-Zulu Natal, Mpumalanga and the Limpopo Province of South Africa. In parallel, S. exsertus and S. phaeotrichus occur on mountain peaks in Kenya and Ethiopia respectively.
The last three species in the S. meyeri alliance: S. meyeri, S. kentaniensis and S. modestus again occur in South Africa in the eastern Cape and Transkei. S. kentaniensis is special in that it flowers in winter as opposed to summer in all other species and is therefore of interest to hybridizers in order to extend flowering season. S. meyeri also occurs in the Mpumalanga escarpment area which is about 800 km away from the Eastern Cape localities. Due to this disjunct distribution the species may in future be reclassified as two separate species. However, the Mpumalanga area also harbours other closely related types which have recently been discovered (Weigend & Edwards) and the whole taxonomy of this complex must still be clarified.
Subgenus Streptocarpus, Group C and allies:
These are plurifoliate perennials with a loose, more or less irregular rosette produced from a rhizome; leaves strap-shaped, several times longer than broad; corolla tube-shaped with characteristic lines on the lower lip and throat; stigma with two unequal lips, fruit long often exceeding 100 mm (abbreviated from Hilliard and Burtt, 1971).
This group consists of S. montigena (SA), S. caeruleus (SA), S. longiflorus (SA), S. rexii (SA), S. primulifolius (SA), S. formosus (SA), S. cyaneus (SA, Swa) and subspecies, S. fenestra dei (SA), S. roseoalbus (SA), S. fasciatus (SA), S. parviflorus (SA), S. gardenii (SA), S. johannis (SA) and S. baudertii (SA). Group C is the one from most hybrids were developed.
S. rexii is the southern-most species of the genus and the first to be discovered. It often grows quite near the sea in areas such as Nature’s Valley and the area around Plettenberg Bay, areas of great scenic beauty. Together with S. primulifolius, it formed the basis of the first hybrids and the distinct lines in the throats of many hybrids derive from these species. S. formosus was previously classified as a subspecies of S. primulifolius. The S. cyaneus group has also recently undergone a taxonomic revision and has been split into a number of separate species and some subspecies such as S. fenestra-dei, S. roseoalbus, and S. cyaneus ssp. cyaneus, S. cyaneus ssp. nigridens and S. cyaneus ssp. junodii. These species grow in the Mpumalanga escarpment area which is very mountainous and inaccessible and there is some speculation that more closely related species occur in this area. S. parviflorus is a beautiful little plant which together with the striking S. caeruleus and S. longiflorus are the northernmost representative in this group in that they grow in the Zoutpansberg and Blouberg areas in the Northern Province of South Africa.
S. johannis is a species which requires special mention. It grows in the Transkei area of South Africa in which some areas are very remote and inaccessible. There are numerous forms of this species and it is more than likely that some of these forms will be reclassified as separate species. The floriforous nature of this species makes it very attractive for hybridization as has been seen by the recent development of such hybrids in Britain.
Subgenus Streptocarpus, Group D:
Monocarpic unifoliates or plurifoliate perennials; leaves not longer than twice their width; corolla tube narrow, curved, mouth characteristically much compressed from the side; limb oblique, self-coloured or with darker patches on lower lip (flower type referred to as key-hole shaped); stigma usually undividedand gelatinous tipped, rarely bi-lobed (abbreviated from Hilliard and Burtt, 1971).
This group consists of S. polyanthus (SA), S. prolixus (SA), S. silvaticus (SA), S. daviesi (SA), S. pentherianus (SA), S. haygarthii (SA), S. confusus (SA), S. semijunctus (Mad) and S. tsimihetorum (Mad). This small group harbours many attractive unifoliate species most of which have small flowers. Although unifoliate, a few of these do develop new phylomorphs enabling the plants to survive into another growth cycle, rendering them at least semi-perennial.
The subgenus Streptocarpella consists of a large number of species which can be subdivided into the African and the Madagascan species. The African species are S. parensis (Tan), S. schliebenii (Tan), S. euanthus (Tan), S. bambuseti (Tan), S. stomandrus (Tan), S. kirkii (Tan), S. saxorum (Tan), S. hirsutissimus (Tan), S. buchanani (Tan), S. caulescens (Tan), S. pallidoflorus (Tan), S. holstii (Tan), S. glandulosissimus (Tan), S. inflatus (Tan), S. kimbozanus (Tan), S. gonjaënsis (Tan), S. elongatus (Tan), S. muscicola (Cam), S. nobilis (Senegal, Nigeria, Cam), and S. insularis (Burkino Faso). The Madagascan species include S. hilsenbergii, S. thompsoni, S. muscosus, S. venosus, S. oliganthus, S. leandrii, S. integrifolius, S. linguatus, S. prostratus, S. tanala, S. levis, S. mandrerensis, S. beampingaratrensis and subspecies, S. andohahelensis, S. tsaratananensis, S. campanulatus, S. macropodus, S. suffruticosus, S. glabrifolius, and S. papangae. Only a few of these are readily available and some make charming potplants because they flower over a long period. These include S. caulescens, S. kirkii, S. saxorum, S. glandulosissimus and S. hilsenbergii. I have grown S. glandulosissimus with good success in my garden and found that it’s larger very pretty blue flowers, which have the lower lip sharply bent forwards, appear over a long period [*, will supply pic].
The Madagascan species S. mandrerensis, S. beampingaratrensis and subspecies and S. andohahelensis have short internodes between leaves and these species in particular appear to be the closest relatives of the Saintpaulias (*, link to one of these species?).
Four species that have been known as Streptocarpus occur in Asia and they should not be classified as true Streptocarpus. They are S. burmanicus, S. orientalis, S. sumatranus and S. clarkeanus.