This article is reproduced from Flowering Plants of the World (Updated Edition), V. H. Heywood ed. Flowering Plants of the World is an excellent general reference to the flowering plants, with many detailed and beautiful illustrations by some of the most accomplished of modern botanical artists. It is published by Oxford University Press (New York, 1993).
As with most plant families, there is an absence of universal agreement on the taxonomy of gesneriads not all of the classification detail in this article is universally accepted, although the general outline is valid, thorough and useful.
African Violets and Gloxinias
Dr. B. Morley
The Gesneriaceae is a large family comprising mostly tropical herbs and shrubs. It includes many popular cultivated ornamentals, such as gloxinias and African violets.
The 125 genera and 2,000 or so species are mostly pantropical, but some are temperate, in the Americas from Mexico to Chile, East, West and South Africa, Madagascar, Southeast Asia, Polynesia, Australasia, China, Japan and southern Europe.
Gesneriads are rare in Africa, but in America are often regarded as tropical counterparts of the essentially temperate family Scrophulariaceae, and are herbs and shrubs, rarely trees, with opposite or alternate, sometimes basal leaves (rarely a single leaf) ,which are simple, entire or toothed (rarely pinnatisect), and without stipules. The underground parts may be fibrous, woody tubers, scaly rhizomes or aerial stolons. The flowers are bisexual, irregular and borne in racemes, cymes or singly. There are five sepals, free or tubular at the base, with five petals also fused into a basal tube, the free ends being oblique, two-lipped or rarely rotate. The two or four stamens often cohere in pairs and release pollen by longitudinal slits. The ovary is superior or inferior and has a single locule containing numerous ovules, usually on two parietal or intrusive placentas. The style is single, crowned with a two-lobed or mouth-shaped stigma. An annular, lobed or one-sided nectary lies between the ovary and petals. The fruits are rounded or elongated capsules or rarely berries, and contain many small seeds, with or without endosperm, and with straight embryos.
Evolution of about half of the New World gesneriads has been partly by co-adaptation with bird pollinators, notably the hummingbird family which is restricted to the Americas. Typical hummingbird flowers are two-lipped, often red as in Columnea, Asteranthera and some Sinningia species. Other pollinators such as bees, bats, butterflies, moths and flies have also been active in gesneriad evolution. In Hypocyrta, Besleria and Alloplectus some species have pouched corollas with constricted throats, the significance of which is still not clear. The Old World genus Aeschynanthus is considered a parallel development with Columnea in being bird-pollinated. Flowers are an important part of the pollination system in gesneriads but extra-floral attraction also exists in some species, such as strikingly colored leaf- and sepal-hairs, or leaf pigmentation with stained-glass-like optical properties when viewed against the light.
Notable New World genera include Columnea, l50 species of shrubs and climbers, often epiphytic; Sinningia, 60 species of herbs, some popularly known as gloxinias; Achimenes, 20 species of often hairy herbs with red to blue flowers; Episcia, 40 species of small trailing evergreens; Gesneria (47 species) and Rhytidophyllum (20 species), two related genera with yellow-green, white or red flowers; Gloxinia (not to be confused with the popular gloxinias), 15 species of herbs with lilac bell-flowers or cinnabar-red pouched flowers; Smithiantha, four Mexican species with green or purple-brown velvety leaves and pyramids of orange-red or yellowish tubular flowers; Phinaea, 10 species with whitish flowers; Kohleria, 20 species often with racemes of orange-red flowers patterned inside with contrasting spots and with brown-green velvety hairy leaves.
Notable Old World genera include Ramonda, three species of stemless. hairy herbs from southern Europe with showy flowers on leafless scapes; Saintpaulia, 12 East African species mostly of rosette herbs; Aeschynanthus, 70 species of trailing or climbing shrubs from the Far East; Streptocarpus, 130 African species of evergreen herbs often with foxglove-like flowers; Cyrtandra, 350 species from Southeast Asia and Oceania; Jankaea (one species) and Haberlea (one species), rosette alpines with lilac or violet flowers native to southern Europe; Chirita, 80 species of tropical Asian herbs with fleshy, often transparent parts and large whitish, blue, purplish or yellow clustered flowers; Titanotrichum, with a single species from China and Taiwan, with tubular flowers bright yellow outside, blotched red-brown with a narrow yellow margin inside; Conandron, three Japanese species of alpine rosette herbs regarded as the counterpart of Ramonda; Petrocosmea, 15 species from Southeast Asia similar to Saintpaulia.
About half the genera are placed in the Old World subfamily Cyrtandroideae, with cotyledons of unequal length, unlike the New World Gesnerioideae with equal cotyledons. This division is supported by data from pigment chemistry and chromosome number patterns. Each subfamily is divided into tribes as follows, with representative genera given in parentheses.
CYRTANDROIDEAE: CYRTANDREAE (Cyrtandra), TRICHOSPOREAE (Trichosporum = Aeschynanthus), KLUGIEAE (Rhynchoglossum), LOXONIEAE (Loxonia), DIDYMOCARPEAE (Ramonda, Chirita, Streptocarpus).
GESNERIOIDEAE: GESNERIEAE (Gesneria), GLOXINEAE (Achimenes, Sinningia), EPISCIEAE (Episcia, Columnea), BESLERIEAE (Besleria), NAPEANTHEAE (Napeanthus), CORONANTHEREAE (Asteranthera, Mitraria, Sarmienta).
The temperate Andean genera Asteranthera, Mitraria and Sarmienta, all climbers with red flowers, and Rhabdathamnus, a shrubby New Zealand genus with attractive red-striped yellow flowers, do not easily fit into either Old or New World Subfamilies, and their own subfamily MITRARIOIDEAE has been proposed by certain botanists,
Herbaceous families related to the Gesneriaceae are the Scrophulariaceae, Orobanchaceae, and Lentibulariaceae; the chiefly woody Bignoniaceae is also florally similar but has woody fruits that often have two locules, and winged seeds, as well as divided leaves. The gesneriad ovary may be superior, as in Scrophulariaceae, or inferior, but in contrast to the Scrophulariaceae the ovary usually has a single locule not two. The usually parietal placentas of gesneriads with superior ovaries differ from the basal placentas of butterworts (Lentibulariaceae). Orobanches are parasites and lack chlorophyll, so also differ from gesneriads.
Some species have been reported as being used in rural medicine, but the importance of the family lies in its cultivated ornamentals. Popular garden and house-plant genera include Achimenes, Columnea, Episcia, Gesneria, Haberlea, Hypocyrta, Kohleria, Mitraria, Ramonda, Saintpaulia (African violet), Sinningia (Gloxinias), Smithiantha, Streptocarpus (Cape primrose) and Aeschynanthus.