|The Enkianthoideae comprise a single genus, Enkianthus, with about fifteen
species occurring in Japan, China, Taiwan, and Indochina. This genus is
the sister group of all other taxa of the Ericaceae (Anderberg 1993, Kron & Judd
1993, Kron & Chase 1993). Cox (1948) had earlier noted that: ”the
primitive species of Enkianthus are without doubt the most primitive in
the Ericaceae”. Enkianthus was included in the Andromedeae (Hooker,
1876; Drude, 1889; Watson et al., 1967) or Cassiopeae (Cox, 1948) but was
placed in its own tribe, Enkiantheae, of the Vaccinioideae, by Stevens
The species of Enkianthus differ from other Ericaceae in having anthers with fibrous endothecium, monadinous pollen without viscin threads, and seeds with vascular bundles in the raphe, but these features may represent symplesiomorphies and as such they are not diagnostic of the genus. The monophyly of Enkianthus and the phylogenetic interrelationship among its species were investigated by Anderberg (1994a). Evident synapomorphies are difficult to find, but some character states have been mentioned in the literature as being unique in the genus, i.e., trichomes of moderately long cells with a distal end that become necrotic long before the proximal end (Stevens 1971), a nucellar pedestal on which the embryo sac is attached, ear-like projections from embryo sac, and ventral carpellary bundles in the septal plane (Palser 1951, 1952, 1961). Given the position of the Enkianthoideae as the sister group of all other Ericaceae, a character such as perulate buds emerges as a potential synapomorphy of Enkianthus. This feature is characteristic of the Rhodoreae but is generally not found in Arbutoideae or the herbaceous Pyroleae, Monotropeae or Pterosporeae of the Pyroloideae. It is most parsimonious to assume that perulate buds have evolved in parallel in Enkianthus and in the Rhodoreae especially since axillary branches arise from the axils of the perulae, a feature found elsewhere only in a very few species of Rhododendron.
The species of Enkianthus differ in leaf texture, inflorescence structure, corolla shape and seed morphology, but they are rather similar in most other respects. The leaves are flat and rather large. They are spiral but often borne in clusters towards the end of the branches. Flowers are arranged in raceme-like or umbel-like inflorescences and have corollas that are campanulate, tubular or urceolate. The stamens are twice the number of the corolla-lobes and have anthers that are smooth to distinctly papillose, opening with longitudinal slits by means of a fibrous endothecium. The anthers have awned appendages, which gradually narrow from the thecae, and the filaments are variously papillose to hairy. The ovary is superior and the fruit develops into a loculicidal capsule opening with five valves. The seeds in most species are provided with winglike ridges. Based on the cladistic analysis by Anderberg (1994a) Enkianthus seems to have originally been shrubs with deciduous leaves, racemose inflorescences, campanulate corollas, and winged seeds, while the more derived species possess coriaceous leaves, umbellate inflorescences, flowers with tubular or urceolate corollas, and unwinged seeds.
In the early evolution of the Ericaceae two evolutionary lineages emerged, one of these evolved into the Enkianthoideae of which Enkianthus is the only known representative today, and the other to the ancestral group that has diversified into the rest of the family.
|Enkianthoideae Kron, Judd & Anderb., stat. nov. — Type genus:
Enkiantheae Stevens, Bot. J. Linn. Soc. 64: 35. 197l.
Shrubs, leaves evergreen or deciduous, alternate or pseudoverticillate, flat, non-ericoid, serrate or entire, convolute in bud. Leaf epidermal cells not lignified. Indumentum of distinctive, elongate, non-glandular hairs. Winter-buds perulate. Inflorescence racemose or umbellate; bracteoles lacking; calyx not articulated with pedicel. Flowers 5-merous, actinomorphic. Calyx lobes deciduous; corolla sympetalous, campanulate, tubular or urceolate. Stamens 10, included, the filaments ± straight, unicellular pubescent; anthers tetrasporangiate, inverted late in development, papillose or smooth, with a pair of awns, with fibrous endothecium, opening with longitudinal slits. Pollen in monads, without viscin threads. Ovary 5-locular, with axile placentation, superior; style long, impressed; stigma not expanded. Fruit a loculicidal capsule; seeds thin-walled with a few layered testa, with winglike ridges or smooth, with vascular bundle in raphe. Embryo sac on nucellar pedestal with ear-like projections; embryo with two cotyledons.