Loranthaceae Juss.

Flora Zambesiaca. Vol. 9, Part 3. Polygonaceae-Myriaceae. Pope GV, Polhill RM, Martins ES. 2006.

Habit

Shrubs hemiparasitic on the branches (or elsewhere rarely the roots) of other dicotyledons, attached by woody haustoria, with or without surface runners producing secondary haustoria, generally evergreen Shrubs hemiparasitic on the branches (or elsewhere rarely the roots) of other dicotyledons, attached by woody haustoria, with or without surface runners producing secondary haustoria, generally evergreen

Stamens

Stamens as many as petals, epipetalous; anther basifixed in Africa, 2–4-thecous, sometimes locellate, opening lengthwise

Ovary

Ovary (receptacle) inferior, with 1–several obscure locules and lacking differentiated ovules; style and stigma simple

Seeds

Seeds without a testa, normally surrounded by a sticky layer developed from fruit-wall outside the vascular bundles. Seeds without a testa, normally surrounded by a sticky layer developed from fruit-wall outside the vascular bundles

Note

The aerial stem-parasites commonly known as mistletoes belong to several families, of which the major ones are Loranthaceae and Viscaceae. These two families were formerly regarded as subfamilies, but are now considered to have originated separately. They differ principally in that Loranthaceae have hermaphrodite flowers with a calyx and showy corolla, while Viscaceae have small inconspicuous flowers and only one perianth whorl. The precarious mode of establishment is, however, similar in most genera of both. The sticky seeds are dispersed by birds, principally tinkerbirds in Africa. The modified hypocotyl forms a pad that adheres to the host-branch to form the haustorium. This penetrates the host to connect with its xylem vessels to permit further growth of the seedling; secondary haustoria are sometimes produced on runners spreading over the host branches, as seen in Plicosepalus and Vanwykia. The least specialized genera of Loranthaceae occur in southern South America, New Zealand and Australia. Two genera, Helixanthera and Taxillus, occur both in Asia and Africa, but the other 19 genera now recognized in Africa, Arabia and Madagascar are all restricted to the region and seem to have undergone most of their radiation there. They may be attributed to two groups of genera. The Tapinanthoid group (genera 1–11) has simple or irregularly branched hairs. The Taxilloid group (genera 12–16) has hairs with whorls of branches (stellate or dendritic). The bracts of these two groups also tend to differ, cupular in the first, unilateral in the latter. The currently accepted genera are largely coincident with the sections adopted in much of the African literature, notably the masterly accounts by Sprague in the Flora of Tropical Africa (1910–1911) and the Flora Capensis (1915), modified from the earlier works by Engler, who described the majority of the common species from tropical Africa around the turn of the century. Segregate classifications have been advocated by van Tieghem in the 1890s, Danser in the 1930s, Balle in the 1950–1960s and by ourselves and our co-workers since the 1970s. For further discussion of the biology and biogeography see Kuijt, The Biology of Parasitic Flowering Plants (1969); Barlow & Wiens, The cytogeography of loranthaceous mistletoes, in Taxon 20: 291–312 (1971); Visser, South African Parasitic Flowering Plants (1981); Calder & Bernhardt (eds), The Biology of Mistletoes (1983); Feehan, Explosive flower opening in ornithophily: a study of pollination mechanisms in some Central African Loranthaceae, in J. Linn. Soc., Bot. 90: 129–144 (1985); Polhill, Speciation patterns in African Loranthaceae, in Holm-Nielsen, Nielsen and Balslev (eds), Tropical Forests: 221–236 (1989); Polhill & Wiens, Mistletoes of Africa (1998); S. J. Vermeulen, Distribution of Mistletoes in a Patchy Habitat, Ph.D. Thesis, University of London (1999). The distinction of the genera is based nowadays principally on modifications of the flowers related to their mode of pollination. Flowers of the less specialized genera in Africa, notably Helixanthera and to some extent Vanwykia in the Flora Zambesiaca area, open spontaneously. In more advanced genera of both the Tapinanthoid and Taxilloid groups the flowers are increasingly complicated with special mechanisms that can only be manipulated by birds and various signals are developed to attract their attention. In most cases vents appear in the mature bud just below where the anthers are held in the tip of the bud, with the stamens and corolla lobes held under tension. The region is often indicated by contrasting bands of colour on the corolla and on parts visible through the vents. Probing by the beak of the bird causes the corolla to split open explosively, either radially or unilaterally with a distinct V-split, dusting the bird’s head with pollen. Further refinements occur with a more economic and precise direction of the pollen puff. These have occurred with a remarkable degree of convergent evolution in the two groups of genera, such that species of Agelanthus and Phragmanthera have been included within a broad concept of Tapinanthus until recently. The pollination strategy in Globimetula is quite different. Here the distinct head of the mature bud darkens and when pecked the petals coil outwards to reveal vents at the base of the staminal column, probing of which causes the stamens to coil inwards explosively, bending the style in the same direction. The species of Tapinanthus (in the strict sense used here) also have a bud head that darkens at maturity. Pecking causes a V-shaped rent and precise deposition of pollen and inflexion of the style. The divergence of ecogeographic races and species is often marked, among other features, by shifts in signals given to visiting birds. It is noticeable that the modifications in flowers with swollen bud heads tend to occur on that structure, with various ridges, wings and crowns, whereas those with vents show more variation related to colour-banding, shape and internal hardening of the corolla lobes. A number of the more common species have developed physiological races in different parts of their ranges, recognizable by distinct host preferences and phenology, and in some cases by distinct modifications in their habit, flower colours and gross morphology.

Distribution

77 genera and about 950 species, widely distributed from the tropics to temperate regions particularly in the south.

Leaves

Leaves opposite, alternate or whorled, simple, entire, often leathery or rather fleshy, estipulate Leaves opposite, alternate or whorled, simple, entire, often leathery or rather fleshy, estipulate

Flowers

Flowers bisexual (rarely unisexual elsewhere), variously borne singly or in racemes, umbels or heads (elsewhere often in 3-flowered dichasia) in axils, at old nodes or terminally, in Africa often large and brightly coloured; bracts usually 1 per flower, cupular or unilateral, with a small to leafy limb Flowers bisexual (rarely unisexual elsewhere), variously borne singly or in racemes, umbels or heads (elsewhere often in 3-flowered dichasia) in axils, at old nodes or terminally, in Africa often large and brightly coloured; bracts usually 1 per flower, cupular or unilateral, with a small to leafy limb

Calyx

Calyx rim-like to tubular, entire to shortly toothed Calyx rim-like to tubular, entire to shortly toothed

Corolla

Petals free or united, 4–5 in Africa, valvate, radially symmetrical or tube opening with a unilateral split Petals free or united, 4–5 in Africa, valvate, radially symmetrical or tube opening with a unilateral split

Androecium

Stamens as many as petals, epipetalous; anther basifixed in Africa, 2–4-thecous, sometimes locellate, opening lengthwise

Gynoecium

Ovary (receptacle) inferior, with 1–several obscure locules and lacking differentiated ovules; style and stigma simple

Fruits

Fruit a berry in Africa (rarely dry and winged elsewhere) Fruit a berry in Africa (rarely dry and winged elsewhere)