Notes

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, the Loranthaceae related to Olacaceae, the Viscaceae to Santalaceae. 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, in Africa principally tinkerbirds. The modified hypocotyl forms a pad that adheres to the host-branch to form the haustorium. This penetrates the host to connect with its sap stream 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 specialised genera of Loranthaceae occur in southern S. America, New Zealand and Australia. Two genera, Helixanthera and Taxillus, occur both in Asia and Africa, but the other 21 genera now recognised 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 1–10) has simple or irregularly branched hairs. The Taxilloid group (genera 11–16) 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 account by Sprague in the Flora of Tropical Africa (1910–1911), modified from the earlier works of 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. The distinction of the genera is based nowadays principally on modifications of the flowers related to their mode of pollination. Flowers of the less specialised genera in Africa, notably Helixanthera, Taxillus and Vanwykia in the Flora area, open spontaneously or with little probing. 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 directing 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.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.L.S. 90: 129–144 (1985); Polhill, Speciation patterns in African Loranthaceae, in Holm-Nielsen, Nielsen & Balslev (eds), Tropical Forests: 221–236 (1989); Polhill & Wiens, Mistletoes Afr. (1998).