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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.