Korthalsella

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Korthalsella

Description

Aerial stem-parasitic small perennials, entirely glabrous except for the floral cushions. Leaves opposite, rudimentary, each pair forming a border mostly less than 1 mm high at each node and subtending the flower clusters. Flowers developing successively in lateral clusters, usually surrounded and separated by multicellular sparsely branched thick-walled hairs (derived from floral bracts) which often form a raised mound (floral cushion); flower clusters sometimes coalescing and completely encircling the stem at each node; first-formed flower arising in an axillary position and usually male; subsequent flowers developing laterally to the first and often also in further transverse rows below the first, mostly female. Fruit pear-shaped or ellipsoid, seldom reaching 3 mm in length, crowned by the persistent tepals.

Distribution

Asia-Tropical: from Japan to Australia and New Zealand, extending eastwards to several Pacific archipelagos and westwards to Indian Ocean islands and Ethiopia: present
7-25 species (see below) distributed from Japan to Australia and New Zealand, extending eastwards to several Pacific archipelagos and westwards to Indian Ocean islands and Ethiopia. In Malesia 5 species, without a distinct centre of diversity. The genus is exceptional among mistletoes for its range over remote islands, apparently the result of its atypical fruit structure and mode of dispersal (see under ecology, p. 404). The apparently fragmented distributions of some species may simply reflect their cryp- sis in the field (see below).

Morphology

Korthalsella is homogeneous in inflorescence and floral characters, but the species vary strikingly in general appearance owing to differences in vegetative characters and in the degree of differentiation of flower-bearing stems. In species with terete or weakly compressed internodes, the successive leaf pairs may be distichous or decussate. In species with strongly flattened internodes, the successive ones of each stem are always flattened in one plane. In some species virtually all nodes bear flower clusters, whereas in others flowering may be restricted to lateral stems or distal parts of stems. In such cases the flowering stems may be less flattened than the vegetative ones. The extreme of this development is illustrated by K. geminata (), in which strongly flattened cladodes produce spike-like conflorescences in which the internodes are very short and terete. For additional illustrations of plant form in Korthalsella see .
The hairs which form the floral cushion are probably very densely fimbriate floral bracts, and therefore are homologous with the hairs in the inflorescences of some Ginalloa species (see there). In K geminata, which lacks floral cushions, floral bracts are visible and are barely ciliate. Dissected floral bracts can also be seen in K japonica and K papuana.

Taxonomy

The most recently published revision of the entire genus is that of . The Australian species were revised by . Danser recognized 23 species in the genus, and additions by Barlow only slightly outnumber reductions to synonymy, thus bringing the total to 25 species. Danser's and Barlow's treatments were based largely on external morphology, ecology and integrity of geographic occurrence. Touw, in Blumea 29 (1984) 525, reported a study of vascular structure in Korthalsella, and in subsequent unpublished work (Molvray 1990, see above) she used this as a major data set for a taxonomic revision. Although she accepted only 7 species in the genus, the nomenclatural implications for Malesia are minor, affecting only one species. However, Molvray's treatment of the entire genus has raised complex issues regarding species circumscription, and it has not been followed here.

For additional notes on taxonomic history, see and . Apart from the work of Danser the most significant earlier work was that of . He was first to recognize Korthalsella as a genus distinct from Viscum, and subsequently distinguished two more genera, Bifaria and Heterixia, and more than 60 species. The latter genera are now regarded as congeneric with Korthalsella, and most of Van Tieghem's species names have been reduced to synonymy.