Viscaceae

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Viscaceae

Description

Perennial evergreen or leafless plants, hemiparasitic on tree branches; attachment to the host by a single primary haustorium. There are some parallel trends in inflorescence structure in Viscaceae and Loranthaceae, although inflorescences in the former family are generally less complex than in the latter. A common inflorescence unit widespread in Viscaceae is a simple dichasium (triad), although it is often amplified by sequential development of additional flowers in lateral positions, forming cymose fans or clusters, and sometimes it is reduced to a single flower. In monoecious species the individual cymules usually bear both male and female flowers, with the central flower (or flowers) being of one gender and the lateral flowers of the other; however, entirely female cymules are common in many species. In many cases the cymules are aggregated into larger conflorescences, which are mostly racemose or spicate.
Inflorescencess axillary or terminal; uni- florescence a simple cyme or a single flower, these sometimes aggregated to racemose or spicate compound inflorescences or expanded into dense clusters. Flowers small (mostly less than 2 mm long), monochlamydeous, unisexual; plants monoecious or (not in Malesia) dioecious. Tepals 2-4, valvate. Stamens as many as and opposite the tepals, epitepalous or free; anthers basifixed, free or sometimes united into a synandrium, 1- to many-celled, opening by pores or rarely slits; pollen spherical. Ovary inferior, 1-locular, with a short placental column; ovules absent; sporogenous cells located at the base of the placental column; stigma simple, sessile, nipple-like. Fruit berry-like, with a single seed covered by a sticky layer which develops in a zone internal to the vascular bundles.

Distribution

Asia-Tropical: temperate regions: present tropical: present
Seven genera and about 400 species, predominantly tropical with fewer species in temperate regions. In Malesia 4 genera and 26 species, distributed throughout the region, without any distinct centre of species richness. For biogeography see below.

Ecology

Like Loranthaceae, Viscaceae exhibit a suite of remarkable adaptations associated with the hemiparasitic habit; for additional details see under morphology below.

Except for Korthalsella and Arceuthobium, effective seed dispersal is by fruit-eating birds, and involves similar adaptations to those found in Loranthaceae (for details see there). In Viscaceae the fruits and seeds are usually smaller than those of Loranthaceae, but they have the same general architecture. In Korthalsella the fruits are mostly less than 2 mm long, and the minute seeds are released by a weak explosive mechanism (Stevenson 1934); dispersal is probably achieved by attachment to the feet or feathers of birds. In Arceuthobium a more powerful explosive mechanism propels projectile-shaped seeds considerable distances (Hinds et al. 1963; Hinds & Hawksworth 1965), and local dispersal from tree to tree is probably abiotic. See further discussion under plant geography.

Host preference and specificity vary widely within the family. Most Viscaceae grow on dicotyledonous hosts, but one genus (Arceuthobium) is specialized for and confined to coniferous hosts, and some species of Dendrophthora, Korthalsella, Phoradendron and Viscum also occur on conifers. In mixed forests with high tree species diversity many species of Viscaceae tend to have low host specificity, occurring on a broad range of host species; this is the case for several species in Malesia. However, other species show some specificity, including Notothixos cornifolius, which appears to have a strong host preference for species of Sterculiaceae. Also notable is the tendency for many species, especially of Notothixos and Viscum, to grow on other mistletoes, both of Loranthaceae and Viscaceae. The latter situation probably reflects the ease with which mistletoes can form haustorial attachments on related species, but may be adaptive in that it involves shared use of the same seed dispersal agents.

In open forests or woodlands with low tree species diversity the viscaceous flora tends to exhibit higher host specificity, sometimes involving differentiation of host races. Infraspecific variation or differentiation of host races in Arceuthobium and Viscum, for example, is linked with specificity for different hosts (Kuijt 1960; Wiens 1964; Hawks- worth 1987; Nickrent & Stell 1990). In other cases, close adaptation for very unusual hosts occurs, for example in Viscum minimum of South Africa, which parasitizes succulent Euphorbia species, and in which only a fertile shoot c. 3 mm long emerges above the surface of the host (Wiens & Tolken 1979). Viscum minimum may thus be one of the smallest dicotyledons. High host specificity in Viscaceae is not generally associated with close visual resemblance to the preferred host, as occurs in some Loranthaceae (see there), although some species of Phoradendron closely resemble their preferred hosts in North America (Atsatt 1979).

The syndrome of floral characters indicates that pollination is consistently by insects. This contrasts with the adaptations of most Loranthaceae, which are predominantly bird pollinated. Pollinator guilds are unknown, at least for Malesia.

Taxonomy

General acceptance of Viscaceae as a family distinct from Loranthaceae dates from about 1960. As early as 1802 Batsch distinguished the group at this level, and was followed by Miers, Miquel, Agardh and Van Tieghem. Other influential authors, including Engler, Danser and Krause relegated the group to subfamily rank within Loranthaceae. More recent sources of data supporting distinction at family level include embryology (Johri & Bhatnagar 1960; Dixit 1962), karyology (Barlow 1964; Wiens & Barlow 1971) and morphology (Kuijt 1968, 1969). The brief diagnostic key to the two families presented at the end of this section covers all Malesian taxa.

Further to the strong grounds for treating Viscaceae as a family distinct from Loranthaceae, there is some evidence that the two families may not even be directly related (Kuijt 1968). Some genera of Santalaceae and Olacaceae also show reductions in ovary structure, and aerial stem parasitism also occurs in other families of Santalales. Some of the striking features of haustorial structure, ovary structure and growth habit in Loranthaceae and Viscaceae may therefore be parallel developments (and divergent specializations) acquired from different direct ancestors in Santalales (possibly in Olacaceae and Santalaceae respectively), so that their common ancestor may be phyletically more remote.

Within the Viscaceae generic limits have remained relatively stable. However, there is no consensus on the relationships between all genera and their arrangement into subfamilies or tribes. In contrast to Loranthaceae, karyology provides little useful data. Various inflorescence and floral characters have been applied in assessing relationships between genera, but differences in weighting lead to different arrangements. Because of the small number of genera in the family, and their obvious affinities with each other, there seems little merit in establishing subfamilial taxa, and none are utilized here.

At the species level there are difficulties in several genera. Many species show considerable morphological variability, some of which may be responses to different growing conditions. In Malesia the greatest difficulties are in Korthalsella, for which Molvray (1990) has argued a substantial reduction in the number of species recognized by Danser (1937,1940), and in Viscum, in which apparently widespread species show considerable polymorphy. Like Loranthaceae, narrow species concepts in the past have led to segregate taxa being recognized. For Malesia many names have been placed in synonymy, especially by Danser (1931, 1935, 1937, 1940, 1941).

Phylogenetic analysis of Viscaceae is difficult, owing to uncertainties about generic relationships, and to the limited number of characters for which polarity states are evident, and applicable at the generic or natural group level. A possible phylogenetic relationship of the genera is indicated by the biogeography of the family (see below).

Cytology

Chromosomal characters have made a significant contribution to phylogenetic knowledge of the family, especially in relation to the origin and maintenance of dioecy in Viscum. The primary basic chromosome number is x = 14, and the other basic numbers of x = 15, 13, 12, 11 and 10 indicate progressive dysploid increase or reduction, usually within genera (Wiens & Barlow 1971, 1979). Polyploidy is rare, but some species have very large chromosomes, the largest equalling those of Loranthaceae and any others in the plant kingdom.

In Viscum sex-associated and floating chromosome translocation complexes are characteristic of dioecious species, but are virtually absent in monoecious species (Wiens & Barlow 1979; Barlow 1981; Aparicio 1993). This suggests that the translocations are primarily associated with the origin and establishment of dioecy, by bringing non-allelic male- and female-determining factors into genetic linkage. The inception of dioecy appears to have initiated a significant secondary radiation of Viscum, especially in Africa (Barlow 1983).

Uses

Mistletoes, including Viscaceae, feature prominently in folk legend and medicine (Kan- ner 1939; Barlow 1987). For general discussion, see introduction to Loranthaceae. The traditional European mistletoe Viscum album is prominent in mythology, for which it has an extensive literature. The Golden Bough of classical Roman mythology was probably this species, as was the killer of the sungod, Balder, of the Germanic peoples, and the heavenly plant worshipped by the Celts. Some of the beliefs surrounding European mistletoe have been transferred to Phoradendron in the New World.

A detailed review of uses of mistletoes (primarily Viscaceae but also Loranthaceae) was provided by Kanner (1939). Along with many magical uses are therapeutic applications which appear to reflect genuine properties of the plants. Institutes which focus specifically on researching medicinal properties of Viscum album have been established, with a strong emphasis on cancer research; for brief historical summaries and experimental studies see, for example, Salle (1980) and Hiilsen & Mechelke (1982).

Phytochemo

Much of the phytochemical study of mistletoes has been undertaken at a time when the Loranthaceae and Viscaceae were treated as a single family Loranthaceae sens. lat. Furthermore many of the studies have involved comparative work in several genera of both families, identifying similarities and differences between the groups then considered subfamilies. For this reason the phytochemistry of the two families has been considered together, to identify the contribution of chemotaxonomy to the current treatment of the two families. As might be predicted, phytochemical studies have shown a number of shared features between the two families. However they have also revealed significant differences which support their distinction. For details see the introductory section on phytochemistry and chemotaxonomy for the family Loranthaceae (p. 217).

Citation

Kuijt - in Brittonia. 1969: 138
Tiegh. - in Bull. Soc. Bot. France. 1896: 247
Agardh, Theoria Syst. PI. 1858: 114
Miers, Contrib. Bot. 1851: 39
Miq. - in Fl. Ind. Bat. 1856: 803
Barlow - in Proc. Linn. Soc. New S Wales. 1964: 269
Danser - in Bull. Jard. Bot. Buitenzorg. 1931: 236