| # The Families of Angiosperms |

L. Watson and M.J. Dallwitz

### Viscaceae Miq.

~ Santalaceae-Visceae.

Including Arceuthobiaceae Van Tiegh., Bifariaceae Nakai, Ginalloaceae Van Tiegh. ex Nakai, Razoumofskyaceae Van Tiegh.

Habit and leaf form. Chlorophyllous but parasitic shrubs. ‘Normal’ plants, or switch-plants; sometimes with the principal photosynthesizing function transferred to stems. Leaves well developed to much reduced, or absent. Plants rootless (in the normal sense, but with haustoria); parasitic; haustorially parasitic; green and photosynthesizing; parasitic on aerial parts of the host. Stem growth conspicuously sympodial (often pseudodichotomous), or not conspicuously sympodial. Mesophytic, or xerophytic. Leaves opposite; ‘herbaceous’, or leathery, or membranous; simple. Lamina entire; one-veined, or parallel-veined. Leaves exstipulate.

Leaf anatomy. Stomata in Arceuthobium paracytic. Hairs present, or absent; if present, eglandular. The mesophyll containing crystals. The crystals druses. Minor leaf veins without phloem transfer cells (Arceuthobium).

Axial (stem, wood) anatomy. Secretory cavities present, or absent. Cork cambium present, or absent; when present, initially superficial. Primary vascular tissues in a cylinder, without separate bundles (usually), or comprising a ring of bundles; collateral. Internal phloem absent. Secondary thickening developing from a conventional cambial ring, or anomalous (? - young Arceuthobium stems exhibiting two opposite pairs of bundles, on pair more strongly developed than the other). Primary medullary rays wide.

The wood ring porous. The vessels small; variously arranged. The vessel end-walls simple. The axial xylem with libriform fibres.

Reproductive type, pollination. Unisexual flowers present. Plants monoecious, or dioecious, or polygamomonoecious (?). Pollination anemophilous, or entomophilous.

Inflorescence, floral, fruit and seed morphology. Flowers solitary to aggregated in ‘inflorescences’; in cymes, or in racemes, or in spikes. The ultimate inflorescence units cymose. Inflorescences terminal, or axillary; (1-)3-flowered dichasia in spikes or branched inflorescences. Flowers bracteate; minute to small; regular; cyclic.

Perianth sepaline, or petaline, or vestigial (often represented by mere bumps or teeth on the ovary rim); (2–)3, or 4; free, or joined; 1 whorled; persistent, or deciduous (e.g. caducous in female Viscum). Calyx (if P so interpreted) (2–)3, or 4 (females often 3, males 4); 1 whorled; polysepalous, or partially gamosepalous, or gamosepalous; entire, or lobulate, or blunt-lobed, or toothed; regular; persistent, or not persistent; valvate, or open in bud.

Androecium 4. Androecial members free of the perianth, or adnate (to the perianth); all equal; free of one another, or coherent (via the anthers); 1 whorled. Androecium exclusively of fertile stamens. Stamens (2–)4; isomerous with the perianth; oppositisepalous; filantherous, or with sessile anthers. Anthers separate from one another, or cohering (e.g. Korthalsella having a synandrium); dehiscing via pores, or dehiscing via short slits, or dehiscing via longitudinal slits, or dehiscing transversely; introrse; unilocular to many locular; bisporangiate, or tetrasporangiate. Endothecium developing fibrous thickenings. Microsporogenesis simultaneous. Anther wall initially with one middle layer, or initially with more than one middle layer; of the ‘monocot’ type. Tapetum glandular. Pollen grains aperturate; 3 aperturate; 2-celled (in 3 genera).

Gynoecium 3 carpelled, or 4 carpelled. Carpels isomerous with the perianth, or reduced in number relative to the perianth. The pistil 1 celled. Gynoecium syncarpous; eu-syncarpous; inferior. Ovary 1 locular. Gynoecium non-stylate, or non-stylate to stylate, or stylate. Styles 1; apical. Placentation basal (i.e. with a short placental column). Ovules not differentiated; in the single cavity 2; sessile; orthotropous; without integuments (the ovules being scarcely developed). Embryo-sac development Allium-type. Polar nuclei fusing prior to fertilization. Endosperm formation cellular. Embryogeny often by transverse cleavage.

Fruit fleshy; indehiscent; a berry (with viscous tissue within); elastically dehiscent (sometimes), or passively dehiscent; 1(–2) seeded. Seeds endospermic; without a testa. Embryo weakly differentiated. Cotyledons 2. Embryo chlorophyllous (2/2).

Physiology, phytochemistry. Not cyanogenic. Alkaloids present (Viscum, Phoradendron). Arbutin absent. Saponins/sapogenins absent. Proanthocyanidins present, or absent (Viscum); cyanidin and delphinidin (Arceuthobium). Flavonols present, or absent; quercetin and myricetin. Ellagic acid absent (Arceuthobium, Viscum). Aluminium accumulation not found.

Geography, cytology. Temperate to tropical. Cosmopolitan, especially tropical and warm regions. X = 10–14 (polyploidy rare).

Taxonomy. Subclass Dicotyledonae; Crassinucelli. Dahlgren’s Superorder Santaliflorae; Santalales. Cronquist’s Subclass Rosidae; Santalales. APG III core angiosperms; core eudicot; Superorder Santalanae. APG IV Order Santalales (as a synonym of Santalaceae).

Species 450. Genera 7; Arceuthobium, Dendrophthora, Ginalloa, Korthalsella, Notothixos, Phoradendron, Viscum.

General remarks. Distinguishable from Santalaceae sensu stricto (q.v.) by non-orthotropous ovules, berries, and (supposedly) by a weakly differentiated but chlorophyllous embryo (?); also differing in data on ‘esoteric characters’ representing limited sampling (anatomical details of the xylem and anther walls; polyacetylenes).

Economic uses, etc. Viscum album and Phoradendron flavescens are the Yuletide mistletoes.

Quotations.

The trees, though summer, yet forlorn and lean,
O’ercome with moss, and baleful mistletoe
(‘Titus Andronicus’, ii., 3)

Illustrations. • Le Maout and Decaisne: Arceuthobium, Viscum. • Le Maout and Decaisne: Viscum. • Arceuthobium vaginatum var. cryptopodium, as A. cryptopodium: Hook. Ic. Pl. 11 (1867–71). • legend: Arceuthobium vaginatum, Hook. Ic. Pl. 1037. • Dendrophthora cupressoides: Hook. Ic. Pl. 23 (1894). • Notothixos malayanus: Hook. Ic. Pl. 16 (1886). • Phoradendron bolleanum, as Viscum: Hutchinson. • Viscum album: Köhler's Medizinal Pflanzen 1 (1887). • Viscum cruciatum: Bot. Mag. 128 (1902). • Viscum album (B. Ent.).

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications (Dahlgren; Dahlgren, Clifford, and Yeo; Cronquist; APG). See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 8th June 2026. delta-intkey.com’.

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