Including Arceuthobiaceae Van Tiegh., Lepidocerataceae 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, or much reduced. Plants rootless (in the normal sense - with haustoria); partially 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. Minor leaf veins without phloem transfer cells (Arceuthobium).
Stem anatomy. Secretory cavities present, or absent. Secondary thickening developing from a conventional cambial ring (?). Xylem with libriform fibres; with vessels. Vessel end-walls simple.
Reproductive type, pollination. Monoecious, or dioecious, or polygamomonoecious (?). Anemophilous, or entomophilous.
Inflorescence, floral, fruit and seed morphology. Flowers solitary to aggregated in `inflorescences'; in cymes, or in racemes, or in spikes. The terminal inflorescence unit 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 gamosepalous; entire, or lobulate, or 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.
Gynoecium 3, or 4. Carpels isomerous with the perianth, or reduced in number relative to the perianth. Gynoecium syncarpous; eu-syncarpous; inferior. Ovary 1 locular. Styles 1; apical. Placentation basal (i.e. with a short placental column). Ovules not differentiated; in the single cavity 2; sessile; ascending; 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, biochemistry. Not cyanogenic. Alkaloids present (Viscum, Phoradendron). Proanthocyanidins present, or absent (Viscum); cyanidin and delphinidin (Arceuthobium). Flavonols present, or absent; quercetin and myricetin. Ellagic acid absent (Arceuthobium, Viscum). Arbutin absent. Saponins/sapogenins absent. 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. Takhtajan's Subclass Rosidae; Celastranae; Santalales. Species 450. Genera 7; Arceuthobium, Dendrophthora, Ginalloa, Korthalsella, Notothixos, Phoradendron, Viscum.
Economic uses, etc. Viscum album and Phoradendron flavescens are the Yuletide mistletoes.
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