Viscaceae Description

Genera Included: Arceuthobium M. Bieb.; Dendrophthora Eichl.; Ginalloa Korth.; Korthalsella Tiegh.; Notothixos Oliv.; Phoradendron Nutt.;  Viscum L.

Habit: Brittle "shrubs"

Parasitism: All aerial hemiparasites.

Roots: Modified into haustoria. The endophyte of the haustorium composed of a cortical strands (which are adjacent to the cambium) and sinkers (which penetrate to the xylem). In Arceuthobium, host is induced to form systemic or non-systemic witches' brooms.

Stem: Dichasially branched with nodal constrictions. Glabrous or pubescent.

Leaves: Simple, opposite (decussate), entire, curvinerved, developed or reduced to scales (squamate), with silicified &/or sclerified mesophyll; stomates paracytic; exstipulate.

Inflorescence: Axillary or terminal spikes and cymes (simple and compound dichasia) and fascicles. The dichasia are enfolded by a pair of fused bracts to form a bracteal cup. Dichasia may be sessile or borne on short peduncles. In monoecious plants, the staminate flower is usually centrally situated in the inflorescence while the pistillate flowers are arranged laterally. In dioecious plants, the staminate dichasia normally consist of three male flowers while the pistillate dichasia bear a solitary female flower in the bracteal cup.

Plant Sex: dioecious, monoecious, andromonoecious.

Flowers: Small, unisexual, monochlamydous (the single whorl interpreted as the corolla), epigynous.
Calyx: Not present (completely fused to ovary wall).
Corolla: petals typically 3 in pistillate flowers,  4 (-6) in staminate flowers; valvate; nectary disk present in staminate flowers.
Androecium: 3-4 stamens, opposite the petals, adnate at the base, filaments short (or absent - anther then sessile), anthers tetrasporangiate, dithecal, opening by longitudinal slits or reduced to only two sporangia and then opening by terminal pores or slits, or transversely partitioned and opening by a number of transverse slits, or confluent into a synandrium (Viscum).
Pollen: Binucleate or trinucleate, triaperaturate, mostly spherical.
Gynoecium: Ovary inferior of 3-4 connate carpels, unilocular (or nearly solid with no apparent locule owing to a massive mamelon); style short, terminal or absent (hence stigma sessile).
Ovule: "Typical" ovule lacking; embryo sac embedded in mamelon (= placental nucellar complex).
Embryo, etc.: Flower contains two, bisporic, 8-nucleate embryo sacs; Allium type embryo sac development (Arceuthobium); embryo with short suspensor.

Fruit: A berry containing one (sometimes two) seeds. May be explosive (Arceuthobium). Viscid layer inside the vascular bundles of the seed. Viscin composed on polysaccharide threads interspersed with mucillaginous pectic/mucopolysaccharide substances.

Seed: Testa lacking; embryo with one cotyledon, supposedly derived from a dicotyledonous state; endosperm chlorophyllous and starchy, derived from a primary endosperm nucleus of one of the embryo sacs; seeds typically with one or two embryos, although up to four may be present in one seed.

Chromosomes: Base number for family perhaps 14. Chromosome numbers have been reported for several species of Arceuthobium and all have n = 14 (Hawksworth & Wiens, 1977; Wiens & Barlow, 1971). Counts for ten species of Dendrophthora and over 40 species of Phoradendron were reported in Wiens & Barlow (1971). All but one accession, P. henslovii, a tetraploid from the Galápagos Islands, showed a gametophytic number of 14. Three species of Korthalsella were reported by Wiens & Barlow (1971) as n = 13–14 and two species of Notothixos as n = 12–13. Over 24 species of Viscum are known and their gametophytic numbers vary, depending upon the species, from 10, 12, 14, and 20 (Wiens & Barlow, 1971). Several species of dioecious Viscum show translocation heterozygosity that determines plant sexuality and sex ratios in populations (Wiens, 1979; Wiens & Barlow, 1979; Aparicio, 1993).

Alternate Family Names:  The APG II and III classifications "lumped" Viscaceae into Santalaceae. Arceuthobiaceae Van Tiegh., Bifariaceae Nakai, Ginalloaceae Van Tiegh. ex Nakai, Razoumofskyaceae Van Tiegh.

Last updated: 14-May-18 / dln