Viscum L.

M. Thulin. Flora of Somalia, Vol. 1–4 [updated 2008] https://plants.jstor.org/collection/FLOS

Distribution

Genus of approximately 100 species, widely distributed in the tropics of Africa (including Madagascar) and Asia, with significant extensions into the north temperate zones of Europe and Asia; in the south temperate regions only in southern Africa and Australia.

Morphology Reproductive morphology Inflorescences

Inflorescence a typical or modified dichasium subtended by a pair of usually fused bracts (bracteal cup); dichasia sessile or pedunculate, solitary or clustered, axillary, or axillary and terminal-Monoecious plants with central flower usually staminate and lateral ones pistillate, or with all flowers of the dichasium staminate or pistillate

Morphology General Habit

Dioecious plants with staminate dichasia most commonly bearing (2–)3 flowers; pistillate dichasia with mostly a solitary flower in the bracteal cup Shrubby (rarely minute herbs), parasitic on dicotyledons (rarely gymnosperms), glabrous, monoecious or dioecious; branching usually dense and intricate, forked or whorled; internodes rounded or compressed, sometimes ribbed and twisted 90° to form a decussate leaf and branching pattern

Morphology Reproductive morphology Flowers

Pistillate flowers 3–4-merous; style present or absent; stigma sometimes expanded Staminate flowers 3–4-merous; anthers dehiscing by numerous pores

Morphology Reproductive morphology Fruits

Berries white, yellow, orange or red, smooth or warty, pedicelled or sessile in the bracteal cup; style and/or stigma usually persistent in fruit.

Flora Zambesiaca. Vol. 9, Part 3. Polygonaceae-Myriaceae. Pope GV, Polhill RM, Martins ES. 2006.

Morphology Reproductive morphology Fruits

Berries white, yellow, orange or red, smooth or warty, pedicelled or sessile in the bracteal cup; style and/or stigma usually persistent in fruit. Berries white, yellow, orange or red, smooth or warty, pedicelled or sessile in the bracteal cup; style and/or stigma usually persistent in fruit.

Note

Problems may be encountered when keying species of Viscum because of evolutionary loss of various organs, e.g. leaves, and extreme size reduction among the reproductive structures commonly employed in identification. Dioecy complicates the problem of identifying staminate plants because most of the useful traits occur in the pistillate inflorescence and fruit. A more serious problem, however, involves the recognition of monoecy. Dioecy in Viscum, unlike many other such groups, is a consistent characteristic and thus an excellent diagnostic character. Leafless monoecious species, however, generally exhibit a paucity to virtual absence of staminate flowers (<10 percent), thus giving the erroneous impression of dioecy. Staminate flowers are generally identifiable by their larger, rounded stature and their shorter, stouter perianth segments, more yellowish colour, and their greater tendency to occur towards the apices of flowering shoots. In monoecious species they are distributed as monads, dyads, or triads in the bracteal cup or (among the African species) as the central flower of the triad. The stamens of monoecious species occur either at the base of the sepals, as they do in all dioecious species, or as a central synandrium. The relatively small size (<1 mm) of many diagnostic features of the pistillate inflorescence and fruit, even if they are consistent, introduce problems of scaling. Thus a fruit 4 mm high is 25 percent longer than a fruit 3 mm high, but the difference would certainly be more apparent if the units were in cm or dm, even though the statistical relationships remain unchanged. All fruit characters apply to mature structures unless otherwise stated. The length of fruiting peduncles and pedicels is particularly critical in this regard, since some species are inevitably borderline in this respect, and full length is not achieved until fruit maturation. In some instances the pedicel may be partially hidden by the bracteal cup. Fruit length is measured from the point of basal attachment to the ring scar that indicates the point at which the perianth segments originally abscissed from the developing fruit. The diameter and ultimate configuration of these scars are often diagnostic, but are not used in the key because they are too difficult to assesss without adequate magnification and critical measuring equipment. The styles are often angular to some extent, and sometimes expanded in one plane, or they appear to be indistinct or absent. In such cases the stigma apparently emerges directly from the summit of the ovary, i.e. the portion of the ovary occurring inside the ring scar. The summit may be flattened, in which case the ring scar is often swollen, or the summit may be slightly elevated to the point of stylar or stigma emergence, giving a volcanate (conical) or mammillate appearance. The stigma may be virtually undefined structurally or slightly expanded to capitate. A number of species have fruits with conspicuous tubercules, but their abundance always decreases during development and some fruit may be smooth at maturity. Fruit colour is generally consistent and remarkably varied between species (white, yellow, dull pink, orange, red) but, unfortunately, fades upon drying. We have generally used vegetative characteristics in the keys where possible to avoid the problem of using floral characters of one sex or another among the dioecious species. When reproductive structures are necessary for identification, we have mostly relied upon characteristics of the pistillate inflorescence, and then the fruit. Most collectors are prone to collect fruiting material and fruits offer more characteristics than the male flowers.

Distribution

A genus of c. 100 species, widely distributed in the tropics of Africa, Madagascar and Asia, with significant extensions into the north temperate zones of Europe and Asia; in the south temperate regions only in southern Africa.

Morphology General Habit

Shrubby plants (rarely minute herbs), parasitic on dicotyledons (rarely gymnosperms), glabrous, monoecious or dioecious; branching usually dense and intricate, forked or whorled; internodes rounded or compressed, sometimes ribbed and twisted 90° to form a decussate leaf and branching pattern. Shrubby plants (rarely minute herbs), parasitic on dicotyledons (rarely gymnosperms), glabrous, monoecious or dioecious; branching usually dense and intricate, forked or whorled; internodes rounded or compressed, sometimes ribbed and twisted 90° to form a decussate leaf and branching pattern Dioecious plants with staminate dichasia most commonly bearing (2)3 flowers; pistillate dichasia with mostly a solitary flower in the bracteal cup Monoecious plants with central flower usually staminate and lateral ones pistillate, or with all flowers of the dichasium staminate or pistillate.

Morphology Reproductive morphology Inflorescences

Monoecious plants with central flower usually staminate and lateral ones pistillate, or with all flowers of the dichasium staminate or pistillate. Inflorescence a typical or modified dichasium subtended by a pair of usually fused bracts (bracteal cup); dichasia sessile or peduncled, solitary or fascicled, axillary or axillary and terminal. Inflorescence a typical or modified dichasium subtended by a pair of usually fused bracts (bracteal cup); dichasia sessile or peduncled, solitary or fascicled, axillary or axillary and terminal Dioecious plants with staminate dichasia most commonly bearing (2)3 flowers; pistillate dichasia with mostly a solitary flower in the bracteal cup.

Morphology Reproductive morphology Flowers

Staminate flowers 3–4-merous; anthers dehiscing by numerous pores. Pistillate flowers 3–4-merous; style present or absent; stigma sometimes expanded. Pistillate flowers 3–4-merous; style present or absent; stigma sometimes expanded Staminate flowers 3–4-merous; anthers dehiscing by numerous pores