Taxus description

Range of the genus Taxus.

Taxus Linnaeus 1753

Common Names

Yew (1).

Taxonomic notes

Eight species in this treatment (1). The species of Taxus are more geographically than morphologically separable; they were all treated by Pilger (3) as subspecies of T. baccata. All species are poisonous; most contain the anti-cancer agent taxol; and a study of heartwood constituents of T. baccata, T. brevifolia, T. cuspidata and T. floridana found them to be chemically almost identical (4). However, the vast ecological amplitude displayed by the various described species (over 60° of latitude and an impressive temperature and precipitation range) suggests that they are, in a meaningful sense, true species. "Detailed study of the genus (not neglecting the cultivated representatives), including extensive fieldwork, is much needed and long overdue" (2). There are hundreds of yew cultivars (4).

Description

"Trees or shrubs, dioecious or monoecious. Bark reddish brown, scaly. Branches ascending to drooping; twigs irregularly alternate, green or yellow-green when young, reddish brown in age. Leaves often appearing 2-ranked, flexible; stomates abaxial, in 2 broad, pale bands; apex soft-pointed, mucronate, not sharp to touch; resin canal absent. Pollen cones globose, yellowish, with 4 - 16 peltate sporophylls, each bearing 2 - 9 sporangia. Ovule 1. Seed maturing in 1 season, brown; aril scarlet to orange-scarlet, soft, mucilaginous, thick, cup-shaped, open at apex, exposing hard seed coat; albumen uniform. x = 12" (2).

Range

Europe: Britain to N Iran. Asia: Russia, Korea, Japan, China, Taiwan, Himal, India, Burma, Vietnam, Philippines. N America: SE Alaska to California, SE Canada to NE USA, Florida, Mexico, Guatemala, El Salvador (1). The eight species are almost wholly allopatric and largely confined to the middle latitudes of the northern hemisphere, with some intrusion to tropical highlands, the northernmost occurrence being in Norway and the southernmost below the equator in the S Celebes. Plants are found in the understory or canopy of moist temperate or tropical mountain forests. Their elevation ranges from near sea level in northern stations to 3000 m in tropical forests (8).

Big Tree

See Taxus baccata.

Oldest

See Taxus baccata.

Dendrochronology

Ethnobotany

Yew has an extraordinarily high resistance to urban air pollution. Most of the species are used as ornamentals (4).

Observations

None of the species are particularly rare, and most can be found in botanic gardens. Taxus baccata is, within the temperate zone, among the commonest ornamental conifers.

Remarks

Named from the Greek TOXUS, reflective of TOXON (bow) and TOXIKON (poison); a yew extract was used as an arrow poison (4).

Paleobotany: The oldest recognizable yew is the Triassic Paleotaxus rediviva, found in strata 200 ma old. The mid-Jurassic Taxus jurassica (140 ma old) is more recognizable as a member of Taxus, containing features characteristic of T. baccata, T. cuspidata, and T. brevifolia. A Quaternary yew, Taxus grandis, is probably simply T. baccata (4).

"The foliage, bark, and seeds - but not the fleshy red aril - of most Taxus species are toxic due to the presence of taxine (5, 7); this alkaloid, however, was not found in T. brevifolia (6). Two Eurasian species, T. baccata (English yew) and T. cuspidata (Japanese yew), are best known and documented for toxicity. Cattle have been poisoned by T. canadensis planted in British Columbia, but toxicity of T. brevifolia has not been conclusively recorded (7). Although horses, cattle, and humans have been poisoned by ingesting yew leaves and seeds, the fresh foliage of T. canadensis is browsed by deer, and that of T. brevifolia by moose" (2).

Pollination is by wind dispersal. Seed dispersal is primarily by birds, which eat the seeds in the aril and subsequently excrete viable seed (8).

Citations

(1) Silba 1986.

(2) Matthew H. Hils at the Flora of North America web site.

(3) Pilger 1903.

(4) Hartzell 1991.

(5) Cooper, M.R. and A.W. Johnson. 1984. Poisonous plants in Britain and their effects on animals and man. London. [Minist. Agric., Fisheries & Food Ref. Book 161.]

(6) Jones, I. and E. V. Lynn. 1933. Differences in species of Taxus. Journal of the American Pharmaceutical Association 22:528-531.

(7) Kingsbury, J. M. 1964. Poisonous plants of the United States and Canada. Englewood Cliffs.

(8) de Laubenfels 1988.