There are seven species in this treatment:
The species of Taxus are more geographically than morphologically separable; they were all treated by Pilger (1903) 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 (Hartzell 1991). 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" (Hils 1993). There are hundreds of yew cultivars (Hartzell 1991).
"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" (Hils 1993).
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 (Silba 1986). 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 (de Laubenfels 1988).
See Taxus baccata.
See Taxus baccata.
Yew has an extraordinarily high resistance to urban air pollution. Most of the species are used as ornamentals (Hartzell 1991).
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.
Named from the Greek TOXUS, reflective of TOXON (bow) and TOXIKON (poison); a yew extract was used as an arrow poison (Hartzell 1991).
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 (Hartzell 1991).
"The foliage, bark, and seeds - but not the fleshy red aril - of most Taxus species are toxic due to the presence of taxine (Kingsbury 1964, Cooper et al. 1984); this alkaloid, however, was not found in T. brevifolia (Jones and Lynn 1933). 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 (Kingsbury 1964). 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" (Hils 1993).
Pollination is by wind dispersal. Seed dispersal is primarily by birds, which eat the seeds in the aril and subsequently excrete viable seed (de Laubenfels 1988).
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.]
Jones, I. and E. V. Lynn. 1933. Differences in species of Taxus. Journal of the American Pharmaceutical Association 22:528-531.
Kingsbury, J. M. 1964. Poisonous plants of the United States and Canada. Englewood Cliffs.
Chadwick, L.C. and R.A. Keen. 1976. A study of the genus Taxus. Ohio Agricultural Experiment Station Bulletin 1086.
Keen, R. A. and L. C. Chadwick. 1955. Sex reversal in Taxus. American Nurseryman 100(6):13-14.
Keen, R. A. 1956. A study of the genus Taxus. Ph.D. thesis. Ohio State University.
Li, J., C.C. Davis, P. Del Tredici and M.J. Donoghue. 2001. Phylogeny and biogeography of Taxus (Taxaceae) inferred from sequences of the internal transcribed spacer region of nuclear ribosomal DNA. Harvard Papers in Botany 6: 267-274.
Last Modified 2012-11-23