Definitions

irish-yew

Taxus

Taxus is a genus of yews, small coniferous trees or shrubs in the yew family Taxaceae. They are relatively slow growing and can be very long-lived, and reach heights of 1-40 m, with trunk diameters of up to 4 m. They have reddish bark, lanceolate, flat, dark-green leaves 1-4 cm long and 2-3 mm broad, arranged spirally on the stem, but with the leaf bases twisted to align the leaves in two flat rows either side of the stem.

The seed cones are highly modified, each cone containing a single seed 4-7 mm long partly surrounded by a modified scale which develops into a soft, bright red berry-like structure called an aril, 8-15 mm long and wide and open at the end. The arils are mature 6-9 months after pollination, and with the seed contained are eaten by thrushes, waxwings and other birds, which disperse the hard seeds undamaged in their droppings; maturation of the arils is spread over 2-3 months, increasing the chances of successful seed dispersal. The male cones are globose, 3-6 mm diameter, and shed their pollen in early spring. Yews are mostly dioecious, but occasional individuals can be variably monoecious, or change sex with time.

All of the yews are very closely related to each other, and some botanists treat them all as subspecies or varieties of just one widespread species; under this treatment, the species name used is Taxus baccata, the first yew described scientifically.

The most distinct is the Sumatran Yew (T. sumatrana, native from Sumatra and Celebes north to southernmost China), distinguished by its sparse, sickle-shaped yellow-green leaves. The Mexican Yew (T. globosa, native to eastern Mexico south to Honduras) is also relatively distinct with foliage intermediate between Sumatran Yew and the other species. The Florida Yew, Mexican Yew and Pacific Yew are all rare species listed as threatened or endangered.

All species of yew contain highly poisonous alkaloids known as taxanes, with some variation in the exact formula of the alkaloid between the species. All parts of the tree except the arils contain the alkaloid. The arils are edible and sweet, but the seed is dangerously poisonous; unlike birds, the human stomach can break down the seed coat and release the taxanes into the body. This can have fatal results if yew 'berries' are eaten without removing the seeds first. Grazing animals, particularly cattle and horses, are also sometimes found dead near yew trees after eating the leaves, though deer are able to break down the poisons and will eat yew foliage freely. In the wild, deer browsing of yews is often so extensive that wild yew trees are commonly restricted to cliffs and other steep slopes inaccessible to deer. The foliage is also eaten by the larvae of some Lepidopteran insects including Willow Beauty.

Uses and traditions

Yew wood is reddish brown (with whiter sapwood), and is very springy. It was traditionally used to make bows, especially the longbow. Ötzi, the Chalcolithic mummy found in 1991 in the Austrian alps, carried an unfinished longbow made of yew wood. Consequently, it is not surprising that, in Norse mythology, the god of the bow, Ullr's abode had the name Ydalir (Yew dales). Most longbow wood used in northern Europe was imported from Iberia, where climatic conditions are better for growing the knot-free yew wood required. The Eihwaz rune ᛇ is named after the yew, and sometimes also associated with the "evergreen" World tree, Yggdrasil.

Yews are widely used in landscaping and ornamental horticulture. Over 400 cultivars of yews have been named, the vast majority of these being derived from Taxus baccata (European Yew) or Taxus cuspidata (Japanese Yew). The hybrid between these two species is (Taxus x media). A popular fastigiate selection of the European Yew (Taxus baccata 'Fastigiata') is often called the Irish Yew, which often complements the fact of the difficulty's with common names. A few cultivars with yellow leaves that are being propagated, collectively are known as golden yews, which is another nomenclature blunder.

The Pacific Yew Taxus brevifolia, native to the Pacific Northwest of North America, and Canada Yew Taxus canadensis are the sources of paclitaxel, a chemotherapeutic drug used in breast and lung cancer treatment and, more recently, in the production of the Taxus drug eluting stent by Boston Scientific. Over-harvesting of the Pacific Yew for this drug has resulted in it becoming an endangered species, though the drug is now produced semi-synthetically from cultivated yews, without the need to further endanger the wild populations. The more common Canada yew, Taxus canadensis, is also being successfully harvested in northern Ontario, Québec and New Brunswick, and has become another major source of paclitaxel. Other yew species contain similar compounds with similar biochemical activity. Docetaxel, an analogue of paclitaxel, is derived from the Taxus baccata.

The yew tree can often be found in church graveyards and is symbolic of sadness. Such a representation appears in Lord Alfred Tennyson's poem "In Memoriam A.H.H." (2.61-64).

The yew tree is a frequent symbol in the Christian poetry of T. S. Eliot, especially his Four Quartets.

On January 18, 2008, the Botanic Gardens Conservation International (representing botanic gardens in 120 countries) stated that "400 medicinal plants are at risk of extinction, from over-collection and deforestation, threatening the discovery of future cures for disease." These included Yew trees (the bark is used for cancer drugs, paclitaxel); Hoodia (from Namibia, source of weight loss drugs); half of Magnolias (used as Chinese medicine for 5,000 years to fight cancer, dementia and heart disease); and Autumn crocus (for gout). The group also found that 5 billion people benefit from traditional plant-based medicine for health care

Hybrids

Taxus x media = Taxus baccata x Taxus cuspidata

Taxus x hunnewelliana = Taxus cuspidata x Taxus canadensis

References

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