Taxus is agenus ofconiferoustrees orshrubs known asyews in the familyTaxaceae.[1] Yews occur around the globe in temperate zones of the northern hemisphere, northernmost inNorway and southernmost in theSouth Celebes. Some populations exist in tropical highlands.[2]
The oldest known fossil species are from the Early Cretaceous.[3]
They are relatively slow-growing and can be very long-lived, and reach heights of 2.5–20 m (8.2–65.6 ft), with trunk girth averaging 5 m (16 ft).[4] They have reddishbark, lanceolate, flat, dark-greenleaves 10–40 mm (1⁄2–1+1⁄2 in) long and2–3 mm (3⁄32–1⁄8 in) 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.[5]
The male cones are globose,3–6 mm (1⁄8–1⁄4 in) across, and shed theirpollen in early spring. Yews are mostlydioecious, but occasional individuals can be variablymonoecious, or change sex with time.[5][6]
Theseed cones are highly modified, each cone containing a singleseed4–7 mm (5⁄32–9⁄32 in) long partly surrounded by a modified scale which develops into a soft, bright redberry-like structure called anaril,8–15 mm (5⁄16–19⁄32 in) long and wide and open at the end. The arils are mature 6–9 months after pollination, and with the seed contained are eaten bythrushes,waxwings and otherbirds, 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.[5]
Taxus is the Latin word for this tree and its wood was used to makejavelins.[7] The Latin word is probably borrowed, via Greekτόξονtóxon, fromtaxša, theScythian word used for "yew" and "bow"[8] (cognate ofPersianتخشTaxš meaning bow)[9][10] because the Scythians used its wood to make their bows.[9]
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 isTaxus baccata, the first yew described scientifically.[11]
Taxus species appear similar. Attempts at taxonomy vary from describing all yews as subspecies ofT. baccata, as did RKF Pilger in 1903, to splitting species by even very small morphological differences, as did R. W. Spjut in 2007 with 25 species and over 50 varieties. Some species have traditionally been recognized by geographic distribution, but Asian species have been more difficult to classify.Taxus contorta in theWestern Himalaya andTaxus sumatrana inMalesia are now generally agreed upon, but overlapping ranges in theEastern Himalaya,China, and subtropical southeast Asia have led to greater confusion, with the speciesTaxus chinensis,Taxus mairei, andTaxus wallichiana being elucidated only in the 21st century with the aid ofmolecular phylogenetics.[2]
The most distinct is the Sumatran yew (T. sumatrana, native toSumatra andCelebes north to southernmostChina), distinguished by its sparse, sickle-shaped yellow-green leaves. The Mexican yew (Taxus globosa, native to easternMexico south toHonduras) 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.[12][citation needed]
Yews typically occur in the understory or canopy of moist temperate or tropical mountain forests. Elevation varies by latitude from 3,000 m (9,800 ft) in tropical forests to near sea level in its northernmost populations.[2]Yews are common in landscape architecture, giving rise to widespreadnaturalized populations in the United States. There, bothT. baccata andTaxus cuspidata are common ornamental shrubs.[13]
T. baccata appears throughout Europe and into western Asia.[2]T. cuspidata occurs over much of East Asia, in China, Japan, Korea, andSakhalin.[14]Taxus brevifolia ranges in the United States from California to Montana and Alaska,[13] whileTaxus canadensis appears in the northeastern United States and southeast Canada.[2]
All species of yew contain highlypoisonoustaxinealkaloids, with some variation in the exact formula of the alkaloid between the species. All parts of the tree except thearils contain the alkaloid. The arils are edible and sweet, but the seed is dangerouslypoisonous; unlike birds, thehuman stomach can break down the seed coat and release the toxins into the body. This can have fatal results if yew 'berries' are eaten without removing the seeds first. Grazing animals, particularlycattle andhorses, are also sometimes found dead near yew trees after eating the leaves, thoughdeer 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 thelarvae of someLepidopteraninsects including the mothwillow beauty.[citation needed]
All parts of a yew plant are toxic to humans with the exception of the yew berries (which however contain a toxic seed); additionally, male and dioecious yews in this genus releasecytotoxic pollen, which can cause headaches, lethargy, aching joints, itching, and skin rashes; it is also a trigger for asthma. These pollen granules are extremely small, and can easily pass through window screens. Male yews bloom and release abundant amounts of pollen in the spring; completely female yews only trap pollen while producing none.[23]
Yews in this genus are primarily separate-sexed, and males are extremely allergenic, with anOPALS allergy scale rating of 10 out of 10. Completely female yews have an OPALS rating of 1, and are considered "allergy-fighting".[23]
Yew wood is reddish brown (with whiter sapwood), and is very springy. It was traditionally used to make bows, especially thelongbow. These longbows were used byScythian people who were part of the police force in ancientAthens. This use was lent into theAncient Greek word for "bow"[9] and later probably borrowed into the Latin word and now generic name ofTaxus.
Ötzi, the Chalcolithic mummy found in 1991 in the Italian Alps, carried an unfinished bow made of yew wood. Consequently, it is not surprising that inNorse mythology, the abode of the god of the bow,Ullr, had the nameYdalir (Yew Dales). Most longbow wood used in northern Europe was imported fromIberia, where climatic conditions are better for growing the knot-free yew wood required.[24] The yew longbow was the critical weapon used by the English in the defeat of the Frenchcavalry at theBattle of Agincourt, 1415. British yews tend to be too gnarly, and thus the wood for English longbows used at the Battle of Agincourt was imported from Spain or northern Italy.[25]
It is suggested that Englishparishes were required to grow yews and, because of the trees' toxic properties, they were grown in the only commonly enclosed area of a village – the churchyard.[26] 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 can be very long-lived. TheFortingall Yew has been considered to be the oldest tree in Europe, at something over 2,000 years old. Tradition has it thatPontius Pilate slept under it while on duty before 30 AD. Claims for an older tree have been made for theDefynnog Yew in the churchyard ofSt Cynog's Church, Defynnog, Wales,[27] but this view is contested.[28] Such old trees usually consist of a circular ring of growths of yew, since their heart has long since rotted away.
Foliage of Irish yew; note the leaves spreading all round the erect shoots
Yews are widely used in landscaping and ornamentalhorticulture. Over 400cultivars of yews have been named, the vast majority of these being derived from European yew (Taxus baccata) or Japanese yew (Taxus cuspidata). The hybrid between these two species isTaxus ×media. A popular fastigiate selection of the European yew (Taxus baccata 'Fastigiata') is often called the Irish yew, illustrating the difficulties with common names. A few cultivars with yellow leaves are collectively known as golden yews.
However, methods were developed to produce the drug semi-synthetically from the leaves of cultivated European yews. Those can be sustainably harvested without the need to further endanger wild populations, and the Pacific yew is no longer at risk.[32] The more common Canada yew is also being successfully harvested in northernOntario,Quebec andNew 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 European yew (Taxus baccata).
Oldest Polish specimen of European yew (1200 years)
^abGarland, Tam; Barr, A. Catherine (1998).Toxic plants and other natural toxicants. International Symposium on Poisonous Plants (5th : 1997 : Texas). Wallingford, England: CAB International.ISBN0-85199-263-3.OCLC39013798.
^Kvaček, Z. 1984. Tertiary taxads of NW Bohemia. 1982 Acta Univ. Carol., Geol., Pokorny 4: 471–491.
^abSpjut, R. W. (2007). "Taxonomy and nomenclature ofTaxus (Taxaceae). A phytogeographical analysis ofTaxus (Taxaceae) based on leaf anatomical characters".J. Bot. Res. Inst. Texas.1 (1):291–332. 203–289.T. brevifolia andT. globosa var.floridana thought to evolve from ancestralT. globosa by loss of stomata and papillae;T. canadensis recognized in Europe based on leaf fossils from late Tertiary deposits
^Corneanu, G. C. , M. Corneanu and R. Bercu. 2004. Comparison between some morpho-anatomical features at fossil vegetal species and at their actual correspondent species. Studia Universitatis Babeş-Bolyai, Geologia, XLIX: 77–84.
^Manchester, S.R. (1994). "Fruits and Seeds of the Middle Eocene Nut Beds Flora, Clarno Formation, Oregon".Palaeontographica Americana.58:30–31.
