Taxus

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Taxus
Taxus baccata MHNT.jpg
Taxus baccata (European yew) shoot with
mature and immature cones
Scientific classification
Kingdom: Plantae
Division: Pinophyta
Class: Pinopsida
Order: Pinales
Family: Taxaceae
Genus: Taxus
L.
Species
  • see text

Taxus is a genus of 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 5 m.[1] 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.[2]

Morphology[edit]

Seeds of Taxus baccata

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.[2]

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.[3] Other source, however, recognize 9 species, for example the Plant List.

Foliage of Mexican yew

The most distinct is the Sumatran yew (T. sumatrana, native to 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.

Male cones of Taxus baccata

Uses and traditions[edit]

Yew wood is reddish brown (with whiter sapwood), and is very springy. It was traditionally used to make bows, especially the longbow.[4] Ötzi, the Chalcolithic mummy found in 1991 in the Italian alps, carried an unfinished longbow made of yew wood. Consequently, it is not surprising that in Norse mythology, the abode of the god of the bow, Ullr, 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.[citation needed] The yew longbow was the critical weapon used by the English in the defeat of the French cavalry at the Battle of Agincourt, 1415. It is suggested that English parishes 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.[5] 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. The Fortingall Yew is considered to be the oldest tree in Europe, at something over 2000 years old. Tradition has it that Pontius Pilate slept under it while on duty before 30 CE. This has recently been topped by a tree in the church yard of a small Welsh Village called St Cynog. It has been dated to 5000 years old by dendrologist Janis Fry.[6] Such old trees usually consist of a circular ring of growths of Yew, since their heart has long since rotted away.

The Eihwaz rune ᛇ is named after the yew, and sometimes also associated with the "evergreen" world tree, Yggdrasil.

Foliage of Irish yew; note the leaves spreading all round the erect shoots

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 difficulties 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 or Taxol, 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 led to fears that it would become an endangered species. However, methods were developed to produce the drug semi-synthetically from cultivated yews, without the need to further endanger the wild populations, and the Pacific yew is not endangered.[7] 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 Taxus baccata.

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).[8]

Oldest Polish specimen of Taxus baccata (1200 years)

Species and hybrids[edit]

Names not linked have been mentioned in publications but not yet validated according to the International Code of Botanical Nomenclature

Fossil (extinct) species

  • Taxus engelhardtii – Oligocene, Bohemia, twig-leaves, similar to T.mairei[9][10]

Commonly reported hybrids

Taxus x media = Taxus baccata x Taxus cuspidata[13]

Taxus x hunnewelliana = Taxus cuspidata x Taxus canadensis[14]

The Species Problem in Taxus: What is a species of Taxus?[edit]

There are as few as 1 to as many as 25 species in the genus Taxus (yew) distributed across the northern temperate forests, extending south into montane forests of tropical regions of Central America and Southeast Asia (Sumatra). Many varieties also recognized bringing the total number of taxa (taxon) to 57; 55 have been differentiated in a taxonomic key[15] (accompanied by images of herbarium specimens).[16] Also many hybrids recognized; 50 cultivars in one study all had unique DNA banding patterns.[17]

What is a species of Taxus? Defining yew species fall under three systematic approaches: (1) geographical—one that conforms to a biological species concept (Theodosius Dobzhansky, Ernst Mayr), (2) pattern recognition of morphological characteristics that define the fewest number of species for the entire genus Taxus,[10][18][19] and (3) numerical taxonomy—morphological and molecular data of Taxus analyzed statistically on a geographical basis.[20] These approaches are further explained as follows.

Geographical species concept in Taxus. The low end of the spectrum in the number of yew (Taxus) species is based on major discontinuous gaps in geographical occurrence in the genus, treated as subspecies[21] or species.[22] For example, in North America, yew is found in the forested regions separated by extensive grassland and desert. The yew regions are (1) southern coastal Alaska and western Montana south to central California (in northern Idaho but absent east of the Cascade Range in Oregon), represented by the Pacific yew (2) the Appalachian Region represented by the Canada yew, (3) a localized mixed mesophytic forest in western Florida represented by the Florida yew and (4) temperate montane forests in Mexico to Central America represented by the Mesoamerican yew.[23]

Each yew region in North America is considered to have its own subspecies[21] or species[24] that conform to the traditional biological species concept (Theodosius Dobzhansky, Ernst Mayr). They are reproductively isolated by geographic and climate barriers, and known to reproduce sexually within each region. The geographical aspect of the biological species concept in Taxus is given considerable weight because Taxus among the different geographic regions look more alike than unalike. This is the primary reason why Robert Knud Friedrich Pilger, in 1903,[21] recognized only one species in the genus. Also, because the species are dioecious with different maturation periods for the release of pollen and seed, specimens in a herbarium rarely have both sexes represented for a particular collection. This makes it difficult to draw conclusions about the reproductive biological differences in Taxus. An example is release of pollen in the spring preceded by microsporogenesis in autumn in Taxus brevifolia.,[25] in contrast to microsporogensis and pollen release during the same year in Taxus wallichiana var. yunnanensis.[26]

Pattern recognition species concept in Taxus. The greater number of species in Taxus is weighted on morphological differences that subjectively recognize the fewest number of species and varieties for the whole genus without regard to whether the species are known to be geographically or ecologically isolated.[10] Evolutionary lineages are taken into consideration based on the biogeographical relationships of qualitative and quantitative character data and the fossil record.[10] The higher taxonomic level characters include the shape of the leaf epidermal cells as seen in transverse section (egg-shaped vs. rectangular to square), number of rows of stomata on the undersurface of leaves in each of two stomata bands, and the number of marginal cells adjacent to the stomata bands lacking papillae. Species are classified into three groups, two subgroups and two alliances.[10] Alliances reflect geographical distinction; for example in the T. baccata Group, the T. baccata Alliance occurs in Europe and in the Mediterranean Region, and the T. cuspidata Alliance is restricted to eastern temperate Asia.[27] The lower level taxonomic characters that are applied to distinguish species and varieties include branching (divide equally or unequally), leaf arrangement (spread equally all around stem vs. along two sides), leaf shape and color in dried specimens, and seed shape.

Based on this morphological approach, yew in the four North American regions comprise three species and eight varieties. The Florida yew is an example of one regarded as a variety of the Mesoamerican yew because the two varieties partially overlap in a key morphological difference—the number of stomata rows in a stomata band (5–9 Florida, 7–11 Mexico[10]). The two varieties are further distinguished by the distribution and prominence of papillae on the undersurface of leaves along the leaf marginal area (less papillose in Florida yew[22]). As a result, the Florida yew is seen as occurring in northern Mexico where yew leaves have stomata in 7 rows bordered by 6 rows of marginal cells without papillae.[10]

Also, the three North American species of yew are not all related to one another. The Canada yew (Appalachian Region) is suggested to be more closely related to yew species in eastern Asia (T. cuspidata Alliance) than to species in North America—by the elliptical shape of the leaf epidermal cells in transverse section.[10] The two other North American species of Taxus have quadrangular to tall rectangular shaped epidermal cell. They are considered related to species in the eastern Himalayas (T. wallichiana Group). There are many examples of higher plants that show these types of biogeographical relationship.[10]

These relationships are strengthened by yew fossils that scarcely differ from extant species in leaf shape and in leaf epidermal features. Two from late Tertiary deposits in Europe closely resemble the Canada yew more than the typical European yew.[9] Another of Middle Miocene age from the Latah Formation in northern Idaho hardly differs from T. brevifolia.[28]

The value of the morphological approach is seen from independent molecular studies that also show the Canada yew to be more closely related to yew in eastern Asia (T. cuspidata Alliance) than to the North American species[29] and the other two species American species are more closely related to the eastern Himalayan yews than to European yew.[30]

Although fewer species (and more varieties) are recognized for North America based on morphological differences than geographical differences, a major disagreement exists with the greater number of yew species recognized to occur in China, 13 species (+ 1 variety),[10] vs. 4 species,[22] while 19 different haplotypes have been distinguished.[31] The more widely distributed Taxus species in Asia that overlap in their geographical range show ecological differences; for example, Taxus mairei occurs in lowland broad leaved forests from 300 to 1,300 m, T. chinensis at the higher elevations in mixed mesophytic forests, 1000 to 2800 m, and T. wallichiana in conifer forests, mostly from 2300 to 3200 m.[10]

The phytogeography of the character relationships of Taxus leaves, including those in the fossil records, are seen as evidence of the present day species having already evolved by late Pliocene. In addition to the leaf fossils similar to the Canada yew, another similar to T. mairei, an endemic to southern China, hardly differs from one that was described from a deposit of Oligocene age in Bohemia. The greater morphological variation in China is believed to be the result of less impact by glaciers during the Pleistocene than in other regions of the world, and that more hybridization events had occurred there than in North America as the climate changed with the advance and retreat of glaciers during the Pleistocene.[10]

Numerical taxonomy of Taxus. The species of Taxus are statistically delimited by principal component analysis (PCA) of morphological and molecular character data. Species of Taxus are recognized if character attributes of their leaves show separation; if these differences can be correlated with molecular data, and if they are geographically or ecologically isolated from other species, although an exception was recognized for T. mairei reported to occur in the Himalayas and in southern China.[32] As many as 26 characters of a Taxus leaf were employed in the principal component analysis.

A weakness to this numerical (PCA) approach is that it tends to recognize the character traits of the most common taxa in regard to the relative species abundance, whereas the bulk of the species are likely to be rare as exemplified by hollow curve distribution patterns.[33][34]

Botanical nomenclature problems in Taxus[edit]

The extensive literature on Taxus before 2007 is largely based on a geographical species concept with exception to yew in the Himalayas. As long as one knew where the yew grew, its species was automatically determined by its geography, not by its morphology. But that philosophy seems to be changing; for example, Taxus florinii, a species found in northwest Yunnan and southwest Sichuan, which is recognized by having 8–11 rows of stomata per stomata band, in contrast to more than 11 rows of stomata in related species, occurs geographically within the range of Taxus chinensis and Taxus wallichiana; it is accepted by two sources.[10][32]

When more than one species of Taxus exists in the same region (sympatric species), then it is necessary to have a reference specimen, which in the practice of taxonomy is referred to as a type (biology) specimen, usually the one used by the author who named the species. Types were hardly known in Taxus until 2007. Images of type specimens for many plants have since become available online, but often not to the general public; access usually requires one to pay a fee, or to be affiliated with an institution that pays for the access.[35]

At the center of the nomenclature problems in Taxus is the Himalayan species. The first to be described from that region was Taxus wallichiana by Joseph Gerhard Zuccarini in 1843 based on specimens distributed by Nathaniel Wallich. Most specimens were listed in a catalogue arranged by genus and species with reference to a species number. Of relevance is number 6054 assigned the name Taxus nucifera (=Torreya nucifera), which included 6054a reported to have been collected in 1822 from Nepal (collector not indicated), and 6054b reported to have been collected from Kumaon division by Robert Blinkworth (without date of collection). Taxus nucifera was described by Linnaeus in his Species Plantarum for a species that occurs in Japan; this was later recognized by Philipp Franz von Siebold and Zuccarini (in 1846[36] to belong to the genus Torreya. The specimens of Taxus at Botanische Staatssammlung München had either been mislabeled or switched for numbers 6054a and 6054b. Additionally, Wallich sent Zuccarini another specimen of Taxus from northeastern India (not listed in his catalogue), one that compares closely to Zuccarini’s illustration for the description he gave of Taxus wallichiana. At the time it seems that Wallich[37] and Zuccarini recognized only one Himalayan species.

William Griffith, who traveled with Wallich on occasion, collected in the Himalayas where he recognized three species of Taxus, two in Bhutan without names and one in Afghanistan he named “Taxus contortus?” (Taxus contorta),[38] with reference to his earlier description[39]

When Pilger published his taxonomic revision of Taxus in 1903,[21] he recognized only one species, which he divided into seven subspecies, one of which occurred in the Himalayas, subsp. wallichiana, extending to China and south to Sumatra; the West Himalayan yew may have been considered the same as the European yew, described by Carl Linnaeus, T. baccata. In the 1970’s, the Himalayan yews were distinguished as two species by botanists publishing for the Flora of China (Chinese version,[40]) and for the Flora Malesiana.[41] Taxus yunnanensis was described as a new species, typified by specimens from southeastern Tibet, and reported to occur from the East Himalayas to China. The Flora Malesiana species was referred to T sumatrana that included the East Himalayan yew, T. yunnanensis, apparently under the assumption that the West Himalayan yew belonged to T. wallichiana.[42] Pilger had included Cephalotaxus sumatrana, a synonym of T. sumatrana under subspecies wallichiana. The Wallich specimen sent to Zuccarini from northeastern India was annotated in 1995 as lectotype (biology).[43]

In November 1997, Taxus fuana was described as a new species from West Himalayas, typified by specimens from southwestern Tibet (Jilong, 3000 m, Qingzhang Expedition 7032).[44] However, many specimens of Taxus in herbaria in London, Kew, Paris, and in the United States that were collected in the West Himalayas, had been annotated Taxus contorta in October 1997, while many others at the Harvard University Herbaria were also annotated by that name in July 1996.

The earliest name is the one that must be applied according to the International Code of Botanical Nomenclature. Those who prefer to recognize just one species in Southeast Asia, T. wallichiana is the earliest name; those who distinguish the West Himalayan yew from the East Himalayan, Taxus contorta is the correct name for the West Himalayan yew. If the yew in the East Himalayas is defined as occurring only within that region, then that allows possibilities for other names to be applied to species in Southeast Asia.

Other species of Taxus are recognized in the East Himalayas. These include T. sumatrana (undescribed variety),T. kingstonii, which is related to T. sumatrana,T. suffnessii, related to T. wallichiana,T. phytonii and T. obscura, both related to T. chinensis.[10] It appears that these have been overlooked as T. wallichiana; however, T. sumatrana and T. kingstonii have also been interpreted by some as T. mairei, but without distinction from an earlier name, T. sumatrana.[32]

References[edit]

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