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Yew, Linnaeus  1753

Taxus - Yew description


Evergreen trees and shrubs with single or multiple trunks. Bark smooth, thin, fibrous to corky, reddish brown, flaking and peeling in longitudinal strips to reveal purple-red patches. Crown densely branched in youth, the lower branches persistent in open-grown plants but large forest trees ultimately with a clear trunk. Branchlets all elongate, without distinction into long and short shoots, hairless, remaining green for the first year or two, clothed by and shallowly grooved between the elongate, attached leaf bases. Winter buds well developed, the thin, flexible scales often persistent at the base of the grown twigs. Leaves spirally attached but twisting at the base to lie flat (distichously) along either side of the twig (except forming a V in Taxus cuspidata), persisting several years. Each leaf needlelike, flattened top to bottom, sword-shaped, often slightly curved, leathery and flexible, dark green above, with two paler, often yellow-green or grayish green, stomatal bands along either side of the midrib beneath, abruptly narrowed to a distinct petiole and gradually or abruptly narrowed to the stiff, pointed tip. Midrib prominently raised above and prominently to inconspicuously raised beneath, the blade flat or bowed on either side and straight or slightly turned down or rolled at the edges.

Plants dioecious (or monoecious in Taxus canadensis). Pollen cones numerous along the branchlets, each single in the axil of a needle, short-stalked, emerging from a short-stalked, scaly bud. Reproductive portion of each cone spherical to oblong, with 4-16 densely clustered pollen scales. Each scale shieldlike, nearly radially symmetrical, with (two to) four to nine pollen sacs arranged all around a common stalk. Pollen grains small (20-40 µm in diameter), roughly spherical to somewhat flattened or irregular, without air bladders, with an ill-defined germination bump, and minutely bumpy over the whole surface. Seeds not in obvious cones, single at the ends of short stalks in the axils of the needles, maturing the first season, then reappearing in the same location for several years. Each smooth, round, hard, dark seed seated in an open, red, fleshy cup (aril) attached only to the seed stalk and with a few small scales at the base. Cotyledons usually two, each with one vein. Chromosome base number x = 12.

Wood heavy, hard, strong, and durable, with an abrupt transition between the creamy white to pale yellowish brown sapwood and the red to orange-brown hardwood. Grain fine and fairly even, with generally obvious growth rings (somewhat obscure in Taxus baccata) marked by a gradual to abrupt transition to a narrow to broad band of darker latewood. Resin canals and individual resin parenchyma cells both absent, the tracheids (wood cells) singly spirally thickened.

Stomates confined to the stomatal bands beneath in crowded or more open and irregular lines. Each stomate sunken beneath and largely hidden by the four to six surrounding subsidiary cells, which are topped by a prominent, steep, interrupted, knobby Florin ring and with most other epidermal cells in the stomatal bands covered with prominent, knoblike papillae. Midvein single, not reinforced by sclereids, without accompanying resin canals, and flanked by small bands of transfusion tissue. Photosynthetic tissue with one to three layers of palisade cells lining the whole upper surface directly beneath the epidermis without an intervening hypodermal layer and without sclereids among the rounded or slightly horizontally elongated spongy mesophyll cells that extend from the palisade parenchyma to the lower epidermis.

Nine species in North America and Eurasia. Yews are important in cultivation because of their tolerance for severe pruning, allowing them to be maintained at any size and to be shaped into dense hedges and topiaries. Their ability to sprout from dormant buds in bare wood after pruning is responsible for this horticultural prominence and is rather unusual among conifers. The more than 200 cultivars selected since the late 18th century cover the gamut of expected variations in habit (such as narrowly upright, like the well-known Irish yew, to spreading, drooping, and various shapes of dwarfs), in foliage form (shorter, more compact, and distorted) and color (mostly variations on yellow, gold, copper, silver, and bluish green), and in aril color (yellow or orange instead of red).

Despite the visual appeal of the colorful, juicy arils, the plants are extremely toxic, only the bird-dispersed arils lacking cyanide-yielding compounds and taxane alkaloids. Some of the latter, especially paclitaxel (sold commercially as taxol), have proven effective in treatment of certain cancers, leading to heavy commercial exploitation of some species and a search for other ways to produce the compounds.

Although the species are very difficult to identify and were sometimes considered varieties of a single species, most cultivated yews belong to only two species (Taxus baccata and Taxus cuspidata) and their hybrid (Taxus x media). These two species are probably each other’s closest relatives, but relationships within the genus are fairly obscure. There are few visually obvious taxonomic characters, and microscopic features, which have been suggested to fill the gaps, give some results that are difficult to reconcile with other lines of evidence. Despite the pharmaceutical importance of the genus, the available DNA data are rather limited, and the relationships they imply conflict with those suggested by distribution of taxane alkaloids. Both kinds of studies, nonetheless, distinguish most Eurasian species from most North American ones, even if they differ in the implied relationships within and between these continental species groups. Most DNA evidence also favors a monophyletic family Taxaceae with a position within the conifers as sister family to the Cupressaceae. Within Taxaceae, Taxus is closest to the similar-looking white cup yew (Pseudotaxus), which differs in several obscure ways and one conspicuous one, a white aril rather than the red (or yellow in some cultivars) one found in yews.

The solitary, terminal seed surrounded by a cup-shaped free aril led Florin in 1948 to postulate that the yews are not conifers. He bolstered his argument with fossil specimens from Jurassic of Yorkshire, about 165 million years old, that now seem more closely related to Amentotaxus of the subfamily Amentotaxoideae. Secure fossil shoots of Taxus extend back only to the Oligocene in Europe, some 30 million years ago, and this relatively recent appearance of the genus accords well with its generally derived features in comparison to other conifers and even to other members of the Taxaceae.




Attribution from: Conifers Garden