| Taxaceae | |
|---|---|
 | |
| Foliage and mature arils of a yew plant | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Gymnospermae |
| Division: | Pinophyta |
| Class: | Pinopsida |
| Order: | Cupressales |
| Family: | Taxaceae S.F.Gray |
| Genera | |
| Synonyms | |
| |
Taxaceae (/tækˈseɪsi.iː,-ˌaɪ/), commonly called theyew family, is aconiferousfamily which includes sixextant and two extinct genera, and about 30 species ofplants, or in older interpretations three genera and 7 to 12 species.
They are many-branched, smalltrees andshrubs. The leaves areevergreen, spirally arranged, often twisted at the base to appear 2-ranked. They are linear to lanceolate, and have pale green or white stomatal bands on the undersides.[1]
The plants aredioecious, or rarelymonoecious. The catkin like malecones are 2–5 millimetres (0.079–0.197 in) long, and shedpollen in the early spring. They are sometimes externally only slightly differentiated from the branches. The fertile bracts have 2-8 pollen sacs.[1][2]
The female 'cones' are highly reduced.[1] Only the upper or uppermost bracts are fertile and bear one or rarely two seeds.[2] The ovule usually exceeds the scale, although ovules are sometimes rarely enclosed by it. They may be found on the ends of branches or on the branches. They may grow singly or in tufts or clumps.[2]
As the seed matures, a fleshyaril partly encloses it. The developmental origin of the aril is unclear, but it may represent a fused pair of swollen leaves.[1] The mature aril is brightly coloured, soft, juicy and sweet, and is eaten bybirds which then disperse the hard seed undamaged in their droppings. However, the seeds are highly poisonous to humans, containing the poisonstaxine andtaxol.[3]
Species are mostly found in the tropics and temperate zones in the northern temperate. There are only a few species in the southern hemisphere.[2]
Taxaceae is now generally included with all other conifers in the orderPinales, asDNA analysis has shown that the yews are phylogenetically nested in the Pinales,[4] a conclusion supported bymicromorphology studies.[5] Formerly they were often treated as distinct from other conifers by placing them in a separate orderTaxales.Ernest Henry Wilson referred to Taxaceae as "taxads" in his 1916 book.[6] Taxaceae is thought to be the sister group toCupressaceae, from which it diverged during the early-midTriassic. The clade comprising both is sister toSciadopityaceae, which diverged from them during the early-midPermian.[7] The oldest confirmed member of Taxaceae isPalaeotaxus rediviva from the earliestJurassic (Hettangian) of Sweden. Fossils belonging to the living genusAmentotaxus from the Middle Jurassic of China indicate that Taxaceae had already substantially diversified during the Jurassic.[8]
The broadly defined Taxaceae (includingCephalotaxus) comprises six extant genera and about 30 species overall.Cephalotaxus is now included in Taxaceae, rather than being recognized as the core of its own family,Cephalotaxaceae. Phylogenetic evidence strongly supports a very close relationship betweenCephalotaxus and other members of Taxaceae,[9][10][11] and morphological differences between them are not substantial. Previous recognition of two distinct families, Taxaceae and Cephalotaxaceae (e.g.,[12]), was based on relatively minor morphological details: Taxaceae (excludingCephalotaxus) has smaller mature seeds growing to 5–8 millimetres (0.20–0.31 in) in 6–8 months, that are not fully enclosed by the aril; in contrast,Cephalotaxus seeds have a longer maturation period (from 18–20 months), and larger mature seeds (12–40 millimetres (0.47–1.57 in)) fully enclosed by the aril. However, there are also very clear morphological connections betweenCephalotaxus and other members of Taxaceae,[13][14] and considered in tandem with the phylogenetic evidence, there is no compelling need to recognizeCephalotaxus (or other genera in Taxaceae) as a distinct family,.[9][10]
Phylogeny of Taxaceae.[15][16]
AmentotaxusPilg. – Catkin-yew
AustrotaxusCompton – New Caledonia yew
CephalotaxusSiebold & Zucc. ex Endl. – Plum yew
PseudotaxusW.C.Cheng – White-berry yew
TaxusL. – Common yew
TorreyaArn. – Nutmeg yew
