Taxaceae

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Taxaceae (Template:IPAc-en), commonly called the yew family, is a coniferous family which includes six extant and two extinct genera, and about 30 species of plants, or in older interpretations three genera and 7 to 12 species.

Description

They are many-branched, small trees and shrubs. The leaves are evergreen, 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.<ref name="Dörken Nimsch Rudall pp. 133–143" />

The plants are dioecious, or rarely monoecious. The catkin like male cones are Template:Convert long, and shed pollen in the early spring. They are sometimes externally only slightly differentiated from the branches. The fertile bracts have 2-8 pollen sacs.<ref name="Dörken Nimsch Rudall pp. 133–143" /><ref name=":SAGenera">Template:Cite book</ref>Template:Rp

The female 'cones' are highly reduced.<ref name="Dörken Nimsch Rudall pp. 133–143" /> Only the upper or uppermost bracts are fertile and bear one or rarely two seeds.<ref name=":SAGenera" /> 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.<ref name=":SAGenera" />

As the seed matures, a fleshy aril partly encloses it. The developmental origin of the aril is unclear, but it may represent a fused pair of swollen leaves.<ref name="Dörken Nimsch Rudall pp. 133–143">Template:Cite journal</ref> The seeds are highly poisonous, containing the poisons taxine and taxol, but the mature aril that surrounds them is brightly coloured, soft, juicy and sweet, and is eaten by birds which then disperse the hard seed undamaged in their droppings. However, if damaged the seeds could release their poisons, which is dangerous also for humans.<ref>Yew Poisoning: MedLine Plus Medical Encyclopedia</ref>

Distribution

Species are mostly found in the tropics and temperate zones in the northern temperate. There are only a few species in the southern hemisphere.<ref name=":SAGenera" />

Classification

Taxaceae is now generally included with all other conifers in the order Pinales, as DNA analysis has shown that the yews are phylogenetically nested in the Pinales,<ref name="Chase Soltis Olmstead Morgan 1993 p=528">Template:Cite journal</ref> a conclusion supported by micromorphology studies.<ref name="Anderson Owens 2003 pp. 233–234">Template:Cite journal</ref> Formerly they were often treated as distinct from other conifers by placing them in a separate order Taxales. Ernest Henry Wilson referred to Taxaceae as "taxads" in his 1916 book.<ref name="Wilson 1916 p.">Template:Cite book</ref> Taxaceae is thought to be the sister group to Cupressaceae, from which it diverged during the early-mid Triassic. The clade comprising both is sister to Sciadopityaceae, which diverged from them during the early-mid Permian.<ref>Template:Cite journal</ref> The oldest confirmed member of Taxaceae is Palaeotaxus rediviva from the earliest Jurassic (Hettangian) of Sweden. Fossils belonging to the living genus Amentotaxus from the Middle Jurassic of China indicate that Taxaceae had already substantially diversified during the Jurassic.<ref>Template:Cite journal</ref>

The broadly defined Taxaceae (including Cephalotaxus) 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 between Cephalotaxus and other members of Taxaceae,<ref name="Quinn Price Gadek 2002 p=513">Template:Cite journal</ref><ref name="Rai Reeves Peakall Olmstead 2008 pp. 658–669">Template:Cite journal</ref><ref name="Springer Science and Business Media LLC 2019 pp. 679–685">Template:Cite journal</ref> and morphological differences between them are not substantial. Previous recognition of two distinct families, Taxaceae and Cephalotaxaceae (e.g.,<ref name="JSTOR">Template:Cite journal</ref>), was based on relatively minor morphological details: Taxaceae (excluding Cephalotaxus) has smaller mature seeds growing to Template:Convert 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 (Template:Convert) fully enclosed by the aril. However, there are also very clear morphological connections between Cephalotaxus and other members of Taxaceae,<ref name="Doyle 1998 pp. 567–599">Template:Cite journal</ref><ref name="Stützel Röwekamp 1999 pp. 145–157">Template:Cite journal</ref> and considered in tandem with the phylogenetic evidence, there is no compelling need to recognize Cephalotaxus (or other genera in Taxaceae) as a distinct family.<ref name="Quinn Price Gadek 2002 p=513"/><ref name="Rai Reeves Peakall Olmstead 2008 pp. 658–669"/>