Taxaceae


Taxaceae, 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.
The plants are dioecious, or rarely monoecious. The catkin like male cones are 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.
The female 'cones' are highly reduced. Only the upper or uppermost bracts are fertile and bear one or rarely two seeds. 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.
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. 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.

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.

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, a conclusion supported by micromorphology studies. 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. 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. The oldest confirmed member of Taxaceae is Palaeotaxus rediviva from the earliest Jurassic 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.
The broadly defined Taxaceae 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, and morphological differences between them are not substantial. Previous recognition of two distinct families, Taxaceae and Cephalotaxaceae, was based on relatively minor morphological details: Taxaceae has smaller mature seeds growing to in 6–8 months, that are not fully enclosed by the aril; in contrast, Cephalotaxus seeds have a longer maturation period, and larger mature seeds fully enclosed by the aril. However, there are also very clear morphological connections between Cephalotaxus and other members of Taxaceae, and considered in tandem with the phylogenetic evidence, there is no compelling need to recognize Cephalotaxus as a distinct family.

Phylogeny

Phylogeny of Taxaceae.
Amentotaxus – Catkin-yew
Austrotaxus – New Caledonia yew
Cephalotaxus – Plum yew
Pseudotaxus – White-berry yew
Taxus – Common yewTaxus baccata European yewTaxus biternata Delicate branch yewTaxus brevifolia Pacific yew, western yewTaxus caespitosa Caespitosa yewTaxus calcicola Asian limestone yewTaxus canadensis Canada yewTaxus celebica Celebes yewTaxus chinensis China yewTaxus contorta West Himalayan yewTaxus cuspidata Rigid branch yew, Japanese yewTaxus fastigiata Irish yewTaxus floridana Florida yewTaxus florinii Florin yewTaxus globosa Mesoamerican yewTaxus kingstonii Kingston yewTaxus mairei Maire yewTaxus obscura Obscure yewTaxus ocreata Scaly yewTaxus phytonii Phyton yewTaxus recurvata English yewTaxus rehderiana Rehder yewTaxus scutata Scutaceous yewTaxus suffnessii Suffness yewTaxus sumatrana Sumatera yewTaxus umbraculifera Umbrelliform yewTaxus wallichiana Wallich yew, East Himalayan yew
Torreya – Nutmeg yew

Extinct genera

Several genera have been described from the fossil record and placed within Taxaceae