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Amentotaxus

Catkin-yew,  Pilger  1917
Taxaceae


Amentotaxus - Catkin-yew description


 

Evergreen small trees or shrubs. Trunk(s) one or more, cylindrical, straight and erect, usually slender. Bark obscurely fibrous, thin, smooth at first, flaking in small scales, and becoming shallowly and irregularly ridged and furrowed with age. Crown cylindrical to narrowly dome-shaped, rather open to moderately dense, with well-separated horizontal to slightly rising branches in pairs successively at right angles to their predecessors or in near whorls of (three or) four. Branchlets all elongate, without distinction into long and short shoots, hairless, remaining green 1-2 years or more, usually a little squared off in cross section, completely clothed by and prominently grooved between the attached leaf bases up to about 1.5 cm long. Resting buds well developed, squarish in cross section, pointedly egg-shaped, with 5-10 pairs of glossy brown, keeled, triangular bud scales, most of which are shed with bud burst and shoot elongation. Leaves attached in pairs, each pair at right angles to the preceding and following pairs, presented in two flat rows to the sides of the twigs by bending of the petioles and twisting of the internodes between the pairs. Individual leaves broadly sword-shaped, straight or slightly curved forward or S-shaped, flattened top to bottom, leathery. Midrib prominent, sharply or more broadly raised within an ill-defined groove above, slightly raised to slightly grooved beneath within a broadly depressed or raised green band flanked by conspicuous, broad, waxy stomatal bands, these in turn flanked by green marginal bands, uniformly dark green above, the leaf edges often narrowly turned down.

Plants dioecious. Pollen cones arranged in pairs along and in a congested clump at the end of a specialized, leafless, drooping spike (hence the scientific name, Latin for “catkin yew”) with three or four (usually) spikes emerging from buds near or at the tip of the previous year’s growth and surrounding a vegetative bud. Each pollen cone spherical or slightly elongated, with about four or five alternating pairs of pollen scales. Each pollen scales with a central stalk, two to eight pollen sacs on the lower side (sometimes on the upper as well), and usually with a triangular free tip above. Pollen grains small to medium (20-45 µm in diameter), spherical or slightly squashed, without air bladders, very finely (to almost undetectably) bumpy but otherwise almost featureless. Seed cones single at the tips of leafless, drooping stalks in the axils of ordinary foliage leaves near the tip of a growth increment, highly reduced, not at all conelike, consisting solely of 4-6(-10) alternating pairs of thin, rounded bracts tightly packed around the base of a single large seed with a fleshy outer seed coat surrounded by and united with a fleshy aril for most of its length. Seeds maturing in a single season. Cotyledons two, each with one vein. Chromosome base number x = 7.

Wood relatively hard and heavy, fragrant, smooth in texture, yellowish brown. Grain medium and even, with somewhat obscure growth rings marked by a gradual transition to a narrow band of latewood. Resin canals absent but with numerous, conspicuous, individual resin parenchyma cells and tracheids (wood cells) with double spiral thickening.

Stomates densely crowded within the stomatal bands so that they share many of their surrounding subsidiary cells, aligned with the long axis of the leaf but not arranged in distinct lines. Each stomate sunken beneath and surrounded by the (4-)8-10 radiating subsidiary cells, which are flat and not topped by a Florin ring. Midvein single, completely surrounded by a discontinuous cylinder of small sclereids, with one small to large resin canal immediately beneath it, and flanked by small bands of transfusion tissue. Photosynthetic tissue with a single or partially double palisade layer lining the whole upper surface beneath the epidermis and with horizontal spongy mesophyll filling the blade on either side of the midvein and usually accompanied by fibers that produce wrinkles perpendicular to the midrib on drying.

Six species in southeastern Asia from northeastern India (Arunachal Pradesh) through central and southern China to Taiwan, south to southern Vietnam.

While the catkin-yews are not confined to primary forests and can tolerate some disturbance, they are rare plants. They are threatened throughout their range by direct cutting for their wood and even more so by general forest clearance. A few reserves were established to protect them or a broader sample of plants and animals, but these reserves cover only a small part of their morphological variation. Partly as a consequence of their rarity, the catkin-yews have scarcely been investigated for the kinds of cancer-fighting chemicals discovered in so many other members of the yew family. Neither of the catkin-yew species is in general cultivation (they are rarely found even in botanical gardens), and there has been no cultivar selection.

Although the members of this genus share many characteristics with other genera of Taxaceae (for instance, spikes of pollen cones are also found in the New Caledonian Austrotaxus), they still have a distinctive combination of features that makes them easy to recognize. Even the leaves, with their conspicuous, broad stomatal bands bearing densely packed stomates sharing many subsidiary cells, are readily identified as Amentotaxus (although they were confused with Podocarpus when the latter was less well understood). There have been considerable taxonomic uncertainties surrounding the genus. While it was sometimes included with plum yews (Cephalotaxus) in Cephalotaxaceae or with nutmeg yews (Torreya) or by itself in Amentotaxaceae, DNA studies strongly support the traditional view that all these genera belong with Taxus, Pseudotaxus, and Austrotaxus in the Taxaceae. Within the Taxaceae, it joins Torreya to form one of three groups of genera (botanical tribe or subfamily). The structure of the seed and aril are very similar to those of Torreya, while the paired leaves are also found in Cephalotaxus. (The leaf arrangement in Torreya and Cephalotaxus, with spiraling pairs, is actually somewhat different from that of Amentotaxus, which has crisscross pairs.)

More controversial is the number of species in the genus. The genus was generally considered to be monotypic, consisting only of Amentotaxus argotaenia, until H. L. Li divided it into four species in 1952. It has been common to recognize three or four species in the genus ever since, with some authors accepting as many as six or seven. The genus is divided into two species (Amentotaxus argotaenia, Amentotaxus yunnanensis) here because, even with large geographic gaps in the range, there appear to be few real character discontinuities and some proposed distinguishing features are of uncertain value. Much more needs to be known about the genetic and evolutionary relatedness of the different populations. There simply is not enough information about variation within the genus to accept many of the proposed segregates as separate species. Unfortunately, the continuing depletion of these intriguing plants with habitat loss makes obtaining a coherent range-wide understanding ever more difficult.

Because of the distinctiveness of the leaves, even in the absence of seed and pollen cones, there is a well-established fossil record of Amentotaxus extending from the Paleocene to the upper Miocene (about 60 million to 10 million years ago) in Europe and from the mid-Cretaceous to the Miocene (about 100 million to 10 million years ago) in North America, but without a known corresponding record in Asia except for a possible pollen record from the Miocene of Taiwan.

 

References

  • Farjon, A. (2010). A Handbook of the World's Conifers. Koninklijke Brill, Leiden.
  • Eckenwalder, J.E. (2009) Conifers of the World: The Complete Reference. Timber Press, Portland.
  • IUCN Red List of Threatened Species, International Union for Conservation of Nature and Natural Resources. Cambridge, UK /Gland, Switzerland

Copyright © Aljos Farjon, James E. Eckenwalder, IUCN, Conifers Garden. All rights reserved.


 

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