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Torreya, G. Arnott  1838

Torreya description


Evergreen trees and shrubs with one to few cylindrical to off-center trunks. Bark fibrous, smooth at first, later flaking, and finally peeling in long, thin strips and becoming variously ridged and furrowed. Crown dense and conical in youth, broadening and becoming more irregular with age, initially with regularly spaced tiers of three to seven horizontal to gently rising branches bearing pairs of horizontal to drooping side branchlets. Branchlets all elongate, without distinction into long and short shoots, hairless, remaining green for up to 2 years or turning brown within the first year, completely clothed by and slightly to prominently grooved between the elongate, attached leaf bases. Resting buds well-developed, with about eight crisscross pairs of triangular, brown bud scales that are shed during shoot expansion. Leaves attached in opposite pairs with successive pairs spiraling around the twigs but presented in two ragged rows by bending and twisting of the petioles, needlelike, variously sword-shaped, straight or slightly curved forward, flattened top to bottom, stiff and leathery with a hard, often prickly tip, usually strongly and often pungently aromatic. Midrib usually inconspicuous above, sometimes flanked by grooves or ridges, broadly and shallowly to prominently raised beneath and flanked by flat or slightly depressed whitish green or brownish stomatal bands that are flanked, in turn, by green marginal bands, the leaf edges flat or slightly and narrowly turned down.

Plants dioecious. Pollen cones forming a double row on the lower side of the current growth increments in the axils of ordinary foliage leaves, extending from the lowest pair of leaves to the tip or only along a portion of the branchlet. Each pollen cone with five to eight alternating pairs of tightly packed bud scales on the stalk beneath five to nine alternating whorls of pollen scales and often one terminal one. Each pollen scale with three to five dangling pollen sacs and a ragged upturned tip (and up to seven pollen sacs all around the terminal pollen scale). Pollen grains small (25-35 µm in diameter), roughly spherical but a little squared off and pyramid-shaped, with an ill-defined germination zone at the narrower end, seemingly smooth, but with extremely small, complexly spiny lumps all over the surface. Seed cones forming a short, often interrupted double row on the lower side of reduced or relatively unmodified current growth increments, or sometimes just a single pair. Each seed cone highly reduced, not obviously conelike, without seed scales, consisting of a short reproductive axis with a pair of bracts in the axil of an ordinary foliage leaf. Each of these primary bracts with an axillary fertile shoot bearing two crisscross pairs of bracts at the base of a single upright seed, usually only one seed of the pair maturing, but sometimes with a third seed at the tip of the reproductive axis. Seeds large, the thin fleshy outer seed coat surrounded by and united with a resinous, fleshy aril for most of its length, maturing in two seasons after which the fleshy layers split along one side to release the remainder of the seed within the hard middle stony layer. Cotyledons two, each with one vein. Chromosome base number x = 11.

Wood light, variably soft to moderately hard, silky in texture, with a modest contrast between the creamy white to yellowish or light brown sapwood and the yellowish brown to dark brown heartwood. Grain fine and fairly even, with obvious growth rings generally marked by a very narrow band of darker latewood. Resin canals absent but with a very sparse scattering of individual resin parenchyma cells, the tracheids (wood cells) with double or triple spiral thickenings, or these too closely spaced to tell.

Stomates densely crowded within the bands so that they often share some of their single circle of 8-10 surrounding subsidiary cells, aligned with the long axis of the leaf but arranged in distinct lines. Each stomate surrounded by a Florin ring and many other epidermal cells also bearing papillae. Midvein single, surrounded by upper and lower caps of sclerenchyma, with one resin canal embedded among these small, round sclereids immediately beneath the midvein, which is flanked by small bands of transfusion tissue. Photosynthetic tissue with a single or double palisade layer lining the whole upper surface and with horizontal spongy mesophyll filling the blade on either side of the midvein and accompanied by branched sclereids in some species.

Six species in eastern Asia and southern North America. The wood of most species of Torreya is quite durable in contact with soil, so they are frequently used for fence posts, while other timber uses, such as furniture or framing, are much restricted by the generally small size of the trees. Sprouting from cut stumps can ensure a continuing supply of the timber under a coppice system. The large seeds have been roasted and eaten in both eastern Asia and North America, and cultivars were developed in both Japan and China for different seed qualities, including oil content and uses in herbal medicine. A modest number of additional cultivars were selected for dwarf or spreading growth habits and for yellowish foliage. The genus name honors John Torrey (1796-1873), a prominent plant taxonomist at the New York Botanical Garden, who formally named and described California nutmeg about 15 years after the genus was named for him based on the stinking cedar.

The nutmeg yews derived this common name from a modest resemblance of their seeds (visually but not in flavor) to the fruits of nutmeg (Myristica fragrans), an unrelated plant. The similarity rests on the splitting husk (aril and fleshy layer of seed coat in nutmeg yew, fruit wall in nutmeg), revealing a hard shell (stony layer of seed coat in nutmeg yew, seed coat in nutmeg) surrounding a convoluted nutritive tissue (female gametophyte in nutmeg yew, endosperm in nutmeg). Nutmeg also has an aril (the mace), but this lies between the husk and the hard shell. The convoluted female gametophyte is quite variable in the different species of Torreya and, in some, is quite smooth. Whether it is smooth or wrinkled was used to divide the genus into two botanical sections, but DNA studies show that such a division does not accord with the phylogeny of the species. Instead, the two New World species, one of which has the wrinkles (Stinking cedar, Torreya taxifolia) and one of which does not (California nutmeg, Torreya californica), are each other’s closest relatives, to the exclusion of the Old World species, which also intermix species with convoluted and smooth gametophytes. Two of the contrasting Asian species, Japanese nutmeg yew (Torreya nucifera), with fairly smooth female gametophytes, and Farges torreya (Torreya fargesii), with the most convoluted female gametophytes in the genus, are essentially identical to each other in one genetic region (the internal transcribed spacer, ITS, region of the ribosome genes) sampled for all species. In fact, all four Asian species are much more similar to one another in their ITS DNA sequences than the two New World species are to each other, suggesting a much more recent divergence.

While even the divergence between the North American and Asian species suggests a separation no longer than about 30 million years ago, the genus appears to be much older. There are fossils with leaf shape and arrangement and epidermal structures just like those of modern species of Torreya from mid-Jurassic sediments (more than 160 million years old) in Europe and eastern Siberia and from Lower Cretaceous sediments (more than 100 million years old) in North America. Despite their similarities to extent Torreya, it is possible that these ancient fossils are some related genus because there is a long hiatus before the next fossil occurrences. A fairly continuous record began in the Oligocene (about 30 million years ago) in North America, Europe, and eastern Asia. The genus only became extinct in Europe after the Pliocene (less than 2 million years ago) and is still present in North America and eastern Asia.

The bijugate leaf arrangement found in Torreya, with spiraling pairs of leaves, is rather rare in plants and, among the conifers, is found elsewhere only in plum yews (Cephalotaxus), in another subfamily within the Taxaceae. The closest relatives of the nutmeg yews, as demonstrated by DNA studies and some morphological features, the catkin yews (Amentotaxus), with which they share a subfamily, have the more common decussate leaf arrangement, with crisscross leaf pairs forming four rows.




Attribution from: Conifers Garden