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Dawn redwood, Miki ex H. H. Hu & W. C. Cheng  1948


Deciduous trees with smoothish, thin, fibrous bark peeling in long strips on a strongly tapering trunk with a flaring, deeply fluted base. Branches numerous, angled upward to form a conical crown, broadening and flattening with age. Shoot system differentiated into permanent long shoots and deciduous short shoots that are shed together with their leaves in the autumn, both types produced in opposite pairs. Leaves in alternating pairs, initially at right angles to one another but coming to lie in a single plane on either side of the twig by twisting of the petioles and of the shoot axis alternately to the left and the right. Leaf blade needlelike, commonly linear, flat, soft in texture, abruptly narrowed to the short petiole, the tip rounded, bright green above.

Plants monoecious. Pollen cones single and well-spaced in the axils of leaves on branched, dangling, specialized reproductive shoots. Each cone with about seven alternating pairs of bud scales at the base and about 20 irregularly arranged, elliptical pollen scales on thin stalks, each with three pollen sacs. Pollen grains small (20-35 µm in diameter), flattened spherical, with a straight or hooked germination papilla, the surface covered with minute bumps. Seed cones squarish spherical to top-shaped, woody, single at the ends of specialized reproductive short shoots bearing five or six pairs of short leaves that are shed before maturity of the cone in its first season. Cones emerging directly from buds on the long shoots that bear seven or eight alternating pairs of bud scales. Each cone with 8-14 alternating pairs of shield-shaped cone scales (of which the lower and upper two or three pairs are small or sterile, or both). Each cone scale with a transversely diamond-shaped, pentagonal, or hexagonal external facet, the fertile and bract portions about equal and intimately fused, with a single row of (five or) six to eight (or nine) seeds. Seeds elliptical, flattened, with a pair of flat wings derived from the seed coat and slightly longer and about two to four timer wider than the seed body. Cotyledons two, each with one vein. Chromosome base number x = 11.

Wood not notably fragrant, light and soft, with pale whitish brown sapwood sharply contrasting with reddish brown heartwood of varying shades. Grain even and fairly coarse, with well-defined growth rings marked by a gradual transition to a fairly wide band of much smaller and thicker-walled latewood tracheids. Resin canals absent, but with sparse to abundant individual resin parenchyma cells scattered though some growth increments and concentrated in open bands in others, generally more abundant in younger stems.

Stomates longitudinally to irregularly oriented, confined to two paler bands beneath, each band with 4-7(-10) lines. Individual stomates sunken beneath the (four or) five or six (to eight) surrounding subsidiary cells, which are topped by a thick Florin ring and often carry other papillae as well. Leaf cross section with a single-stranded midvein above a smaller resin canal, flanked by wings of transfusion tissue, and with an additional resin canal lying on either side out at the leaf margin. Photosynthetic tissue of fairly homogeneous spongy mesophyll without a distinct palisade layer of columnar cells beneath the upper epidermis and accompanying, almost continuous hypodermis.

One species in central China. After Metasequoia glyptostroboides was discovered in the 1930s, it was introduced into cultivation on a massive scale, at least in botanical gardens initially. Perhaps, in part, because it proved to be slow to reach full reproductive maturity (seed cones are formed many years before pollen cones, so seed produced until the 1970s was generally nonviable), there has been little cultivar selection. The few available cultivars emphasize ease of vegetative propagation and narrower crowns.

Metasequoia is one important link in the unity of the family Cupressaceae because it joins the decussate leaf arrangement (alternating pairs) found in cypresses and junipers with the typical needlelike leaf form found in many redwoods. Although the genus shares annual shedding of its foliage with Taxodium (bald cypress), it is actually much more closely related to Sequoia (redwoods) and Sequoiadendron (giant sequoias). In the past, some suggested that Metasequoia might have been partly ancestral to Sequoia through hybridization followed by polyploidy, but this view is not now generally accepted. Nonetheless, the evident similarity to Sequoia explains the scientific name, using Greek for “near”.

It was a dominant tree in many parts of the northern hemisphere throughout the late Cretaceous and Tertiary (but not, apparently, in western Europe). The fossil species were also deciduous, and this allowed them to grow in the arctic far north of related evergreen trees like the redwood (to almost 80°N on Axel Heiberg Island and northernmost Greenland). The earliest recognizable species, from the mid-Cretaceous of Alaska and Siberia, had not yet fully established the opposite leaf arrangement that characterizes all later species. Several of these were described, based on variations in shape and arrangement of various organs and on different kinds of characters available for comparison because of different modes of preservation. Since there is just a single living species, however, and since this species consists of just a few thousand individuals in a small region of China, compared to the millions of hectares inhabited by Tertiary Metasequoia, nobody knows how much variation to expect within a species of this genus. Thus there is no certainty that the number of described fossil species correspond to the actual number of Tertiary species represented by these fossils. After the northern hemisphere cooled following the Eocene (from about 35 million years ago) and the arctic became too cold to support forests, the range of the genus became progressively more restricted. Metasequoia finally disappeared from North America and most of Eurasia after the Miocene, persisting longest (outside of its west-central Chinese last refuge) in Japan, where it lasted into the Pliocene.




Attribution from: Conifers Garden