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Rimu, Solander ex J.G. Forster  1807


Evergreen trees and shrubs of varying habit. Trunk varying from short and contorted to tall and columnar, sometimes fluted and buttressed. Bark fibrous, thin and hard, peeling in long strips or irregular to elongate flakes and becoming pockmarked or shallowly and narrowly ribbed and furrowed. Crown conical to dome-shaped, often with extra branches between the main tiers of whorled branches. Branches and branchlets strongly rising to weeping. Branchlets all elongate, without distinction into short and long shoots, usually hairless, remaining green for at least the first year, evidently grooved between the elongate, attached leaf bases. Resting buds not well developed, consisting of a cluster of arrested ordinary foliage leaves or slightly differentiated scale leaves. Leaves spirally arranged and radiating all around the twigs, fairly uniform in length along a growth increment. Leaves strongly differentiated in size and often in form between juvenile and adult foliage, the transition either abrupt or via a gradual progression through intermediate leaf forms. Juvenile leaves needlelike, thin and narrow, tapering very gradually but continuously from the base to the blunt or sharp tip, straight or a little curved forward. Adult leaves scale- to needlelike, straight or variously curved forward, often circular, semicircular, or diamond-shaped in cross section. Leaf tip blunt or pointed but rarely prickly, the base usually running onto the twig without a definite petiole.

Plants dioecious. Pollen cones egg-shaped to cylindrical, single at the tips of branchlets or in the axils of ordinary foliage leaves, or a few loosely clustered. Each pollen cone with a few bracts at the base and 40 or more densely spirally arranged pollen scales, each scale bearing two pollen sacs and with a scarcely to enormously elongated tip. Pollen grains medium (body 30-50 µm long, 33-70 µm overall), with two small, round air bladders, or with a slightly inflated fringe extending all around the body, leaving just a small area of the furrow uncorvered. Surface fairly smooth in the furrow region, minutely bumpy or pitted over the thick cap, and coarsely sculptured with thick wrinkles on the bladders or fringe and internal sculpturing completely filling the space. Seed cones single at the tips of ordinary branchlets in the axils of foliage leaves, highly modified and reduced. Each cone with a few small free bracts at the base and with about 12 bracts in the reproductive part that become a podocarpium. These bracts fused with the cone axis and with each other, becoming swollen, red, and juicy only just as the seed matures, even then retaining conspicuous free tips. Fertile bracts one to three, each with a single plump, unwinged seed encircled at the base (at maturity) by a thin, green, cuplike seed scale (the epimatium). Opening of the ovule pointing down into the cone axis at the time of pollination, when fully enveloped by the epimatium, but gradually straightening and outgrowing the epimatium so that at maturity, in the second year, the seed points away at an angle to the cone axis. Seeds more or less egg-shaped, conspicuously flattened so that they have a ridge going up each side, the opening of the micropyle at the seed tip short, straight, and blunt. Cotyledons two, each with two veins. Chromosome base number x = 10.

Wood moderately hard and heavy, fragrant, often with a strong distinction between sap- and heartwood but often also with a gradual transition. Sapwood narrow or broad, white to light brown. Heartwood yellow to reddish brown or bright red, often streaked. Grain very fine and even, lacking obvious growth rings in most species, although narrow annual bands of scarcely differentiated latewood are often present and annual growth tings may be well marked in Dacrydium cupressinum. Resin canals absent but with abundant individual resin parenchyma cells.

Lines of stomates on both inner (upper) and outer (lower) surfaces or (less often) only on the inner, usually arranged in more or less well defined stomatal bands (each containing two to six lines), those of the outer surface often short and confined to the region near the base. Each stomate sunken beneath and partly hidden by the one or two circles of four or five (or six) subsidiary cells, plugged by wax and surrounded by a Florin ring. Outer subsidiary cells and ordinary epidermal cells often with highly wrinkled cells walls. Leaf cross section with a single, prominent midvein located centrally, accompanied beneath by one resin canal and flanked by transfusion tissue at the sides and (often) above. Photosynthetic tissue radiating all around the midvein.

Twenty-one species in southeastern Asia, Malesia, New Caledonia, Fiji, and New Zealand.

The overall modern distribution of Dacrydium is similar to that of its closest relatives, Dacrycarpus and Falcatifolium, although the latter does not extend as far in the northwest (only in southern Malaya in mainland Asia) and in the southeast (it is not in Fiji or New Zealand). A few of the species, including Dacrydium cupressinum and Dacrydium elatum, are important timber trees producing some of the finest softwood lumber in their native regions. The scientific name of the genus, Greek for “little tear”, refers to droplets of resin exuded from the bark. Rimu (Dacrydium cupressium) is the only species in the genus with a (modest) history in horticulture, and there has been no cultivar selection in this essentially tropical genus.

Dacrydium is the second largest genus in the family Podocarpaceae and, like the large Podocarpus, it was long used in a broad sense that included many species now placed in separate genera, most of which are thought to be only distantly related to the species still retained in Dacrydium. The genus Dacrydium was the species still retained in Dacrydium. The genus Dacrydium was then used as a catchall for podocarp species with scale leaves and upright seeds with a reduced epimatium, in contrast to the broad leaves, inverted seeds, and full-blown epimatium of Podocarpus (in the broad sense). Even before the initiation of dismemberment of the genus in 1969, it was recognized as heterogeneous, with at least three groups of species differing in pollen types, chromosome base number, and in details of foliage and of seed cones. With the removal of Falcatifolium in 1969, followed by Halocarpus, Lagarostrobos, and Lepidothamnus in 1982, what remains in Dacrydium is much more coherent.

Despite this coherence, there has been a proposal (Melikian and Bobrov 2000) to further subdivide Dacrydium into four genera so that rimu (Dacrydium cupressinum) would be the sole remaining species in Dacrydium, but this proposal has not been accepted by other conifer taxonomists. Basically, these segregate genera would all be each other’s closest relatives (although the proposing authors think that Dacrydium cupressinum stands somewhat apart from the others) and the groupings conflict with some of the other available evidence. That said, there are precious few data known that could be used to investigate the relationships among species of Dacrydium so a convincing phylogenetic arrangement of species with the genus is not yet possible.

Like the related Dacrycarpus and Falcatifolium, Dacrydium has a fossil record of leafy shoots in Australia, from which the genus is now absent. These fossils belong to several species that extend from the mid-Eocene (about 45 million years ago) to the Miocene (about 20 million years ago), after which the genus disappeared from Australia, presumably because of the increasing aridity of the continent (living species of Dacrydium all have a high moisture requirement). These Australian records are the only known fossilized shoots of Dacrydium, but pollen grains assigned to the genus have a slightly wider range in time and space. Pollen records of Dacrydium extend from the Paleocene (more than 56 million years ago) to the Pliocene (less than 5 million years ago) in Australia and are also found in Pliocene deposits in New Guinea.




Attribution from: Conifers Garden