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Larch, P. Miller 1754


Deciduous trees with a straight, single trunk bearing spirally arranged or regular tiers of three to five slender, short to long, horizontal branches, forming a conical to broadly conical crown. Bark nonfibrous, smooth at first, becoming scaly and sometimes shallowly ridged and furrowed at the base of large trees. Branchlets hairy or not, of two types: long shoots, which extend the growth of the tree and have well-separated needles, and short shoots, long-lived, highly condensed spurs arranged spirally around the long shoots and bearing a new tuft of (10-)20-50(-60) needles each year. Winter buds well developed, usually small, scaly, resinous or not. Leaves spirally arranged, densely so on short shoots and much more sparsely on long shoots, and sticking straight out all around the twigs. Each leaf needlelike, soft and flexible, flatly or a little plumply triangular or horizontally diamond-shaped in cross section, straight or slightly curved, the tip pointed but innocuous, the base abruptly narrowed to a very short petiole.

Plants monoecious. Pollen cones numerous, single at the tips of short shoots, upright, with numerous densely spirally arrange pollen scales, each bearing two pollen sacs. Pollen grains large to very large (55-115 µm in diameter), almost spherical, practically featureless except for a slightly thickened ring around one of the hemispheres. Seed cones spherical or egg-shaped to oblong, single at the tips of short shoots and with a narrower stalk, upright at pollination and at maturity in a single season, remaining intact, the 20-120 densely spirally arranged seed scales then spreading to release the seeds. Seed scales rounded-rectangular or rounded-triangular to heart-shaped, often slightly indented at the tip, the tongue-shaped bracts with a central bristle (continuing the midvein), attached only to the base of the scales and hidden by to strongly protruding between them. Seeds two per scale, oval, tilted with respect to the much larger asymmetric wing derived from the seed scale that cups the seed body on one side and overlaps a little around the edges on the other side. Cotyledons four to eight, each with one vein. Chromosome base number x = 12.

Wood hard, not fragrant, waxy or greasy to the touch, the yellowish brown to reddish brown or even quite red heartwood distinct from the light brown sapwood. Grain uneven, with a sharp transition between earlywood and latewood. Vertical resin canals few, small, and irregularly distributed, with horizontal water-conducting cells (tracheids) in the rays.

With tightly spaced lines of stomates confined to two stomatal bands on the lower side of the needle or with a few, less prominent lines of stomates above. Each stomate sunken beneath the four (to six) elongate surrounding subsidiary cells, which are covered by a thin cuticle and lack a Florin ring. Leaf cross section with a (sometimes interrupted) single resin canal following the edge of the needle on either side of the single-stranded but split midvein. Photosynthetic tissue mostly of spongy mesophyll but sometimes with one palisade layer in the center of the upper face. Hypodermis in one layer above and below the midrib and in one to three layers inside the corners at the outer edge of the leaf.

Ten species across the northern hemisphere in the boreal zone and in mountains south to the northwestern and northeastern United States, the Alps, and the Himalaya.

Larix is the classical Latin name for the European larch (Larix decidua), which the Romans encountered in the Alps. Several larch species and hybrids have long been grown for forestry and horticulture, and a modest number of cultivars have been selected. These include blue foliage, dwarfs (most of which are derived from witches-brooms), and weeping forms.

Larches are by far the most widely distributed, species-rich, and abundant of the five living genera of deciduous conifers today. The other genera (Glyptostrobus, Metasequoia, Pseudolarix, and Taxodium) were more widely distributed in the geologic past but are each now restricted to single (or two) species and a local region in warm temperate portions of China or the southeastern United States. In contrast, Larix is well established across the northern hemisphere boreal forests, and each of the three northern species has an extensive distribution. The seven more southerly mountain species, however, have restricted ranges like those of the species in the other deciduous genera. These deciduous genera are all unrelated to one another (except Glyptostrobus and Taxodium), and each is most closely related to, and phylogenetically nested among, evergreen genera, thus indicating that deciduousness arose repeatedly among conifers.

Each of the deciduous genera lived in the high arctic in the past, which may be where they evolved deciduousness to cope with the darkness of an arctic winter not yet frigid in the early Tertiary. Larix appears to be younger than the other five genera, with earliest known fossils dating only from the Eocene, less than 50 million years ago, while the others date back at least to the Cretaceous, 70 million years ago and more. The earliest larch fossils are in the high arctic, well north of their present northern limit. Almost everywhere that larches grow today in the north they are accompanied by evergreen conifers, mostly true firs (Abies), spruces (Picea), and pines (Pinus). Thus deciduousness in larches may be a holdover from an arctic past rather than a response to current conditions.

Many of the more southerly larches grow near species of Douglas fir (Pseudotsuga), their evergreen closest relatives. This relationship has long been known because of similarities in many aspects of structure and function, including wood, seed cones, and pollen and pollination mechanism. DNA studies and other molecular investigations also point to the close relationship of these two genera and to their relative distance from the other pinoid genera, Pinus, Picea, and Cathaya (although they are closest to the latter).

The closeness of Larix and Pseudotsuga led taxonomists to believe that the southerly larches with three-pointed bracts sticking out between the seed scales (like those of Douglas firs) are more primitive than the boreal species with hidden bracts, and most classifications of the genus emphasized the distinction between long- and short-bracted species by placing them in separate botanical sections. DNA studies suggest, however, that the long-bracted larches of the New World are more closely related to their short-bracted neighbor than to the Old World long-bracted species, which in turn are more closely related to their own short-bracted neighbors. This uncertainty cast upon the traditional view of relationships by DNA studies is itself uncertain because the DNA segments studied to date do not vary much between species, because sampling of species and populations could be improved, and because larches can hybridize naturally and thus confuse molecular results as well as morphological ones. Clearly, more needs to be done on clarifying relationships among the larch species and sorting out their evolutionary history.




Attribution from: Conifers Garden