Hemlock, (Endlicher) Carrière 1855
Pinaceae

Tsuga - Hemlock description

Evergreen, usually single-trunked trees with furrowed bark, a drooping leader, and horizontal to slightly drooping slender branches. Branchlets thin, flexible, all elongate, without distinction into long and short shoots, with well-developed, but small, scaly, winter buds. Leaves spirally arranged, usually predominantly twisted into a single flat row on each side of the twig (except Tsuga mertensiana, in which they angle forward all around the twig), often quite variable in length, with a few shorter ones inverted along the twigs. Individual leaves needlelike, generally broadly sword-shaped, usually flattened and dark green with a shallow central groove above and with a pair of white stomatal bands beneath (except Tsuga mertensiana, which has plump needles with stomates on both sides). Leaf blade with a minutely toothed or smooth edge, narrowing abruptly to a short petiole that attaches to a small woody peg running down onto the twig, and with a bluntly pointed, rounded, or notched tip.

Plants monoecious. Pollen cones sparse, single in the axils of leaves, commonly near the tips of first- and second year-twigs, slender-stalked. Individual cones almost spherical, less than 1 cm long, with numerous spirally arranged pollen scales, each scale with two pollen sacs. Pollen grains large to very large (body 55-95 µm in diameter, 60-110 µm overall), usually surrounded by a circular frill (except Tsuga mertensiana, which has two air bladders instead). Seed cones hanging at the tips of second-year twigs, maturing in a single season and opening to release the seeds, then falling intact the year after seed release. Each cone spherical to ellipsoid before opening, the bracts tiny to moderate and hidden by the circular to oval thin seed scales. Seeds in pairs, each with a long asymmetric wing derived from the scale. Cotyledons two, each with one vein. Chromosome base number x = 12.

Wood soft, odorless, light brown, without a clearly darker heartwood. Grain even, with well-marked growth rings formed by a more or less abrupt transition between small-celled latewood and large-celled earlywood. Vertical resin canals and individual resin cells absent.

Stomates arranged in and oriented with their long axis along definite lines within the stomatal bands. Each stomate sunken beneath and largely hidden by the four surrounding subsidiary cells, which may themselves be sunken beneath the general level of the epidermis. The two subsidiary cells at the ends of the stomate are often shared between adjacent stomates in a line while the two lateral ones are generally not shared between stomates in adjoining rows. Leaf cross section with a single large resin canal touching the epidermis beneath the single-stranded or partially doubled midvein, which is bordered, adjacent to the phloem, by a scattering of transfusion tissue mixed in with other cell types. Photosynthetic tissue with a single well-developed palisade layer directly beneath the upper epidermis without an intervening hypodermis (except sometimes at the leaf edge along the midline groove) and with spongy mesophyll extending from there down to the lower epidermis with its stomatal bands.

Eleven species in eastern and western North America and in southern Asia from India (northern Uttar Pradesh) to northern Vietnam, Taiwan, and Japan. Several species of hemlock are important sources of timber, pulp, and (formerly) tannins from the bark. Their graceful habit makes them attractive ornamentals in regions of ample rainfall. Most are quite shade tolerant. Since they require a lot of moisture, they do not grow well in drought-prone areas. Nonetheless, they are moderately commonly cultivated, particularly in the form of one or another of the numerous cultivars, most selected from Eastern hemlock (Tsuga canadensis). These cultivars are quite varied in overall growth habit, many taking advantage of or exaggerating the natural tendency of the leader and branches to arch gracefully. Other go against the grain in emphasizing stiffly upright growth while still others pull everything into tight buns or show other branching irregularities such as twisting or contortion. Cultivar selection has also emphasized variations in needle size, density, and color, including yellows, golds, and creams in the new needles as well as variegation in mature foliage. The name of the genus is the Japanese name for Tsuga sieboldii.

With one exception, the species of Tsuga are all quite similar to one another Mountain hemlock (Tsuga mertensiana) differs from the others in its leaf form and in having pollen with two bladders. Some taxonomists remove it to its own genus, or botanical section, Hesperopeuce, but accumulating DNA studies place it firmly among the other hemlocks. It is especially close to its neighbor, Western hemlock (Tsuga heterophylla), although the similarities in DNA may be due, in part, to occasional natural hybridization between them. On the other hand, Cathaya and Nothotsuga, whose species are sometimes included in Tsuga, appear amply distinct.

Fossil hemlocks have been found all across the temperate northern hemisphere dating back to the late Cretaceous, perhaps, but most convincingly from the Oligocene, about 30 million years ago. The genus remained in Europe, where it is now absent, until well into the Pleistocene ice ages, as recently as about 300,000 years ago.

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.