Tsuga is a genus of conifers in the pine family Pinaceae. The common name hemlock is derived from a perceived similarity in the smell of its crushed foliage to that of the unrelated plant poison hemlock. Unlike the latter, Tsuga species are not poisonous.
There are eight, nine, or ten species within the genus (depending on the authority), with four species occurring in North America and four to six in eastern Asia.
Description. Hemlocks are medium-sized to large evergreen trees, ranging from 35 to 200 feet (10 – 60 m) tall, with a conical to irregular crown, the latter occurring especially in some of the Asian species. The leading shoots generally droop. The bark is scaly and commonly deeply furrowed, with the color ranging from grey to brown. The branches stem horizontally from the trunk and are usually arranged in flattened sprays that bend downward towards their tips. Short spur shoots, which are present in many gymnosperms, are weakly to moderately developed. The young twigs as well as the distal portions of stem are flexible and often pendent. The stems are rough due to pulvini that persist after the leaves fall.
The winter buds are ovoid or globose, usually rounded at the apex and not resinous. The leaves are flattened to slightly angular and range from 0.2 to 1.4 inches (5 – 35 mm) long and 0.04 to 0.12 inch (1 – 3 mm) broad; they are borne singly and are arranged spirally on the stem. The leaf bases are twisted so the leaves lie flat along either side of the stem or more rarely radially. Towards the base, the leaves abruptly narrow to a petiole set on a forward-angled, pulvinus. The petiole is twisted at the base so that it is almost parallel with the stem. The leaf apex is either notched, rounded, or acute. The undersides have two white stomatal bands (in T. mertensiana they are inconspicuous) separated by an elevated midvein. The upper surface of the leaves lack stomata, except in T. mertensiana. They have one resin canal that is present beneath the single vascular bundle.
The pollen cones grow solitary from lateral buds. They are 0.12 to 0.4 inch (3 – 10 mm) long, ovoid, globose, or ellipsoid, and yellowish-white to pale purple, and borne on a short peduncle. The pollen itself has a saccate, ring-like structure at its distal pole, and rarely this structure can be more or less doubly saccate. The seed cones are borne on year-old twigs and are small ovoid-globose or oblong-cylindric, ranging from 0.6 to 1.6 inches (15 – 40 mm) long, except in T. mertensiana, where they are cylindrical and longer, 1.4 to 3.2 inches (35 – 80 mm) in length. They are solitary, terminal or rarely lateral, pendulous, and are sessile or on a short peduncle up to 0.16 inch (4 mm) long. Maturation occurs in 5 to 8 months, and the seeds are shed shortly thereafter; the cones are shed soon after seed release or up to a year or two later. The seed scales are thin, leathery and persistent. They vary in shape and lack an apophysis and an umbo. The bracts are included and small. The seeds are small, from 0.08 to 0.16 inch (2 – 4 mm) long with a 0.32 to 0.48 inch (8 to 12 mm) long wing. They also contain small adaxial resin vesicles. Seed germination is epigeal; the seedlings have four to six cotyledons.
Mountain hemlock, T. mertensiana is unusual in the genus in several respects. The leaves are less flattened and arranged all round the shoot, and have stomata above as well as below, giving the foliage a glaucous colour; and the cones are the longest in the genus, 1.4 to 3.2 inches (35 – 80 mm) long and cylindrical rather than ovoid. Some botanists treat it in a distinct genus as Hesperopeuce mertensiana (Bong.) Rydb., though it is more generally only considered distinct at the rank of subgenus.
Another species, Bristlecone hemlock, first described as Tsuga longibracteata, is now treated in a distinct genus Nothotsuga; it differs from Tsuga in the erect (not pendulous) seed cones with exserted bracts, and pollen cones clustered in umbels, in these features more closely allied to the genus Keteleeria.
The species are all adapted to (and are confined to) relatively moist cool temperate areas with high rainfall, cool summers, and little or no water stress; they are also adapted to cope with heavy to very heavy winter snowfall and tolerate ice storms better than most other trees. Hemlock trees are more tolerant of heavy shade than other conifers; hemlocks are, however, more susceptible to drought.
The two eastern North American species, T. canadensis and T. caroliniana are under serious threat by the sap-sucking insect Adelges tsugae (Hemlock Woolly Adelgid or HWA). This adelgid, related to the aphids, was introduced accidentally from eastern Asia, where it is only a minor pest. Extensive mortality has occurred, particularly east of the Appalachian Mountains. The Asian species are resistant to this pest, and the two western American hemlocks are moderately resistant. Tsuga species are also used as food plants by the larvae of some Lepidoptera species including the Autumnal Moth and the Engrailed, and older caterpillars of the Gypsy Moth. Once these infest a tree, they can do more than simply kill one tree. Larger hemlocks that are infected have large relatively high root systems that can bring other trees down if one falls. The foliage of young trees is often browsed by deer, and the seeds are eaten by finches and small rodents.
Old trees are commonly attacked by various fungal disease and decay species, notably Heterobasidion annosum and Armillaria species, which rot the heartwood and eventually leave the tree liable to windthrow, and Rhizina undulata, which may kill groups of trees following minor grass fires that activate growth of the Rhizina spores.
The wood obtained from hemlocks is important in the timber industry, especially for use as wood pulp. Many species are utilized in horticulture, and numerous cultivars have been selected for use in gardens. The bark of the hemlock is also used in tanning leather. The needles of the hemlock tree are sometimes used to make a tea. In 2001 Ormonde Jayne Perfumery became the first perfume house to make a western perfume from an extract of Black Hemlock Absolute oil and named it Ormonde Woman. It has since been named as one of the 100 great classic by Luca Turin in his book The Little Black Book of Perfume: 100 Great Classics. Since Ormonde Woman’s launch several other scents have been launched using Black Hemlock.Attributed from: Wikipedia, The Free Encyclopedia