Tsuga
Tsuga is a genus of
conifers in the family
Pinaceae. The common name hemlock is derived from the perceived similarity in the smell of the crushed foliage to that of the unrelated herb
Poison hemlock; see hemlock for other senses of the word. Unlike the herb, the species of
Tsuga are
not poisonous. The name
Tsuga is the Japanese name for
Tsuga sieboldii.
The genus includes nine species, four in
North America and five in
Asia. They are medium-size to large
evergreen trees, 20-65 m tall, with conical to irregular crown shape and drooping branch tips.
Encyclopedia
Tsuga is a genus of
conifers in the family
Pinaceae. The common name
hemlock is derived from the perceived similarity in the smell of the crushed foliage to that of the unrelated herb
Poison hemlock; see hemlock for other senses of the word. Unlike the herb, the species of
Tsuga are
not poisonous. The name
Tsuga is the Japanese name for
Tsuga sieboldii.
The genus includes nine species, four in
North America and five in
Asia. They are medium-size to large
evergreen trees, 20-65 m tall, with conical to irregular crown shape and drooping branch tips. The
leaves are needle-like, 8-40 mm long and 1.5-3 mm wide, arranged spirally on the stem but twisted at the base to lie flat on either side of the shoot; they are green above without
stomata , and with two white stomatal bands below. The
cones are pendulous, small , ovoid to cylindrical, and mature in autumn, 6-8 months after pollination. The
seeds are very small, 2-4 mm long, with an 8-12 mm wing.
Western Hemlock Tsuga heterophylla is the largest species, reaching heights of 70 m. It is a particularly common timber tree in the Pacific Northwest of
North America. It is also planted for timber in northwest
Europe and other temperate areas with high rainfall and cool summers.
The two species in eastern North America,
Eastern Hemlock T. canadensis and Carolina Hemlock
T. caroliniana, are threatened by a sap-sucking
insect, the Hemlock Woolly Adelgid . This
aphid was introduced accidentally from eastern Asia. Extensive mortality has occurred, particularly east of the
Appalachian Mountains. The Asian species, and also the two west American hemlocks, are relatively resistant to this pest.
Tsuga species are used as food plants by the
larvae of some
Lepidoptera species including
Autumnal Moth and
The Engrailed.
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, 35-70 mm long and cylindrical rather than ovoid. Some botanists treat it in a distinct genus as
Hesperopeuce mertensiana, 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 cones with exserted bracts, and male cones clustered in umbels, in these features more closely allied to the genus
Keteleeria.
External links
