Tsuga - Description

Description

They are medium-sized to large evergreen trees, ranging from 10–60(–79) m tall, with a conical to irregular crown, with 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 colour 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 5–35 mm long and 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 either side of the stem or more rarely radially. Towards the base the leaves narrow abruptly 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 3–5(–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 15–40 mm long, except in T. mertensiana, where they are cylindrical and longer, 35–80 mm in length; they are solitary, terminal or rarely lateral, pendulous, and are sessile or on a short peduncle up to 4 mm long. Maturation occurs in 5–8 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 2 to 4 mm long, and winged, with the wing being 8 to 12 mm in length. They also contain small adaxial resin vesicles. Seed germination is epigeal; the seedlings have four to six cotyledons.