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Tsuga heterophylla is a large evergreen coniferous tree growing to 165-230 ft (50–70 m) tall, exceptionally 270 ft (82 m), and with a trunk diameter of up to 9 ft (2.7 m). It is the largest species of hemlock, with the next largest (mountain hemlock T. mertensiana) reaching a maximum of 194 ft (59 m). The bark is brown, thin and furrowed. The crown is a very neat broad conic shape in young trees with a strongly drooping lead shoot, becoming cylindric in older trees; old trees may have no branches in the lowest 100-130 ft (30–40 m). At all ages, it is readily distinguished by the pendulous branchlet tips. The shoots are very pale buff-brown, almost white, with pale pubescence about 1 mm long. The leaves are needle-like, 5–23 mm long and 1.5–2 mm broad, strongly flattened in cross-section, with a finely serrated margin and a bluntly acute apex. They are mid to dark green above; the underside has two distinctive white bands of stomata with only a narrow green midrib between the bands. They are arranged spirally on the shoots but are twisted at the base to lie in two ranks on either side of the shoot. The cones are small, pendulous, slender cylindrical, 14–30 mm long and 7–8 mm broad when closed, opening to 18–25 mm broad. They have 15–25 thin, flexible scales 7–13 mm long. The immature cones are green, maturing gray-brown 5–7 months after pollination. The seeds are brown, 2–3 mm long, with a slender, 7–9 mm long pale brown wing.