Yews are small to medium-sized evergreen trees, growing 10–20 m (35–65 ft) (exceptionally up to 28 m or 92 ft) tall, with a trunk up to 2 m (6 ft 7 in) (exceptionally 4 m or 13 ft 1 in) in diameter. The bark is thin, scaly brown, and comes off in small flakes aligned with the stem. The leaves are flat, dark green, 1–4 centimetres (1⁄2–1+1⁄2 in) long, 2–3 mm (3⁄32–1⁄8 in) broad, and arranged spirally on the stem, but with the leaf bases twisted to align the leaves in two flat rows on either side of the stem, except on erect leading shoots where the spiral arrangement is more obvious. The leaves are poisonous.[4][7]
The seed cones are modified, each cone containing a single seed, which is 4–7 mm (3⁄16–1⁄4 in) long, and partly surrounded by a fleshy scale which develops into a soft, bright red berry-like structure called an aril. The aril is 8–15 mm (5⁄16–9⁄16 in) long and wide and open at the end. The arils mature 6 to 9 months after pollination, and with the seed contained, are eaten by thrushes, waxwings and other birds, which disperse the hard seeds undamaged in their droppings. Maturation of the arils is spread over 2 to 3 months, increasing the chances of successful seed dispersal. The seeds themselves are poisonous and bitter, but are opened and eaten by some bird species, including hawfinches,[14] greenfinches, and great tits.[15] The aril is not poisonous; it is gelatinous and very sweet tasting. The male cones are globose, 3–6 mm (1⁄8–1⁄4 in) in diameter, and shed their pollen in early spring. Yews are mostly dioecious, but occasional individuals can be variably monoecious, or change sex with time.[4][7][16]
