Giant sequoias are the world's largest trees in terms of total volume. They grow to an average height of 50–85 metres (160–279 ft) and 6–8 metres (20–26 ft) in diameter. Record trees have been measured to be 94.8 metres (311 ft) in height and over 17 metres (56 ft) in diameter. The oldest known giant sequoia based on ring count is 3,500 years old. Sequoia bark is fibrous, furrowed, and may be 90 centimetres (3.0 ft) thick at the base of the columnar trunk. It provides significant fire protection for the trees. The leaves are evergreen, awl-shaped, 3–6 mm long, and arranged spirally on the shoots. The seed cones are 4–7 cm long and mature in 18–20 months, though they typically remain green and closed for up to 20 years; each cone has 30-50 spirally arranged scales, with several seeds on each scale giving an average of 230 seeds per cone. The seed is dark brown, 4–5 mm long and 1 mm broad, with a 1 mm wide yellow-brown wing along each side. Some seed is shed when the cone scales shrink during hot weather in late summer, but most seeds are liberated when the cone dries from fire heat or is damaged by insects.