The swamp gum grows about 90m tall. Among the gum trees widely used for timber are the jarrah, the messmate, and the blackbutt. The leaves of the Tasmanian blue gum provide an oil used as a medicine and to help refine petroleum. The forests of the tropics are the world's richest plant habitat. Temperate forests rarely have more than a dozen different trees.
Most tropical forests have several distinct layers.
Towering above the main bulk of the forest are isolated emergent trees, growing up to about 60m tall. Below them, 30-50m above the ground, is a dense, unbroken canopy of leaves and branches at the top of tall, straight trees. In the gloom beneath is the understorey, where young emergents, small conical trees and shrubs grow. Conifers and cycads Conifers and their close relatives the cycads and ginkgos are among the oldest of all plants, first appearing over 275 million years ago. They are called gymnosperms and all make seeds in cones.