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When a forest is composed wholly of one species of tree, it is said to be a "pure" forest. If trees of several varieties grow within its borders it is called a "mixed" forest. Both classes are found in Western Australia. There are great stretches of forest in the South-West, where jarrah trees only are to be seen, others containing nothing but karri. Tuart, too, is found in a "pure" state, and so are some other trees. Outside these areas where "pure" forests exist, are regions of mixed forest, where one type of tree may be most plentiful, but growing with it are others of different varieties. Trees, like man, have habits or likings, and they grow best where the conditions are favourable for the indulgence of these habits or likings. For instance, some trees are "light-demanders," that is, they need plenty of light if they are to become big and productive. The eucalypts are of this class. If, in walking through a jarrah forest, one looks around for young jarrah seedlings, it will be observed that they are found very seldom inedeed under the shade of, or in close proximity to, grown jarrah trees. But they will be observed in plenty in the clear spaces upon which the sun's rays directly fall between the trees. Others, again, will thrive under more or less shade and attain their best when they receive this shade from trees of other species. Many of the acacia, family are of this type, and many of the tall shrubs of our karri country provide good examples of "shade bearers."
