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When the leaves sprout afresh in the spring on trees that have lain dormant or shed their leaves in the winter, there is a great demand by the trees for water, and the pipes and cells which are then formed have thin walls and large openings. Later on, in summer, there is not so much want of water, and smaller openings will do; at the same time the bark begins to press on the wood, so that the cell walls must be capable of resisting this pressure. Therefore in summer the pipes have thick walls and small openings. In winter growth stops, as has been said, and it begins again next spring with the pipes with large openings. These being formed next to the thick-walled ones give the appearance of a ring when the wood is cut across.
Running across the annual rings from the centre of the tree to the bark there are some thin lines. These are called the medullary rays, and to them mainly is due the figure possessed by many woods. In order to see these medullary rays at their best, one has to cut the wood in a particular way, that is, along a radius, or as the timber man says "on the quarter." The breadth of the rays varies in different species of trees, being difficult to see in eucalypts and pines, but in some others, such as Casuarinas, Sheoaks and Banksias, standing out quite clearly. These medullary rays consist chiefly of soft cells which keep up a connection through the harder timber from the centre of the tree to the bark. They, as well as some other parts, are used by the tree as storehouses for any excess of starch that may be in the food, and, it is believed, are concerned in the series of changes whereby sapwood is ultimately converted into heartwood.
Reproduction.—Nature makes plentiful provision for the preservation of tree growth. All trees when they come to maturity bear flowers, although sometimes these are not very conspicuous. These flowers in due time develop into fruit and seeds, which fall to the ground and give birth to new trees. Besides reproduction by means of seeds, many trees are able to spring up again by means of shoots from the stump after the tree has been cut down, or by suckers from the roots. This power varies in the different species, and upon it depends the production of coppice woods, about which something will be said when we come to speak of forests.
