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the tree—its root-system.

We may now suppose the young oak-plant to be rapidly developing into a tree. Technically the seedling is said to be a plant after the first year, and when it reaches the height of a few feet the young tree is called a sapling; these ideas are by no means well defined, however, and we may regard them as arbitrary terms of little or no scientific value.

The principal changes which are noticeable as the little tree grows larger are the gradual increase in the length and thickness of the stem, and in the number and spread of the branches put forth year after year. Corresponding with these increments, each spring sees a greater number of leaves than the one before, and it is easy to prove that the roots also become more numerous and complex each season.

The above simply expresses certain facts of observation, but it is more accurate to link them together as follows:

In each successive season of growth the young oak develops more leaves than it did before—in other words, the total area of the leaf-surface exposed to the air and