obtained by their dissection or material supplied by nature for their sustenance.

Thirdly, each part of a tree generates itself in such a way that the maintenance of any one part depends reciprocally on the maintenance of the rest. A bud of one tree engrafted on the twig of another produces in the alien stock a plant of its own kind, and so also a scion engrafted on a foreign stem. Hence we may regard each twig or leaf of the same tree as merely engrafted or inoculated into it, and so as an independent tree attached to another and parasitically nourished by it. At the same time, while the leaves are products of the tree they also in turn give support to it; for the repeated defoliation of a tree kills it, and its growth thus depends on the action of the leaves upon the stem. The self-help of nature in case of injury