of the organs is followed by constant renovation, and the foliage which covers it the present summer is as new and as young as that which adorned it a hundred or a thousand years ago. Trees which shed their leaves annually, or at longer intervals as do the evergreens,
grow by formation of new wood in layers upon their outer surface, and just beneath the bark. These constitute the class Exogens, or outside growers, as shown in Fig. 1. This plate, with others used to illustrate this article, are from Figuier's "Vegetable World," and have been placed at our disposal by the publishers of that interesting work.
A layer represents the growth of a year. Where these are accessible, there is no difficulty in ascertaining the age of a tree, or the rate
