^22 



THE POPULAR SCIENCE MONTHLY. 



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, 



Fm. 1. 



Section of Trunk of Fib-Tree, showing the Annual Kings of Growth. 



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. 



Fig. 2. 



Section of Palm, without Annual Rings. 



A layer represents the growth of a year. Where these are acces- 

 sible, there is no difficulty in ascertaining the age of a tree, or the rate 



