18 



A PRIMER OF FORESTRY. 



THE GROWTH OF A TREE. 



The addition of new material in the way described in 

 tlie preceding pages is the foundation of growth. Ex- 

 cept in the buds, leaves, fruit, and the twigs less than 

 a year old, this material is deposited in a thin coat over 

 the whole tree between the wood and the bark. The 

 new twigs grow in length by a kind of stretching, but 

 only during the first year. Thus it is only by means of 



these youngest twigs that a 

 tree increases in height and 

 in spread of branches. After 

 the first year their length is 

 fixed, younger twigs stretch 

 out from the buds, and the 

 older ones grow henceforth 

 only in thickness. (See fig. 

 9.) The fresh coat of new 

 material mentioned above 

 covers them year by year. 

 There are two layers in this 

 coat, separated by a third 

 one of tender forming tissues 

 called the cambium, in which the actual making of the 

 new substance goes on. The inner side of the cambium 

 layer forms new wood, the outer side new bark. Be- 

 sides the true cambium, which forms both wood and 

 bark, there is another cambium which makes the corky 

 outer bark, and nothing else. This cork cambium may 

 encircle the whole tree, like the true cambium, as in the 

 Eed Cedar, or it may form little separate films in the 

 bark, but in either case it dies from time to time, and 

 is re-formed nearer the wood. (See figs. 10-13 and Pis. Y 

 and Yl.) 



Fig. 18.— Cross section of a fallen 

 Black Oak. Milford, Pa. The 

 slabs shown in figs. 19 and 22 were 

 sawed lengthwise from this tree, 

 beginning where the black lines 

 arc seen on the cross section. 



