GROWTH OF TREES AND FORESTS 189 



the quality of the soil for any species, and consequently a table 

 showing the average height of trees of different diameters and 

 on different sites furnishes a ready means of determining the 

 relative value of the sites for the species in question. In fact 

 the three quality sites commonly recognized are usually deter- 

 mined by a classification of heights. 



i. Age of Trees. 



The age of a second-growth pine tree and of some other species 

 up to fifty or sixty years can be obtained with fair accuracy by 

 counting the whorls of branches, since the pine makes but one 

 whorl each year. The buds of winter develop in the spring 

 into branches and leader. Height growth is made entirely 

 at the top. One can only approximate the age of most trees 

 without cutting and counting the rings on the stump. Every 

 fall, in our climate, the growth ceases and the tree remains at 

 rest until spring. When activity in the cambium (the tissue 

 just inside the bark) recommences, large wood cells are made 

 with comparatively thin walls. Throughout the growing season 

 new layers of cells are formed, but as the season advances these 

 cells become smaller and their walls relatively thicker. On 

 account of this difference in structure the wood formed in spring 

 and early summer is lighter colored than that formed at the 

 end of the season. A sharp, well-defined line in most species 

 separates the dark growth of the fall from the light growth 

 of the next spring, and the belt between these dark lines is 

 called the annual ring. In the tropics where there are no well- 

 defined seasons, no distinct rings occur, and the age of trees 

 cannot be ascertained in this way. The distinctness of the 

 rings in trees varies with the texture of the wood. In such 

 woods as the spruce, pine, oak, ash, and chestnut the rings are 

 easily distinguishable, although slow growth in some cases may 

 require the use of a magnifying glass. With such fine-grained 

 trees as beech, birch and maple, on the other hand, it is often 

 very difficult to distinguish the rings. In obtaining the age of 

 a tree by counting the rings on the stump it must be remembered 



