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Light is an important factor in tree growth, and the form of the tree 

 varies with the light supply. In the forest the trees grow tall, with long, 

 clear trunks and few branches, while in the open the tree is short and 

 branchy with a large crown. The belief that the growth of the tree push- 

 es its branches higher up on its trunk is erroneous ; the bareness of the for- 

 est tree is caused by the loss of the lower branches for want of light. The 

 open-ground tree retains its branches lower on the trunk, and consequently 

 does not produce as good lumber, the limbs, that all start from the centre 

 of the tree, each year's wood growth burying their beginning deeper and 

 deeper, if not killed and shed early, produce knots, which injure the 

 strength and value of the sawn lumber. In dense forests the lack of light 

 causes the lower branches to die and fall and the trunk is left clear. In 

 forest planting, trees are set densely to shut out the light and kill the lower 

 branches, in order to produce later, clear, valuable lumber. 



Trees grow not only in height, but also in. diameter, the growth taking 

 place in the soft cambium layer between bark and wood. Every spring 

 this layer of living cells begins to grow and divide, at first very rapidly 

 with the rapid height-growth, but gradually more slowly as summer ad- 

 vances. The first quickly-formed wood cells are thin-walled with large 

 openings, forming the pores seen in the spring wood of the oaks and ashes. 

 The later summer-formed cells are closely crowded and compressed, with 

 thick walls and small openings, the wood appearing denser and darker 

 colored from this crowded condition of the cells. This succession of dif- 

 ferent spring and summer wood zones allows the easy recognition and 

 counting of the annual layers or rings of wood, and this variation in the 

 ring structure serves to identify various species of trees, and to indicate 

 the comparative strength of their wood. After cutting a tree, therefore, 

 its age can be found by counting the annual rings on the stumps, and from 

 the record preserved in these rings the history of its growth can be read. 



The annual ring is formed in all countries where there is a temporary 

 cessation of growth, caused by distinct summer and winter seasons. Ex- 

 ceptionally trees fail to deposit wood over the whole trunk on account of 

 loss of foliage, etc., and no ring is formed, or, where the growth has been 

 disturbed during the season, a second ring may form, which can mostly be 

 readily distinguished from the true rings. 



To the forest manager the study of ring growth is of great importance, 

 because from the rings of growth the progress of the crop may be seen 

 the amount of wood formed, and the time when it is most profitable to har- 

 vest calculated. Since also the proportion of spring and summer wood 

 largely determines the quality of the timber, the ring growth furnishes an 

 index for regulating the quality of the crop, since, by management of spe- 

 cies, and adapting them to soils, the proportion of the spring and summer 

 wood may be influenced. It is a mistake to consider that the lumber of the 

 more rapidly grown trees of any species is weaker, for it is apparent that 



