F. PRINCIPLES OF SILVICULTURE. 



How TREES GROW. 



Trees, like most other plants, originate from seed, build tip a body of cell tissues, form foliage, 

 flower, and fruit, and take up food material from the soil and air, which they convert into cellulose 

 and other compounds, from which all their parts are formed. They rely, like other plants, upon 

 moisture, heat, and light as the means of performing the functions of growth. Yet there are 

 some peculiarities in their behavior, their life and growth, which require special attention on the 

 part of a tree grower or forest planter. 



FOOD MATERIALS AND CONDITIONS OF GROWTH. 



Trees derive their food and solid substance in part from the air and in part from the soil. 

 The solid part of their bodies is made up of cellulose, which consists largely of carbon (44 per 

 cent of its weight), with hydrogen and oxygen added in almost the same proportions as in water. 

 The carbon is derived from the carbonic acid of the air, which enters into the leaves, and under 

 the influence of light, air, and water is there decomposed; the oxygen is exhaled; the carbon is 

 retained and combined with elements derived from the water, forming compounds, such as starch, 

 sugar, etc., which are used as food materials, passing down the tree through its outer layers to 

 the very tips of the roots, making new wood all along the branches, trunk, and roots. 



This process of food preparation, called " assimilation," can be carried on only in the green 

 parts, and in these only when exposed to light and air; hence foliage, air, and light at the top 

 are essential prerequisites for tree growth, and hence, other conditions being favorable, the more 

 foliage and the better developed it is and the more light this foliage has at its disposal for its 

 work, the more vigorously, will the tree grow. 



In general, therefore, pruning, since it reduces the amount of foliage, reduces also for the 

 time the amount of wood formed; and just so shading, reducing the activity of foliage, reduces 

 the growth of wood. 



SOIL CONDITIONS. 



From the soil trees take mainly water, which enters through the roots and is carried through 

 the younger part of the tree to the leaves, to be used in part on its passage for food and wood 

 formation and in part to be given up to the air by transpiration. 



In a vigorously growing tree the solid wood substance itself will contain half its weight in 

 the form of water chemically combined, and the tree, in addition, will contain from 40 to 05 per 

 cent and more of its dry weight in water mechanically or hygroscopically held. This last, 

 when the tree is cut, very largely evaporates; yet well seasoned wood still contains 10 to 12 per 

 cent of such water. The weight of a green tree a pine, for instance is made up in round 

 numbers of about 30 per cent of carbon and 70 per cent of water, either chemically or hygroscop- 

 ically held, while a birch contains a still larger percentage of water. 



The largest part of the water which passes through the tree is transpired i. e., given off to the 

 air in vapor. The amounts thus transpired during the season vary greatly with the species of 

 tree, its age, the amount of foliage at work, the amount of light at its disposal, the climatic 

 conditions (rain, temperature, winds, relative humidity), and the season. These amounts are, 

 however, very large when compared with the quantity retained; so that while an acre of forest 

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