OUR FORESTS 6 



The roots, which hold the tree in place and take np from the soil, 

 water and certain mineral substances needed in the tree's growth; 

 the trunk or stem, which supports the crown and supplies it with 

 water and food from the roots; and the crown, which has much to 

 do with the life of a tree, for in this part take place the most im- 

 portant processes in the digestion of its food and the reproduction 

 of the tree (fig. 1). 



The materials upon which a tree feeds are derived from the soil 

 and from the air. Those from the soil are collected by the roots, 

 which extend down into the ground. At the end of the roots and 

 rootlets are countless root hairs reaching out between particles of 

 soil for water and the various substances which it holds in solution. 

 The water and food materials thus collected move upward through 

 numerous channels in the roots, trunk, and branches to the leaves. 



The leaves serve as factories where the foods necessary for the 

 tree's growth are manufactured. This food making takes place in 

 numberless tiny cells of the leaf where by aid of chlorophyll bodies 

 and the action of sunlight, the carbonic acid gas taken from the air 

 is broken up into its elements, oxygen and carbon. While the oxygen 

 is returned to the air, the carbon, still through the action of sunlight 

 and chlorophyll, is combined with the oxygen and hydrogen of the 

 water from the roots, forming new chemical compounds, in which 

 nitrogen and various earthy substances from the water are present. 

 Thus the raw food materials which reach the tree through the roots 

 and the leaves are digested in the leaves somewhat as food is digested 

 in the human body. They are then sent to all living parts of the 

 roots, stem, and crown, where they are either used at once in growth 

 or stored away for later use. 



Like all other plants and like animals, trees breathe. This breath- 

 ing process goes on both day and night. The breathing is done 

 through the leaves and through tiny openings in the bark called 

 lenticels. 



The amount of water taken up by the roots is usually very much 

 larger than is required in the chemical processes which go on in the 

 leaves. There is thus a surplus of water which cannot be held in 

 the leaves, but must make way for fresh supplies carrying the 

 mineral constituents necessary to the tree's growth. The tree rids 

 itself of this unused water by a process known as transpiration, 

 which is the evaporation of water from all parts of the tree above the 

 ground, but principally from the leaves. In this way trees give 

 off great quantities of water vapor, which tends to keep the air in 

 the forests humid and favorable to growth. 



HOW A TREE GROWS 



Most trees grow in height and spread of branches by sending out 

 shoots formed by the development of new wood cells. The growth 

 in height each year is made at the terminal bud of the main stem 

 or stems. The " candles " of the pines are showy evidences of this 

 new growth. 



A tree grows in thickness or girth through the addition each year 

 of a coat of new wood cells (fig. 2). The layer of wood thus de- 

 veloped is known as an " annual ring ", which, after it is once 

 formed, does not change in size or place during the life of the tree. 



