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BULLETIN 8 6 3, U. S. DEPARTMENT OE AGRICULTURE 



How the branches lengthen and the tree trunk increases in size; 

 the location, color, and structure of the living tissue or cambium 

 layer. What are annual rings, heartwood, and sapwood? 



Requirements for growth : Air, light, moisture, and heat. 



Figure 8. — How the tree trunk grows. All growth takes place in the cambium, lying 

 between the inner bark and sapwood. This is a very thin layer of living cells which 

 divide and subdivide, forming on the outside bark and on the inside wood (A), The 

 inner bark, or last tissue, is soft and moist. Its function is to carry The food prepared 

 in the leaves- to all growing parts of the tree (B). By a gradual change the inner bark 

 passt s into outer bark, a corky layer composed of dry, dead cells. This serves to protect 

 the living stem against evaporation and mechanical injury (C). The woody growth 

 daring one season is called an annual ring. In the spring the newly formed cells are 

 thin-walled and spongy, while in midsummer and fall the walls of the cells become 

 thicker and denser. This difference can be distinguished in many kinds of trees as 

 light-colored spring wood and darker colored summer wood. Sapwood (D) is the lighter 

 colored band of wood beneath the bark, often from 1 to 2 inches thick. It carries the 

 sap from the roots to the leaves. Heartwood (E) is formed by a gradual change in the 

 sapwood by which it becomes darker, heavier, and often more lasting. Most of the trees. 

 but not all. form heartwood. Pith is the soft tissue on the innermost part of the stem. 

 about which the first woody growth takes place in the newly formed twig (F). From 

 it ( j xtnnd the pith rays (G). These are flat, vertical bands of tissue which connect the 

 pith with the various layers of wood and the inner bark. They transfer and store up food 



Trees in association — a stand. Influence of trees upon each other. 

 Difference, if any, between shape of crowns of open-growth trees and 

 those grown in close stands. Influence of different light and soil- 

 moisture supply. 



Effect of tree density (number of trees in a given area) upon 

 growth of the indiyidual tree. Natural dying out of trees in close 

 stands with advancing age. Understocked, well-stocked, and over- 



