6 Georgia State College of Agriculture. 



often slippery, light-colored, living-wood-tissue (cambium) in which 

 the cells are capable of division. The division of the cambium cells 

 results in diameter growth. All other wood tissue is dead or nearly 

 dead material. When first formed from the cambium all cells 

 contain protoplasm, but they soon lose their living contents. 



The outer layers thus form a coating or shell of living tissue 

 over the dead inner portion of the roots, stem and branches. Often 

 the entire heart-wood at the base of the tree may be destroyed and 

 yet the tree be growing vigorously, indicating that the life pro- 

 cesses are carried on in the sapwood and bark only. 



Annual Rings. If the bark, sap-wood and heart-wood are care- 

 fully examined, each will be found to be made up of layers which 

 in cross-section appear as concentric rings of tissue, one of which 

 is produced each year. These concentric layers are known as annual 

 rings, and it is possible to determine the age of the tree by counting 

 them. As a result of the division of the cambium cells a layer of 

 new cork tissue is produced on the inside of the bark and a layer 

 of new wood tissue is laid on the outside of the sap-wood, the 

 cambium layer always remaining between the layers of new tissue. 

 The amount of bark produced each year is much less than the 

 amount of woody tissue. The entire tree, roots, trunk and branches, 

 is covered each year with a new coat of wood and bark. 



Spring and Summer Wood. If an annual ring of oak wood is 

 examined it will be found to be made up of two layers, a soft, open, 

 light layer (spring wood), and a dense, hard, darker-colored layer 

 (summer wood). In the spring a large amount of water is re- 

 quired by most trees to produce the new leaves and to supply the 

 growing parts, with the result that the cells produced during the 

 early part of the growing season are large and thin-walled. In oak 

 wood the openings of the cells in the spring wood are visible to the 

 naked eye. As soon as the leaves have developed, much less water 

 is required and the cells produced later in the season are small and 

 thick-walled. This difference in growth gives rise to the marked 

 contrast between spring and summer wood in many trees, results 

 in the "grain" of sawn wood, and separates one year's growth from 

 another. The rate of growth of the various species is not the same, 

 nor is it the same from year to year in any one species. A favor- 

 able, moist season will produce a wide annual ring; an unfavorable, 

 drouthy season, a narrow ring. In general trees grow slowly during 

 the first few years of life, the rate then increases until middle life, 

 then falls off gradually. In an old tree the annual rings may be so 

 narrow as not to be distinguishable with the naked eye. 



Height GroAvth of Trees. Height growth and growth of branches 

 in length are produced by the development of the terminal buds. 

 In this growing tissue the division of cells takes place transversely 

 (in contrast to the longitudinal division of the cambium cells), re- 

 sulting in an elongation. Along with this there is .a stretching pro- 

 cess as the cells become filled with water. New shoots do not grow 

 in length after the first season. They grow in thickness only, and 

 other new shoots are produced from new terminal buds to increase 



