FORESTRY LESSONS ON HOME WOODLANDS 23 



Lesson X. GROWTH OF TREES AND FORESTS 



Problem. — To learn how trees and forests grow. 



Sources of information. — Farmers' Bulletins 134, 173, and 1071; 

 Forestry Bulletin 92; Department Bulletin 308. 



Illustrative material. — Potted seedlings, pots or boxes, and seeds 

 of trees. A chart showing roots, stem, and leaves of a tree. A chart, 



Fig. 14.— 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 passes 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 during one season 

 is called an annual ring. In the spring the newly formed cells are thin-vv ailed and spongy, while in 

 midsummer and fall the walls of the cells become thicker and denser. This difference can be distin- 

 guished in many kinds of trees as light-colored spring wood and darker colored summer wood. Sap- 

 wood (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 extend 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 



or better, an actual cross section of a tree stem showing different 

 parts of the stem, such as annual rings, heartwood, sapwood, bark, 

 and cambium. Leaves mounted so that their structure can be 

 studied. Branches showing bud and twig arrangement. Drawings 

 showing shapes of crowns of trees grown in the open and grown in 

 close stands. 



