FORESTRY FOR 4-H CLUBS 



Growth of Trees and Forests 



9 



Aim. — To find out how trees and forests grow. 



Sources of information. — Farmers' Bulletins 1256, 1486, 1517, 1671, and 1989; 

 Miscellaneous Publication 162. 



Illustrative material. — l^otted seedlings, pots or boxes, and seeds of trees. A 

 chart showing roots, stems, and leaves of a tree. A chart, or better an actual 

 cross section of a tree stem, showing different parts such as annual rings, heart- 

 wood, sapwood, bark, and cambium (fig. 5). Leaves mounted so that their 

 structure can be studied. Branches showing bud and twig arrangement. Draw- 

 ings showing shapes of crowns or tops of trees grown in the open and of those 

 grown in close stands. 



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Figure 5.— How the tree trunk grows. All growth takes place in the cambium (a), lying between 

 the inner bark and sapwood. This is a very thin layer of living ceils, which divide and subdivide, 

 forming on the outside bark and on the inside wood. 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. 

 By a gradual change the inner bark passes into outer bark (r), a corky layer composed of dry, dead 

 cells. This serves to protect the living stem against evaporation and mechanical injury. The 

 woody growth during 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 

 ((?) is the result of 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. 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. 



