26 DEPARTMENT BULLETIN 863, U. S. DEPT. OF AGRICULTURE. 



sapling pine or hardwood. At regular intervals of a week or month, 

 mark on it the total height of the growing tree. Keep a record also 

 of the dates and measured heights. 



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 leaves. 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-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 iformed 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 inner- 

 most 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 flat bands of the same 

 tissue connect the pith with the various layers of wood and the bark. They transfer 

 and store up food. 



Lesson XL FOREST REPRODUCTION. 



Problem. — To learn how trees reproduce themselves. 

 Sources of information. — Farmers' Bulletins 134, 173, 423, Til, 

 788, 1071, and 1123; Forestry Bulletins 45, 121, and 244. Forestry 



