20 



A PRIMER OF FORESTRY. 



H 



r: 



Fig. 20.— "Wood of a Spruce, 



greatly magnified. 

 (From Hartig. Anatomie 



mid Physiologic der Pfian- 



zen, Berlin, 1891.) 



season of growth and one of rest, 

 like our own, the cells of the layer 

 of new wood formed each year 

 at the inner surface of the cam- 

 bium are arranged in a definite 

 way. When growth begins in the 

 spring, and the fresh twigs and 

 leaves put out. there is a great 

 demand for water in the crown 

 to supply these moist green new 

 parts of the tree. Water rises in 

 most trees through the newer lay- 

 ers of the wood, and especially 

 through the last ring. Conse- 

 quently, at first the tree makes 

 thin walled cells with wide open- 

 ings, through which water can 

 rise rapidly to the ends of the 

 branches. Later on, when the de- 

 mand for water is not so great, 

 and there is plenty of digested 

 food to supply building material, 

 the cells formed are narrow and 

 thick-walled. (See fig. 20.) Thus 

 the summer wood in each year's 

 growth is heavier, stronger, and 

 darker in color than the spring 

 wood. In the wood of many 

 broadleaf trees, such as Oak and 

 Chestnut, the spring wood is also 

 marked by a band of open tubes 

 of larger size called ducts. In 

 others, such as Maple and Beech, 



