20 



A PRIMER OF FORESTRY. 



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Fig. 20.— Wood of a Spruce, 



greatly magnified. 

 (From Hartig. Anatomic 



nnd riiysiologie der Pfian- 



zen, Berlin, 1891.) 



season of growth and one of rest, 

 like our owd, 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 tbe 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 

 tbrough the last ring. Conse- 

 quently, at first the tree makes 

 thin walled cells with wide open- 

 ings, through M'hich 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 supi^ly 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 sx^ring 

 wood. In the wood of many 

 broadleaf trees, such as Oak and 

 Cliestnut, the spring wood is also 

 marked by a band of open tubes 

 of larger size called dncts. In 

 others, such as Maple and Beech, 



