326 



GENERAL BOTANY 



Structure. The broad outlines of structure in the spruce stem 

 are identical with those of the woody stems of the alder, ash, and 

 other trees and shrubs belonging to the flowering plants already 



studied (Figs. 191 and 

 192). The bark is com- 

 posed of thick layers of 

 cork, which scale off in 

 flakes on the main trunk 

 and the older branches of 

 the tree. This corky bark, 

 like that of other trees, is 

 formed by a special cork 

 cambium which arises in 

 a layer of cortical cells 

 beneath the epidermis dur- 

 ing the first year's growth 

 of the main stem or its 

 branches. The epidermis 

 is soon cut off from its 

 water supply by this new 

 growth of a corky bark, 

 and is then sloughed off. 

 The cortex forms a wide 

 zone of storage tissue in 

 the young stem, but is 

 destroyed later by being 

 crushed between the corky 

 bark and the expanding 

 central cylinder of phloem 

 and xylem. A distinctive 

 feature of the cortex in 

 the spruces and pines is the formation of the large resin canals, 

 which contain the resin common in coniferous trees. The central 

 cylinder presents the same general features as that of other trees. 

 The cambium is flanked on either side by the phloem and xylem 

 rings, and the annual wood rings are also sharply marked by 

 the difference in structure of the spring and summer wood. 



FIG. 190. Virgin forest of red spruce (Picea 

 rubra) in the Adirondack Mountains 



Photograph furnished by the United States Divi- 

 sion of Forestry 



