30 



ECONOMIC WOODS OF THE UNITED STATES 



(e.g., Tsuga, Abies, Sequoia) where normally absent. The formation 

 of these traumatic resin ducts, as they are called, following wound- 

 ing by chipping of the outer layers of the sapwood of Pinus 

 palustris, is the source of most of our turpentine and other naval 

 stores. Traumatic ducts can be distinguished from normal ones 



Fig. 8. — Cross section through a portion of two growth rings of Pinus ponderosa 

 (western yellow pine); r. d., resin duct; e., epithelial cells; r., ray; e. to., early 

 wood; I. w., late wood; 6. p., bordered pit. Magnified abc*it 200 diameters. 



by their peculiar localization, usually, as seen on cross section, 

 forming one or more compact rows concentric with the growth 

 ring (Fig. 10). 



Gum ducts occur sporadically in the woods of certain indigenous 

 Dicotyledons, viz., Liquidambar, Swietenia and Prunus. 



