RESISTANCE OF CONIFERS TO CREOSOTE INJECTION. 9 



PENETRATION AS AFFECTED BY WOOD STRUCTURE. 



PENETRATION IN THE RESIN STRUCTURES. 



The resin ducts were first thought to be the most important factor 

 in the penetration of wood by creosote. It was soon found, however, 

 that this was not the case ; many species that had well-developed sys- 

 tems of resin ducts proved much more difficult to impregnate than 

 others that had none. Thus the heartwood of Douglas fir and tama- 

 rack, both of which have highly developed radial and longitudinal 

 ducts, proved extremely resistant to impregnation, while eastern 

 hemlock, white fir, and other species, having no resin ducts, proved 

 fairly easy to penetrate. Close examination showed that the resin 

 ducts of the former received the oil but that the wood surrounding 

 the ducts was not penetrated. (See PL V, fig. 1.) Thus the ques- 

 tion of penetration in the wood fibers was shown to be of fundamental 

 importance, while that in the ducts was of secondary importance. 



On the other hand, when the wood fibers themselves were readily 

 penetrable a system of resin ducts became of great importance. 

 These ducts, forming a network of passages through the wood, caused 

 the oil to be applied at numerous points within the wood, as well as 

 on the surface, and in this manner greatly facilitated penetration of 

 the entire structure. (PI. V, fig. 2.) 



PENETRATION IN RADIAL, TANGENTIAL, AND LONGITUDINAL DIRECTIONS. 



WHEN NO RADIAL RESIN DUCTS WERE PRESENT. 



In this case the penetration longitudinally was between 20 and 120 

 times as great as the penetration radially or tangentially. 



When the oil was applied under pressure to a small area on the 

 surface of a hemlock board 12 inches long, 3 inches wide, and one- 

 half inch thick, it would appear at either end of the board before it 

 would pass through one-half inch of wood radially. Much difference 

 was found in the longitudinal penetration of such species. Eastern 

 hemlock (PL VI) is an excellent example of easily penetrated wood 

 which contains no resin canals, while Alpine fir (PL VII, fig. 1) is 

 an example of a species having similar structure but very resistant 

 to impregnation. 



WHEN RADIAL RESIN DUCTS WERE PRESENT. 



In this case the oil followed the ducts and spread longitudinally in 

 each summerwood band as it was traversed. Also at numerous inter- 

 sections of the radial and longitudinal ducts the oil changed its 

 course and penetrated longitudinally. (PL VII, fig. 2.) In this 

 case the average radial penetration varied from one-fourth to three- 

 fourths of the average longitudinal penetration. 

 41702°— Bull. 101—14 2 



