RESISTANCE OF HARDWOODS TO CREOSOTE. 17 



various elements of wood structure on the penetration of creosote, it 

 was necessary, so far as possible, to eliminate these other factors. 

 The following conclusions, which are based only on structural con- 

 siderations, should not be applied to commercial practice, such as the 

 grouping of timber for treatment, 1 without considering the other 

 factors that are involved. 



1. The most important of the structural factors affecting the pene- 

 tration of the hardwoods is the condition of the vessels in the wood. 

 When the vessels are open, it is comparatively easy to secure a good 

 penetration. If the vessels are closed by tyloses or gummy substances, 

 they are usually rendered more or less impermeable to creosote. 



2. Next in importance is the ability of the wood prosenchyma 

 (fibers, etc.) to absorb creosote. In some species having numerous 

 and well-developed tyloses in the pores (hickory, for example), it 

 was found possible to obtain a deep penetration on account of the 

 comparative ease with which the wood prosenchyma could be treated. 

 The penetrability of this part of the wood structure is therefore of 

 great importance in species having the vessels closed by tyloses or 

 other material. The wood prosenchyma in the sapwood took treat- 

 ment much more easily than that in the heartwood, probably because 

 the heartwood contained infiltrating substances in the cell walls 

 which tended to make the wood substance less permeable. Woods 

 which were penetrated in both the vessels and wood prosenchyma 

 generally showed the heavier absorption and deeper penetration. 

 Those species which took very irregular absorptions in the cylinder 

 tests were found to possess unusual structural characteristics, such as 

 an irregular distribution of tyloses, interlaced fibers, or cross-grained 

 structure. 



The experiments indicate that even in species of very similar 

 structure the manner of taking treatment may vary widely. This is 

 illustrated in the treatment of hickory and white oak. Both of these 

 woods are ring-porous and the vessels are closed by abundant tyloses. 

 Hickory, however, took a fairly good treatment, whereas white oak 

 was practically impermeable. The variation in the permeability of 

 red heart and white heart beech is also an example of the different 

 manner in which woods of similar structure may take treatment. 



3. Penetration and absorption of the preservative is much less 

 uniform in woods of the diffuse-porous group than in the ring- 

 porous, probably because the tyloses in the vessels, and the gums and 

 infiltrating substances are less uniformly distributed in diffuse-porous 

 woods. In most of the species treated little or no penetration of 

 the preservative occurred in the medullary rays and other paren- 

 chyma cells. 



1 Some of tne other factors affecting the treatment of wood are taken up in Forest 

 Service Bulletin 118, " Prolonging the Life of Crossties." 



11961°— 18— Bull. 606 3 



