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 t}doses 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 the 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 
