RESISTANCE OF HARDWOODS TO CREOSOTE. 3 



wood. These rings are more or less conspicuous in the hardwoods of 

 temperate regions, especially in those of the ring-porous group. In 

 general, the springwood is more open and has fewer thick- walled 

 cells than the summerwood. 



Medullary rays.— The medullary rays (silver grain) which extend 

 like the spokes in a wheel from the bark toward the pith at the cen- 

 ter of the tree, are very conspicuous in some hardwoods, such as oak, 

 beech, and sycamore. 



MICROSCOPIC STRUCTURE. 



The microscopic structure of the hardwoods is illustrated hy Plates 

 I, II, III, and IV. 



Plate I is a photograph, taken through the microscope, of a section 

 of maple. This wood belongs to the diffuse-porous group of hard- 

 woods, which is so called because the pores or vessels (V) are scat- 

 tered with considerable uniformity throughout the annual ring 

 (AR). In this group the pores in the springwood (SP) are gener- 

 ally but little larger than those in the summerwood (SW). 



Plate II shows red oak, an example of ring-porous wood. In the 

 springwood of this species there is a noticeable group of large pores. 

 When seen on the cross-section of a log these make conspicuous con- 

 centric rings around the pith. In this wood the pores are not dis- 

 tributed uniformly through the annual ring and there is a very con- 

 siderable difference in the sizes of those in the springwood and the 

 summerwood. The springwood pores in a ring-porous wood are 

 usually considerably larger than the corresponding pores in a diffuse- 

 porous wood. 



Although these two species are typical of the two groups, yet in 

 each group it is possible to find variations from the types of structure 

 shown. Some diffuse-porous woods, for example, are more porous 

 than maple ; that is, they have more pores per square inch in propor- 

 tion to the number of fibers and the like than others do. Some of the 

 ring-porous woods have the large springwood vessels blocked with 

 cell-like growths called tyloses 1 (T, Plate IV). These are visible 

 to the naked eye as glistening fragments in the pores of such woods 

 as white oak or hickory. Tyloses are also present in the diffuse-por- 

 ous woods, although somewhat less abundantly and uniformly de- 

 veloped. They occur in the vessels which are not active in the trans- 

 fer of sap; therefore, they are most numerous and completely de- 

 veloped in the older sapwood and the heartwood. 



Medullary rays. — The medullary rays (ME) are generally rather 

 conspicuous in the hardwoods, although in woods like aspen and 



1 " Tyloses : Their occurrence and practical significance in some American woods," 

 Journal of Agricultural Research, Vol. I, March, 1914. 



