RESISTANCE OE 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 b}^ 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 
(AE). 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, t\\Qj 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. T3 r loses 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. 
