848 ESSENTIALS OF BOTANY 
spring and summer growth (Fig. 236). Among the com- 
monest and most typical of the ring-porous woods are ash 
and oak, and of the diffuse-porous ones, birch and maple. 
419. Wood as Fuel. — Although coal (Sect. 375) is the 
fuel of the world’s great industries, yet there are large 
areas throughout which wood is still the principal fuel. 
All kinds of wood can be burned, but for certain purposes 
those kinds are preferred which make an abundant flame 
or which leave solid heds of glowing coals. In general the 
heating effect of well-dried wood when burned is nearly 
EEG 
180/816), ]°] -fola} fo} 
2} 2) (212 
Sycamore Birch 
Fic. 236. Diffuse-Porous Woods. (Somewhat magnified.) 
ar, annual rings. 
proportional to its weight per cubic foot. The fuel value 
per cord is therefore dependent on the weight per cord, 
and the heaviest woods, such as hickory, most of the oaks, 
hackberry, and some kinds of ash, are the best for burning. 
For certain purposes, where a concentrated smokeless fuel 
which lights easily and does not readily go out is required, 
charcoal is employed. Generally the heaviest woods make 
a dense charcoal of great heating power. 
420. Forestry. — Forestry as a science deals with the 
factors which concern the occurrence and growth of for- 
ests. Forestry as an art deals with the rules of forest 
