348 



ESSENTIALS OF BOTANY 



spring and summer growth (Fig. 236). Among the com- 

 monest and most typical of the ring-porous woods are asli 

 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 beds of glowing coals. In general the 

 heating effect of well-dried wood when burned is nearly 



ar- 



\~ar 



Sycamore Birch 



Fig. 236. Diffuse-Porous Woods. (Somewhat magnified.) 

 ar, annual rings. 



proportional to its weight per cubic foot. Tlie 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 o-rowth of for- 

 ests. Forestry as an art deals with the rules of forest 



