534 USEFUL PLANTS AND PLANT PRODUCTS 



8. FUEL 



507. Nearly all fuel is of vegetable origin. In most civilized 

 countries to-day the principal fuel supply consists of various 

 kinds of coal, that is, of vegetable matter which has been buried 

 in the earth for ages and undergone many changes (Sec. 330). 



Peat, the consolidated material left after the partial decay of 

 certain bog mosses (Sec. 292), in some countries forms a consid- 

 erable part of the available fuel, and the deposits in the north- 

 ern United States are of some importance. 



Wood, in portions of the country, is still the principal fuel. 

 Certain varieties are preferred for household use on account of 

 their furnishing good beds of glowing coals, or for burning in 

 open fires on account of their freedom from any tendency to 

 snap. But in general the fuel value of thoroughly seasoned 

 wood is nearly proportional to its weight per cubic foot, that is 

 to say, the very heaviest woods, such as hickory, the white oaks, 

 black locust, and some kinds of ash, are worth most for heating. 



Other parts of plants besides wood are used to some extent 

 for fuel. In large tanneries the spent bark is often compressed 

 to extract most of the water and then burned. Corncobs are 

 often burned in stoves and under steam boilers. In treeless 

 regions twisted ropes of straw are used as fuel. 



9. ORNAMENTAL PLANTS 



508. Our ornamental plants may be roughly classed into 

 shade trees, shrubs, herbaceous perennials, and annuals. The 

 total number of species and varieties cultivated in the United 

 States runs far into the thousands, but in many cases florists' 

 varieties are distinguished from one another only by color or 

 some other comparatively unimportant characteristic. . 



Most of our cultivated ornamental plants are of foreign origin, 

 and representatives of almost all parts of the earth except the 

 arctic regions are found among them. In a few instances native 



