268 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



different woods vary markedly in mechanical and allied properties 

 and, consequently, in usages. Important features are general 

 strength, hardness, stiffness, toughness, fineness of grain, cleavability, 

 density, moisture content, the commonness of defects, and suscepti- 

 bility to insect damage and to decay. Expression of many of these 

 depends upon the age of the tree from which the wood was cut 

 and the treatment it has received since cutting, and all of them 

 naturally vary with the kind of tree involved. It is said that the 

 annual consumption of wood in the world amounts to some sixty 

 thousand million cubic feet, of which nearly half is used in North 

 America. 



The two main types of timber are softwood, obtained from 

 coniferous trees such as Spruces, Pines, and Larches, and hardwood, 

 from Angiospermous trees such as Oaks, Maples, and Mahoganies. 

 Besides its use as a popular fuel and employment as a raw material 

 for conversion into diverse products whose origin is often unrecogniz- 

 able, wood is used, as such, in the form of structural timber for 

 buildings and bridges, as boarding and flooring, girders and rafters, 

 pit-props and railway ' sleepers ', poles and posts, piling and cooper- 

 age, veneers and plywood, shingles and woodenware, parts or the 

 whole of ships and boats, furniture and vehicles, boxes and crates, 

 fences and hoardings, and for innumerable other purposes of which 

 some have already been mentioned. 



Among specific sheltering materials other than wood, the leaves 

 of such woody plants as Palms, and herbaceous materials such as 

 straw, are widely used for shelters and thatches in different parts 

 of the world. Also extensively employed are various soft or com- 

 minuted plant products for insulating and packaging, familiar 

 examples being Bog-mosses and hay for insulation, and sawdust and 

 shavings for packaging. 



Industrial Uses and Extractives 



Many of the items discussed under other captions, such as fuels, 

 fats, and fibres, might be included here, but the chief classes to be 

 considered in this section are those whose chemurgical treatment or 

 processing makes them important sources of industrial derivatives 

 — namely, sugars, starches, cellulose, and some distillation products. 

 To take this last item first, the destructive distillation of wood- 

 waste produces such valuable materials as charcoal, tar, oil, turpen- 

 tine, wood alcohol, acetic acid, and wood gas. 



