WOOD IN GENERAL 247 



responds to the bony skeleton of animals in contrast with 

 their tendons and hairs to which we may liken internal and 

 external vegetable fibers respectively. 



A definition of wood in the economic sense requires that 

 it be distinguished principally from fiber, because of the 

 especially close similarity between them. Fibers, we have 

 seen, are sometimes woody, while all true woods, as will 

 presently appear, are fibrous. Cellulose is the main con- 

 stituent of each. Woods and Avoody fibers contain in addi- 

 tion to cellulose more or less of a substance (or mixture of 

 substances) known as lignin. This is of uncertain chemical 

 composition though known to consist of the same elements 

 as cellulose. Like that substance it permits water and gases 

 to pass readily through it. It is distinguished from cellulose 

 by turning yellow instead of blue when treated with sul- 

 phuric acid and iodine. It is the fact that wood is used in 

 comparatively large, firm masses which chiefly distinguishes 

 it from fibers; while it is the fibrousness of wood that most 

 readily distinguishes it from cork and other massive materials 

 to be presently studied. Let us then for our present purpose 

 define wood as the comparatively hard mass of fibrous ma- 

 terial which serves mainly for mechanical support in plants 

 and in various artificial structures. 



From earliest times wood has been the most widely useful 

 material of construction. Our civilization has been developed 

 largely upon its possil)ilities. In prehistoric times wherever 

 it was abundant, wood was used almost exclusively for build- 

 ings, utensils, and implements; though in regions less favor- 

 ably situated various substitutes of course had to be found. 

 Even before skill in metal-working had been acquired men 

 were able to shape wood by means of their rude stone tools 

 into many highly useful forms. Thus, only the rudest means 

 are necessary for making from a single log a "dugout" canoe 

 capable of holding many men: a fire kept alive along the top 

 of a fallen trunk burns or chars the wood so that it may be 

 scraped away till the desired form is reached. With the 

 coming of metal tools and their improvement from time to 

 time, more extensive use could be made not only of wood, 

 but also, and for the same reason, of stone and other hard 



