222 PRINCIPLES OF AMERICAN FORESTRY. 



the sap from which is obtained turpentine, resin, gums, 

 and rubber, sugar, liquors, medicines; the pith, which 

 gives us food and fibre for clothing and other purposes; 

 the fruit and leaves, which are also used for food, medic- 

 inal and chemical extracts. 



Wood is Made Up of Cells which vary in form and in 

 arrangement in different species. The cells are arranged 

 in the form of irregular, concentric cones, so that a cross 

 section of a tree shows a series of concentric rings. For 

 ordinary purposes, however, a log may be considered as 

 being made up of a series of concentric cylinders, each 

 cylinder representing one year's growth. If each ring 

 is examined closely, it will be noticed that it is made up 

 of an inner, softer, lighter-colored portion, and an outer 

 portion that is firmer and darker colored. The inner 

 portion was formed in the early part of the season, when 

 growth was rapid; this portion is termed the spring wood. 

 The outer portion, where the cells are packed firmly to- 

 gether, grew in. the summer, when the growth was slow, 

 and this is termed the summer wood. Since the latter 

 portion is very heavy and firm, it to a large extent 

 determines the weight and strength of the wood. 



Wood a Structure. On account of the peculiar arrange- 

 ment in its structure, wood should not be regarded as a 

 homogeneous mass, but rather as a mechanical structure, 

 the arrangements of the units of which in each case should 

 be carefully considered in estimating its strength and 

 its value for various purposes. 



Methods of Sawing. In sawing wood the relation of 

 the saw cut to the annual rings is an important matter, 

 and its reference in relation to them has given rise to the 

 names cross-cut, tangential, and quarter-sawing. 



Cross-cut Sawing simply refers to cutting across the 

 grain. This method of sawing is seldom adopted other 

 than for purposes of division, but occasionally it is used 



