50 



they become functionless and die. The sapwood comprises the 

 peripheral zone of wood which lies next to the cambium and contains 

 the only living elements of the wood. The heartwood comprises all 

 the wood inside of this zone. The elements of the latter are dead 

 and usually dark in color. The line of demarcation between the 

 two regions is usually sharp. The width of the sapwood is variable. 

 In some species like Sassafras it is very narrow, while in other 

 species like Hickory it is wide. The depth of color of the heartwood 

 is also variable. In some species like Persimmon it is very dark 

 in color while in other species like Hemlock there is very little dif- 

 ference in color between the heartwood and sapwood. 



The cross-section also shows that the wood is divided into numer- 

 ous concentric zones or rings. These are known as annual rings 

 since each one usually represents the growth of a season (Plate XI, 

 1, 3). Certain disturbances like frost, drought, and insect damage 

 may cause the formation of a second ring in the same season. These 

 rings are known as false or fictitious growth rings. Growth rings 

 have a physiological origin. They represent alternating periods of 

 rest and activity, and occur in practically all trees of the temperate 

 region, characterized by an active vegetative period in summer and a 

 resting period in winter. As one approaches the equator the growth 

 rings disappear, since the seasonal changes are not so sharp. Each 

 growth ring may be divided into two parts, the inner, called early 

 or spring wood, and the outer, called late or summer wood (Plate 

 XI, 3). 



The cross-section further shows radial lines crossing the growth 

 rings at right angles. These are known as medullary or pith rays, 

 or simply as rays. A few of them originate in the pith and extend 

 through the wood into the bark. Such are known as primary rays. 

 As the stem increases in size additional rays are necessary. These 

 originate in the wood, extend into the bark and are known as sec- 

 ondary rays. The rays are very valuable in distinguishing the wood 

 of many of our common trees since the different woods possess rays 

 which vary in height, width, and structure. The very wide rays of 

 the Oaks enable one to distinguish their wood from that of all other 

 species. These large rays are a valuable asset to Oak wood since 

 they give rise to the beautiful figure which one finds on some oak 

 furniture and interior finishings. The best figure is obtained by 

 quarter-sawing i. e. cutting it radially. 



The end of a freshly cut log of pine is often covered with small 

 drops of resin, which were given forth from small openings in the 

 wood. These openings are known as resin ducts (Plate XI, 1, 2). 

 They are long intercellular channels bounded by a layer of epithelial 

 cells. Their presence in the wood. of the Pines, Larches, and Spruces 

 enables one to distinguish them from all other trees. Injury may 



