51 



sometimes stimulate the formation of abnormal resin ducts in woods 

 in which they do not occur normally. 



In some woods elements occur, known as vessels, which facilitate 

 the transportation of water in the stem. Their presence or absence 

 and their structure and distribution are among the most valuable 

 characteristics in classifying woods. On the basis of porosity one 

 may divide the woods into three classes, viz: (1) Ring-porous or 

 Unequal Pored, (2) Diffuse-porous or Equal Pored, and (3) Non- 

 porous. Chestnut and Oak wood are excellent examples of the ring- 

 porous class (Plate XI, 4, 5). A zone of large pores is found in the 

 early wood and smaller pores in the late wood. Maple and Beech are 

 common examples of the diffuse-porous class (Plate XI, 6). The 

 pores of this class are approximately of the same size and distributed 

 uniformly throughout the growth ring. Pine and Hemlock are com- 

 mon examples of the non-porous class in which pores are entirely 

 absent. (Plate XI, 3). The wood of this class is also classified as 

 Homogeneous, while that with pores is classified as Heterogeneous. 



The various woods possess other characteristics which are valu- 

 able in distinguishing them and in using them in the arts. The 

 wood of the different species varies almost as widely as do their flow- 

 ers, fruits, and leaves, especially with reference to grain, weight, 

 hardness, color, gloss, smell, shrinkage, durability, penetrability, etc. 

 These variable properties and the manifold uses to which the different 

 woods are put are discussed under each species. 



