198 Bulletin 194 



does not continue to fill the whole cavity within the cell-walls, but 

 forms only a thin lining around the walls. Finally it disappears 

 altogether, leaving the dead cell-walls intact, making the substance 

 called wood. The cells composing wood are not all the same shape 

 or size, but vary greatly according to the function which they serve in 

 the life of the tree. Because of the great variety of the cells com- 

 posing it, wood takes on various characters of structure, graining and 

 coloring, which are distinctive for each kind of wood and which greatly 

 aid in distinguishing the various kinds from one another. 



The wood of conifers is very regular and simple, consisting mainly 

 of cells of one sort. It is this uniformity of structure which makes 

 the wood of conifers valuable for so many purposes. These cells are 

 called tracheids (a, fig. IX). They are vertical, elongated, tapering 

 tubes, one-twentieth to one-fifth of an inch in length, with closed ends 

 and a relatively large cavity. The pointed ends of the tracheids over- 

 lap one another in a dovetailed fashion, breaking the joints and making 

 the wood much stronger than it would be otherwise. They are further 

 identified by the presence of pits or pores in their walls which serve as 

 waterways between the tubes. These pits (b, fig. IX) appear as if 

 made up of two circles, and they are for this reason called bordered 

 pits. The other cells making up the wood of conifers are the ray cells. 

 These extend horizontally, and in a radial-section appear as tiers of 

 regular, brick-shaped cells. The cells communicate vv^ith one another 

 by numerous pits in the walls. In certain genera the medullary rays 

 are bordered by tracheids similar to the ordinary wood tracheids, but 

 designated as ray tracheids. In one of the pines the ray tracheids have 

 teeth-like projections from the walls, which are useful in identifying 

 this species. Scattered here and there on the cross-section of some 

 coniferous woods are to be seen irregular grayish or brownish dots. 

 These are resin ducts and are especially well-represented in the pines. 

 Resin ducts are openings between cells containing resin, which is 

 manufactured by certain trees. If you cut a limb from a pine tree, 

 the cut surface will very quickly become sticky with the resin which 

 oozes from these ducts. On a warm day in the pine woods the resin is 

 quite apparent by its fragrant odor. Some woods have present also 

 certain cells or groups of cells which produce resin, which gives them 

 a distinctive appearance, and therefore they are called resin cells. 

 Their presence or absence aids in the determination of certain woods. 



The wood of broad-leaved trees is much more complex in structure 

 than the wood of conifers. Instead of being made up essentially of 



