200 Bulletin 194 



because of this, be confused with the non-porous coniferous woods, 

 which they may resemble. 



Tracheids in the wood of broad-leaved trees are subordinate 

 elements. They are much smaller and less uniform in size and shape 

 than in conifers and are more abundant in the vicinity of the tubes or 

 vessels. As in conifers, the tracheids bear numerous bordered pits on 

 their walls. In some cases, as in the wood of the ash, tracheids are 

 absent. 



Strength, hardness and toughness are given to broad-leaved woods 

 principally by the wood fibers. These are slender, spindle-shaped, 

 sharp-pointed cells with thick walls and narrow cavities (d, fig. IX). 

 Usually they are provided with oblique, slit-like, simple pits. The 

 wood fibers for the most part lie vertically side by side and parallel 

 to one another, but in some woods, like the sycamore, they are more 

 or less interwoven. This makes the wood cross-grained, difficult to 

 work and hard to split. 



While the medullary rays of coniferous woods are uniformly small 

 and inconspicuous, they are often broad and prominent in broad-leaved 

 woods. They are largest in the oak, where they may become twenty- 

 five to seventy-live cells wide and several hundred cells high ; between 

 these broad rays are numerous smaller ones, mostly uniseriate and 

 from one to twenty cells high. In the sycamore all the rays are broad, 

 while in the beech only a portion of the rays are broad. Ray cells are 

 usually elongated in the radial or horizontal direction and are then 

 termed procumbent. Not infrequently, however, the marginal cells, 

 as in the willow, are elongated vertically ; cells of this kind are said to 

 be upright. These differences in the character of the medullary rays 

 are important means of identification. 



Another kind of tissue which is more or less prominent in broad- 

 leaved woods is wood parenchyma. Parenchyma consists of vertical 

 groups of short cells, the end ones of each group tapering to a point 

 (e, fig. IX). The cells of parenchyma resemble the cells of the medul- 

 lary rays ; the walls are invariably pitted with rounded, simple pits. 

 The function of the parenchyma is the distribution and storage of the 

 food products manufactured by the leaves. The distribution of paren- 

 chyma on the cross-sections of different species is subject to more or 

 less variation, making this feature of considerable importance in clas- 

 sifying woods. Commonly wood parenchyma is grouped around the 

 pores or vessels. In the basswood it occurs in somewhat broken, 

 tangential lines, forming, with the wood fil)ers, a tier-like arrangement 



