ECONOMIC WOODS OP THE UNITED STATES 



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other. In structure and arrangement they exhibit variations of 

 considerable taxonomic value. For example, in Ilex the fibres 

 are rather thin-walled and marked with spirals and bordered pits, 

 and closely resembling tracheids except for their greater size. 

 In IAquidambar (Plate VI, Fig. -1) the fibres are mostly square in 

 cross section and in rather definite radial arrangement. In 



a B 



Fig. 2. — Typical Wood Cells. A, Wood fibre with very narrow lumen; B, 

 wood fibre with larger lumen and showing oblique, slit-like simple pits (s. p.) ; 

 C, end of wood fibre showing saw edge; C", end of wood fibre showing forked 

 structure; D, ends of two tracheids from Pinus showing numerous bordered pits 

 (6. p.) ; E, Tracheid from Quercus; F, wood-parenchyma strand, showing indiviuuai 

 cells and simple pits (s. p.) ; G, chambered wood-parenchyma strand from Juglans, 

 showing crystals of calcium oxalate; H, conjugate parenchyma cells; K, portion 

 of a vessel segment showing simple perforation (p) ; L, portion of a vessel segment 

 showing scalariform perforation (Sc. p.). Greatly enlarged. 



Robinia (Plate III, Fig. 3) and Toxylon they are in rather large, 

 compact masses in the late wood, separated by groups or bands of 

 pores and parenchyma. In any wood in which they occur they are 

 most abundant in the median portion of the growth ring, and 

 material decrease in the width of a ring is usually at their expense. 

 The ends of most wood fibres are smooth and uniformly 



