16 ECONOMIC WOODS OF THE UNITED STATES 



possible to tell small pores from cross sections of tracheids. In 

 outline pores may be round, elliptical, or angular. The first 

 two cases are the rule where the vessel walls are thick enough to 

 resist the pressure of tiie surrounding elements. This is the case, 

 for example, in the small pores of the red and live oaks (Plate II, 

 Fig. 6), while in the white oaks (Frontispiece; Plate II, Fig. 5) 

 the walls are thin and the pores angular in outline. Sometimes 

 pores are disposed in rings or zones in the early wood of the 

 growth ring, producing ring-porous woods (Plate III) ; in other 

 cases they are scattered singly or in groups throughout the 

 ring or arranged in radial or tangential rows, producing diffuse- 

 porous woods (Plate VI). (See Growth Rings.) In any case 

 the largest pores are almost invaria])ly in the first formed wood 

 of the season. The distribution, size, form, and arrangement of 

 the pores are characters of great importance in classifying woods. 



References 



SotEREDER, H.: Anatomy of the Dicotyledons, Vol. II, p. 1136. 



Mell, CD.: History of the Investigations of Vessels in Wood, Proc. Soc. 



Am. Foresters, Vol. VI, No. 1, 1911, pp. 78-91. 

 DeBary, a.: Comparative Anatomy, pp. 155-171; 503. 

 Mayr, H.: Schlich's Manual of Forestry, Vol. V, 1908, pp. 9-10. 

 Sanio, Carl: Bot. Zeitung, Vol. XXI, No. 15, 1863, pp. 121-128. 

 Hartig, R. : Lehrbuch der Anatomie und Physiologie der Pflanzen, Berhn, 



1891, pp. 79-93. 



TRACHEIDS 



Tracheids are elongated, spindle-shaped, fibre-like elements, 

 determinate in length and characterized by bordered pits in 

 their side-walls. 



In the wood of Gymnosperms the tracheid is the dominant 

 element, performing the dual function of conducting water and 

 providing mechanical support for the tree. Bordered pits are 

 mostly confined to the radial walls, except in late wood, and are 

 most abundant near the ends of the tracheids and in one or two 

 rows (Fig. 2, D). Seen in cross section, the tracheids are polygonal 

 in outline, arranged in radial rows, and, near the periphery of 

 growth ring, with very appreciable increase in thickness of the 

 wall, reduction of the lumen, and tangential flattening (Fig. 8; 

 Plate II, Figs. 1, 2, 4). In a few species, particularly Pseudotsuga, 



