18 ECONOMIC WOODS OF THE UNITED STATES 



elements often entirely wanting. They are much smaller and less 

 uniform in size and shape than in conifers, and are of most common 

 occurrence in the immediate vicinity of vessels. Their ends are 

 often curved, especially when they terminate just above or below 

 a ray. The walls are usually comparatively thin and bear numer- 

 ous bordered pits very irregularly distributed. Intermediate 

 forms of tracheids are sometimes found which show distinct 

 transition to the vessels in the detailed structure of their walls 

 and in the occasional presence of perforations at the ends of the 

 cells. 



References 



PENH ALLOW, D. P.: North American Gymnosperms, pp. 33-58. 



DEBARY, A.: Comparative Anatomy, pp. 164-165. 



THOMPSON, W. P. : On the Origin of Ray Tracheids in the Coniferae, Bot. 



Gazette, Vol. L, 1910, pp. 101-116. 

 KNY, L.: Ein Beitrag zur Entwickelungsgeschichte der "Tracheiden," Ber. 



d. deutschen Bot. Gesellschaft, Vol. IV, 1886, pp. 267-276. 

 SANIO, CARL: Botanische Zeitung, Vol. XXI, No. 14, 1863, pp. 113-118. 



WOOD FIBRES 



Typical wood fibres (Fig. 2, A, B) are slender, spindle-shaped, 

 sharp-pointed cells with thick walls and narrow cavities. They 

 are further characterized by usually oblique and slit-like simple 

 pits, or less frequently by small, indistinctly bordered pits. Wood 

 fibres are not found in Gymnosperms, but are nearly always 

 present in the wood of Dicotyledons. 



Wood fibres are of two types, septate and ordinary (non- 

 septate). The septate forms are divided by cross-partitions 

 formed after thickening of the walls has begun. They are of 

 limited occurrence and of relatively small importance. They 

 are characteristic of Swietenia mahagoni and serve as one means 

 of distinguishing the wood from that of certain others closely 

 resembling it. 



The ordinary forms are very common and are the principal 

 source of strength, hardness, and toughness of broadleaf woods. 

 While their function is largely mechanical, it is probable that 

 they, especially those with bordered pits, play some part, as yet 

 undetermined, in water transportation. 



Wood fibres exhibit transitional forms from the typical to 

 tracheids on one hand, and to wood-parenchyma strands on the 



