OF WOOD IN GENERAL. 



tangential longitudinal sections ; but its walls are not elongated 

 radially. 



As has been said, the wood of broad-leaved trees may contain 

 from three to five of these different elements. Vessels are always 

 present, but in some cases tracheids are a,bsent. The wood of 

 Plane, Ash, and Citrus (Orange, Lemon, etc.), for example, con- 

 sists of vessels, woody fibre, thin-walled fibrous cells and wood- 



Pr. 



Pr. 



WP 



FIG. 25. A piece of dicotyledonous wood, magnified about 100 times. A 

 transverse section is shown above, with a pith-ray (Pr.) crossing the zone of 

 autumn wood (a) which forms the outer boundary of an annual ring. In 

 front is a radial longitudinal section showing wood-parenchyma ( \VP), some 

 large tracheae (T), and much wood-fibre, crossed by another pith-ray. The 

 tangential section is in shadow. 



parenchyma only. That of Holly, Hawthorn, and Pyrus (Apple, 

 Pear, Rowan, etc.) is made up of vessels, tracheids, and wood- 

 parenchyma : that of Maples, Elder, Ivy, Euonymus, etc., contains 

 also thick-walled fibrous cells. The wood of Berberis (Barberry) 

 consists exclusively of vessels, tracheids, and thin and thick- 

 walled fibrous cells ; and that of Oaks, Hornbeams, Plum, and 

 Buckthorn of vessels, tracheids, woody fibre, and wood-paren- 

 chyma (Figs. 25 and 26). The most common type of structure, 

 however, occurring in Willows, Poplars, Alder, Birch, Walnut, 



