8o THE PLANT SKELETON 



and the thickness and physical character of their walls are all 

 important factors in the quality of the wood; but the plan of 

 distribution of the tracheal and thin-walled parenchyma ele- 

 ments causes the fibers to occur in larger or smaller groups in 

 the different species, thus making the wood as a whole stronger 

 or weaker in consequence. 



(III '' ^j^ ^ I I h' 



't. ,/L, /(.( ;. E 



-n^ Vr ) r'l ''^ / 



m ^ 



Fig. 36. — ^Photomicrograph of cross section of oak wood. E, early growth; L, late growth; 

 OT, larger medullary ray; «, smaller ray. X25. 



In woody plants the wood fibers are, as a rule, relatively 

 much more numerous in the late than in the early growths. 

 This difference stands out sharply in such woods as the ash 

 and the oak (Fig. 36) where the late growth is a dense, hard, 

 and strong cylinder encasing the relatively weak and porous 

 cylinder of the early growth. In some trees, like the yellow 

 poplar of commerce, the wood fibers are relatively few through- 

 out the entire year's growth, .and the wood is relatively light, 

 porous and weak in consequence (Fig. 37). Although the 



