THE ANATOMY OF WOODY PLANTS 





FIG. 25. Types 

 of fibers. Explana- 

 tion in the text. 



of Ephedra, a low representative of 

 the Gnetales. The pitting is on all 

 walls, and spiral bands are present 

 on the inner surface. In b is shown 

 a fiber-tracheid from a species of 

 Magnolia. Here again the pitting is 

 not confined to the radial aspects of 

 the element, but is also tangential. 



I The apertures of the pits are much 

 elongated and extend beyond the 

 nearly circular outline of the pit 

 membrane. In c appears a type of 

 mechanical element characteristic of 

 the higher dicotyledons. In this form 

 of fiber the pit membrane is exceed- 

 ingly narrow and the mouth extremely 

 elongated. As a result of this situa- 

 tion the pores of the fiber appear 

 to be practically without a border. 

 Mechanical elements of this type are 

 usually known as libriform fibers. 

 It has been considered by Strasburger 

 and others that the libriform mechani- 

 cal element is of a different morpho- 

 logical nature from the fiber-tracheid 

 and the tracheid. The distinguished 

 German morphologist was of the 

 opinion that elements of this type 

 were derived from the fusion of stor- 

 age parenchyma cells. This view, 

 however, as will be pointed out more 

 appropriately later, does not har- 

 monize with the general evolutionary 

 sequence in the development of 

 structures in the wood and, more- 

 over, meets with serious difficulties 



C even from the comparative stand- 



