32 



THE ANATOMY OF WOODY PLANTS 



s 



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. 

 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 

 even from the comparative stand- 



