2-4 



THE STRUCTURE OF ECONOMIC PLANTS 



clearly understood, it seems likely that it may be accomplished by a 

 splitting or modification of the middle lamella (intercellular sub- 

 stance) which permits the ends of the separated elements to slip 

 past one another. 



Wood Fibers. — The wood or xylem fiber is an elongated sclerotic 

 element with tapered ends, which gives mechanical strength and 





. .. Jl'JiJ 



Fig. 8. Tracheae, tracheids, wood-fibers, and xylem parenchyma of a dicotyledon with 

 transition forms between the various elements. Diagrammatic. (From Strasburger, Textbook 

 of Botany, Fifth English Ed.) 



rigidity to the axis. They vary greatly in form, size, and the 

 chemical properties of the wall; but are usually angular in transec- 

 tion, with thick, heavily lignified walls in which there are bordered 

 pits that may become occluded as the wall thickens. (Fig. 8.) 

 In the development of the wood fiber, the thin-walled, paren- 

 chymatous fiber-element grows rapidly, attaining full size before 

 lignification and consequent hardening of the wall occurs. The 

 continued deposition of wall substance results in a reduction in the 

 size of the lumen of the cell, and the protoplast finally dies, leaving 



