108 THE ELEMENTS OF VEGETABLE HISTOLOGY 



idea of the diameter of the fiber and the relative size 

 of the lumen. In sectional view, fibers are rarely 

 circular in outline, as one or more sides are flattened 

 because of the pressure of surrounding tissues. Vari- 

 ous types of fibers are illustrated' in Plates 40 

 and 41. 



Classification. In describing fibers, the following 

 points should be noted : 



1. Presence or absence of pores; 



porous and non-porous types. 



2. Presence or absence of striations; 



striated and non-striated types. 



3. Branching; 



branched and non-branched types. 



4. Presence or absence of crystals; 



crystal-bearing and non-crystal-bearing types. 



Wood and Bast Fibers. In tracing the origin of 

 plant tissues we learned that two types of conducting 

 tissues were developed in the plerom region, and that 

 each of these was associated with a fibrous or support- 

 ing tissue. One of these supporting tissues was termed 

 xylem fiber or wood fiber and the other phloem fiber 

 or bast fiber. There are slight histological differences 

 between wood fibers and bast fibers. Wood fibers 

 are usually thicker-walled and shorter than bast fibers. 

 Pores, when present hi wood fibers, are usually oblique 

 and few in number, whereas in bast fibers the pores 

 are horizontal to the long axis of the cell and rather 

 numerous. As wood fibers are more brittle than bast 

 fibers they are more apt to be broken in grinding, and 

 while complete bast fiber cells may easily be found in 

 powdered materials it is seldom that wood fiber 

 cells remain intact. 



Schlerenchymatic Tissue. Schlerenchymatic tis- 



