122 PHARMACEUTICAL BOTANY 
Fibers, or Wood Fibers; when they appear in the phloem region, 
Phloem Fibers, Bast Fibers or Liber Fibers; when in the pericycle, 
Pericyclic Fibers; when in the cortex, Cortical Fibers. Wood fibers 
are usually cut off by the cambium only and so are generally 
Ti 
, Te 
Fic. 63.—Tangential-longitudinal section of wood of the Black Ash. 170. 
wf, wood fibers; mr, medullary rays. Note how the-tapered ends of the wood fibers 
are spliced over each other. 
seen in the secondary xylem. They differ from the fibers of 
the phloem, pericycle and cortex in that they possess bordered 
pits although in many instances these are so reduced by the 
thickening of the wall as to be simple. Phloem, pericyclic and 
cortical fibers possess simple pits. 
EPIDERMIS 
EpierMis is the outer covering tissue of a plant and is pro- 
tective in function. It provides against mechanical injury and 
