PLANT WORLD CONTINUED 



231 



Fig. 130. Types of Wood and Bast Fibers. 

 A, cross section of bast fibers from stem of Aristolochia 

 Sip/io showing stratification. B, Portion of bast fiber, 

 showing oblique striation. C, Portion of bast fiber show- 

 ing transverse striation. D, Bast fiber from the bark of 

 Cinchona Calisaya, showing longitudinal striae and small 

 tubes connecting the lumen of the cell with the exterior. 

 (From Bastin's "College Botany." Courtesy of G. P. 

 Engelhard & Co.) 



medullary rays, while the 

 unchanged parenchyma in 

 the center of the plerom is 

 the pith. 



In many orders of 

 plants it is these primary 

 tissues which remain with 

 but little change, through- 

 out life, but in the higher 

 orders these primary tissues 

 change to secondary or 

 permanent tissues. (Fig. 

 132.) 



The epidermis is re- 

 placed by a bark structure 

 which originates in the 

 periblem region. 



Some of the primary cortical 

 cells become meristematic, thus 

 constituting the cork cambium or 

 phellogen ( ) ; these 



cells subdivide rapidly to form a 

 new tissue on their outer surface, 

 the cork, and on the inner surface, 

 phelloderm. 



Bark is everything outside of the 

 true cambium (not the cork cam- 

 bium), excluding the cambium and 

 epidermis. 



The phellogen retains its meris- 

 tematic power throughout the entire 

 life of the plant so that new pro- 

 tective tissues can keep pace with 

 the internal growth. 



The primary nbro-vascular bundles consist of xylem and phloem, 

 but in the change to secondary structures, a meristematic tissue called 

 cambium ( ) develops in connection with these. 



The cambium develops on the outer face of the xylem (Fig. 133), 

 and on the inner face of the phloem, so that the cambium arc on each 

 xylem bundle produces xylem on its inner face and phloem on its outer 

 side. Similarly, the cambium arc on the phloem bundle develops xylem 

 on the inner side and phloem upon the outer. 



Fig. 131. Medullary Rays and Pith. 



A, Pinus Virginiana, cross section of 

 two-year-old branch. P, pith ; x, wood, show- 

 ing two annual rings ; cam, cambium ; ph, 

 phloem ; r, resin-ducts in the cortex. B, Pinus 

 insignis, cross-section of the inner part of the 

 wood. P, pith ; t 1 , primary tracheae ; t a , 

 secondary tracheids ; r, resin-ducts ; m, medul- 

 lary ray. (From D. H. Campbell's "A Uni- 

 versity Text-book of Botany," by permission 

 of The MacMillan Co., Publishers). 



