232 



General Biology 



Fig. 130. Types of Wood and Bast Fibers. 



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 

 throughout life with but lit- 

 tle change, 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 



the 



A, cross section of bast fibers from stem of Aristolochia i • 



Siphu showing stratification.^ B, Portion of bast fiber, WfllCll Originates 111 



showing oblique striation. C, Portion of bast fiber show- r><=»riK1p»m -ren-r-ir^n 



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



Some of the primary 



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.) 



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 

 cambium), 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 fibro-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. 1 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-, 

 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.) 



^ylem and phloem both carry water, but the former carries food material as such, while the 

 latter carries food in the water. 



