Plant World Continued 



231 



128.) The layer bordering on the primary epidermis is known as the 

 hypodermis ( )• This layer is made up of thick- 



walled cells which are usually angled. The layer joining the plerome 



zone is the endodermis ( 



Fig. 129. 



). The cells in this layer 

 are also thick-walled and resemble those. of 

 the hypodermal layer. Between hypoder- 

 mal and endodermal layers there are several 

 layers of cells which constitute the primary 

 cortex or cortical parenchyma ( ). 



The cortical parenchyma is made up 

 largely of undifferentiated original periblem 

 tissue. 



It is in the plerom zone (Fig. 129) 

 where the most striking changes in the cell 

 walls take place. Groups of cells have 

 their walls thickened by the deposition of 

 lignin ( ), which forms the 



fibrous elements that give strength to the 

 plant. Such fibrous elements are known as 

 prosenchyma ( ). The 



Diagram to illustrate secondary 

 growth in a dicotyledonous stem 

 which takes place in the plerome 

 zone. 



R, the first-formed bark; p, 

 mass of sieve cells; ifp, mass of 

 sieve cells between the original 



wedges of wood; fc, cambium of conducting elements are developed in the 



wedges of wood; ic, cambium be- m . . n 



tween wedges; b, groups of bast midst of these llgnified Cells. 



cells; fh, wood of the original 



wedges; ifh, wood formed between 



wedges; x, earliest wood formed; 



(c). (After Land.) 



Each group of lignified cells, together 

 with its associated ducts, constitutes the 

 xylem ( ). This 



is usually arranged in a very definite order in the plerom region. There 

 are other cells forming tubes, also in the plerom zone. The end walls 

 of these cells are perforated. These form the sieve tubes. Each group 

 of sieve tubes with its associated companion cells, parenchyma cells, 

 and lignified tissues, constitutes the phloem ( ). 



These bundles are also often arranged in a very definite order. 



The lignified cells of xylem are called wood fibers (Fig. 130), and 

 the lignified cells of phloem are called bast fibers. 



Xylem and phloem are made up of both fibrous and vascular (con- 

 ducting) elements to form fibro-vascular bundles. 



The xylem and phloem are located in a circle near the outer boundary 

 of the plerom region, and as they begin to develop, usually alternate- 

 with one another. 



As there are narrow strips of unchanged plerom parenchyma ex- 

 tending between the fibro-vascular bundles (Fig. 131), these strips 

 present the appearance of rays, and consequently are known as 



