230 



GENERAL BIOLOGY 



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

 zone is the endodermis ( ). 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 



growth in a. dicotyledonous stem fibrous elements that give strength to the 



plant. Such fibrous elements are known as 

 prosenchyma ( ). The con- 



Fig. 129. 



Diagram to illustrate secondary 



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; f c , cambium of ducting elements are developed in the midst 



wedges of wood ; ic, cambium be- 



cells ; fh, wood of the original 



wedges ; ifh, wood formed between 



wedges ; x, earliest wood formed ; 

 (c). (After Land.) 



tween wedges; b, groups of bast of theSC Hgnified Cells. 



Each group of Hgnified 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 asso- 

 ciated companion cells, parenchyma cells, and Hgnified tissue, constitutes 

 the phloem ( ). These bundles are also often ar- 



ranged in a very definite order. 



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

 the Hgnified cells of phloem are called bast fibers. 



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

 ducting) elements to form fibre-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 



