220 Pla?it Biology 



The mature region of a root consists of the following primary tissues: 



(1) epidermis, (2) cortex, (3) stele. The epidermis is an outer pro- 

 tective layer of one cell thickness. Just beneath the epidermis is the 

 cortex or differentiated region which surrounds the central stele. The 

 cortex consists of (1) parenchyma (Fig. 18), composed of thin-walled 

 cells which usually measure about the same in each direction and which 

 possess stored food and water in the large vacuoles of their cytoplasm; 



(2) endodermis, or the inner boundary of the cortex, formed by a layer 

 of cells resembling those of the epidermis; (3) mechanical tissues which 

 are present in certain plant roots and absent in others. The stele, or 

 solid, central cylindrical portion of the root, consists of (1) pericycle, 

 (2) primary xylem (woody tissue), (3) primary phloem, and (4) pa- 

 renchyma. The pericycle surrounds the stele and consists of one or more 

 layers of cubical cells just inside the endodermis. The primary xylem 

 extends lengthwise of the stele. The first cells to be differentiated in the 

 primary xylem are the tracheids (Fig. 18) which are elongated cells 

 with pointed ends and walls thickened in certain places. The three 

 most common types of tracheids are the spiral tracheids, in which the 

 cell wall is thickened along a spiral, the remainder of the wall being thin; 

 the annular tracheids, in which the cell wall has a series of thickened 

 rings; pitted tracheids, in which the cell wall is generally thickened with 

 only pits of thin walls. The protoplasm of the tracheids dies and leaves 

 them hollow in order to transport water through the root. As the root 

 develops, the primary xylem also develops spiral vessels, annular vessels, 

 and pitted vessels. These vessels are formed by joining a number of 

 elongated cells end to end. The end walls where the cells join are dis- 

 solved, thus forming a long tubular structure. The side walls of the ves- 

 sels have thickenings similar to the tracheids. The protoplasm of the 

 vessels dies and thus a long hollow tube is formed for conducting plant 

 liquids (Fig. 18). The primary phloem of the mature root is composed 

 of numerous strands of cells located between the numerous primary xylem 

 strands and alternating with them. The primary phloem consists of ( 1 ) 

 sieve tubes (Fig. 18) and (2) companion cells. A sieve tube is formed 

 by uniting a number of elongated cells end to end in a way similar to 

 that in which a xylem vessel is formed, except that the side walls are not 

 especially thickened and the end walls are not dissolved. These end 

 walls develop sievelike pores through which the cytoplasm of adjacent 

 cells is continuous. Closely associated with the sieve tubes of most plants 

 are companion cells (Fig. 18), the length of which is equal to or shorter 



