STRUCTURE OF THE ROOT 



73 



The most prominent tissues of the stelar column are certain well- 

 defined strands of xylem and phloem. In small roots like those of the 

 Cress or Onion there may be only two of each of these ; and the strands 

 may meet at the centre, there being no pith at all. In larger roots of 

 Dicotyledons there are usually more of them, four or five being common 

 numbers (Fig. 56). But the number is not constant in roots of the 

 same species, or even of the same individual. In Monocotyledons 

 the number is usually larger still, and it may run to a very high figure 



FIG. 55- 



Transverse section of the stele of the root of Acorus a Monocotyledon. cort = 

 cortex. n4 = endodermis. />r=pericycle. phl = ph\oem. #r* = protoxyleiu. 

 />.. = pitted vessel. ( x 150.) 



in roots of Palms, -or Screw Pines. In such roots a pith is present, 

 and it may be of considerable bulk. A typical structure of the stele 

 for a simple Monocotyledon is seen in the root of the Sweet Rush, 

 Acorus (Fig. 55). The cortex and endodermis surround the stele 

 itself, of which the superficial layer is the thin-walled pericycle, here 

 a very regular row of cells. There are seven groups of xylem, and 

 seven of phloem alternating with them. Each group of xylem is 

 composed of smaller vessels of the protoxylem (prxy), which are 

 directed to the periphery, while successively larger, pitted vessels 

 (p.v.} constitute the later-formed metaxylem. The alternating groups 

 of phloem are not strongly developed, and the whole is compacted 



