314 A TEXT-BOOK OF BOTANY. 



from there to the outside of the peripheral vessels of the xylem 

 (Fig. 174) ; thus a continuous cambial zone gradually arises. 

 From this zone secondary tracheae or vessels become developed on 

 the inner face of the primary phloem, while secondary phloem 

 becomes differentiated outside the primary rays of xylem ; or 

 only parenchyma develops outside the primary xylem, resulting in 



FIG. 174. Section in the older part, higher up on the root of pea (Pisum), showing in 

 addition to what has been observed in Fig. 1 73, the beginning of the change from primary to 

 secondary structure: CA, the development of a cambium; SX, secondary hadrome (or 

 vessels), and SP, secondary leptome (or sieve). 



the formation of secondary PARENCHYMA-RAYS (or medullary 

 rays). In other words, the original radial structure of the stele 

 changes to the collateral type (Fig. 175). Owing to this increase 

 within the stele, the peripheral tissues from the endodermis to the 

 epidermis naturally become broken and are subsequently thrown 

 off, but are replaced by the pericambial cork and secondary cor- 

 tex derived from the pericambium. The older roots, then, of 

 Gymnosperms and Dicotyledons thus resemble the structure of 

 stems, except that no pith exists in these roots, at least not usually. 



