Z94 THE STRUCTURE OF ECONOMIC PLANTS 



where subsequent periclinal divisions of the cells of the calyptra 

 may have occurred. (Fig. 146.) 



The cortex originates from the periblem and is composed of 

 large parenchymatous cells whose radial dimensions at first much 

 exceed the axial ones. They decrease in size toward the stele and 

 are limited inwardly by the endodermal layer. This is formed by a 



Fig. 146. Median longisection of root apex showing histogens at growing point : cal- 

 dgn, calyptrogen-dermatogen ; co, cortex ; en, endodermis ; ep, epidermis ; pbm, periblem ; 

 />c/, pericycle; />/, plerome; rr, root cap; j//, stele. (After Grassley.) 



final periclinal division of the innermost row of cortical cells which 

 takes place at about the level where the epidermis is no longer 

 enclosed by the root cap. The plerome produces the cells of the 

 stele which are at first slightly and later greatly elongated. The 

 median cell of the plerome gives rise to the metaxylem elements; 

 while the protoxylem, phloem, fundamental parenchyma, and 

 pericyclic cells are derived from the lateral ones. 



As the maturation of the cells derived from the three histogens 



