458 THE STRUCTURE OF ECONOMIC PLANTS 



ogen layer. The plerome produces the stele; the periblem, the 

 cortical tissues; and the calyptrogen-dermatogen layer, the root 

 cap and epidermis. 



The cortex of the young root consists of several layers of paren- 

 chymatous cells limited outwardly by the epidermis which pro- 

 duces a large number of root hairs. The endodermis is not defined 

 very early in ontogeny by the development of Casparian strips, 

 but its position may be readily determined with reference to the 

 pericycle which lies immediately centrad to it. The pericyclic 

 cells become enlarged and some of them undergo a series of oblique 



xrf pri 



Fig. 134. Left, transection of young primary root showing stele; right, development of 

 lateral root from pericyclic tissue : co, cortex ; en, endodermis ; ep, epidermis ; mx, meta- 

 xylem; d, oil duct; pel, pericycle ; ph, phloem; /ia-, protoxylem ; >-r/?r/, root primordium. 



longitudinal divisions which result in the formation of the charac- 

 teristic primary oil ducts found in this family. These lie in two 

 arcs, each of which contains about nine ducts. The central one, 

 which lies directly outside the protoxylem point, is quadrangular 

 in transection while those lateral to it are usually triangular. 

 (Fig. 134.) 



Trecul (xy) and van Tieghem (i6) have described the develop- 

 ment of these ducts in detail, and de Bary (3) has summarized their 

 work. The first two oil ducts to be cut off in the pericycle are 

 quadrangular in transection and located 180° apart. The two 

 protoxylem points of the diarch xylem plate which later differen- 

 tiate in the plerome lie immediately centrad to them. The forma- 

 tion of the two centrally located primary oil ducts is schizogenous, 



