638 



THE STRUCTURE OF ECONOMIC PLANTS 



are articulate and develop from a linear series of cells through the 

 dissolution of the end walls. They are located in the arcs of the 

 pericycle which lie outside the two groups of primary phloem, and 

 additional ducts may also be differentiated on the centrad surface 

 of the phloem. There are no ducts in the pericycle outside the 

 protoxylem. (Fig. 334, B.) 



The cortical cells are derived from the periblem. The endo- 

 dermal layer adjacent to the pericycle consists of regularly arranged 



Fig. 335. Transection of portion of primary root at initiation of cambial activity showing 

 lactiferous ducts in pericycle and tangential division of an endodermal cell, variety New York 

 Regular : ca, cambium ; co, cortex ; en, endodermis ; ep, epidermis ; Ix d, lactiferous duct ; 

 mx, metaxylem; pel, pericycle; ph i, primary phloem; px, protoxylem. 



cells, and the differentiation of Casparian strips occurs at about the 

 time that secondary thickening of the root is initiated. A unique 

 development in the ontogeny of the endodermis is the tangential 

 division of single cells or sectors of cells. This happens first in 

 arcs lying outside the primary phloem, and finally the entire 

 endodermis may become biseriate. Phillips (xi) has described a 

 somewhat similar situation for Cynara, also a member of the 

 tribe Cichorieae, in which doubling of the endodermis occurs 

 very early in ontogeny, taking place prior to the differentiation 

 of the stelar elements. The remainder of the cortex consists of 



