LACTUCA SATIVA 



637 



divisions in vertical planes predominate in connection with the 

 differentiation of the vascular elements. This, coupled with 

 axial elongation, results in the development of slender, much- 

 elongated cells. The outermost layer of the plerome forms the 

 pericycle, which can be distinguished early by the larger size of 

 its cells. Centrad to the pericycle, activity can be observed at 

 two opposite poles where the cells of the primary phloem are 



Ix d- 



ph 2 



B 



Fig. 334. Lactiferous ducts : A, longisection through cotyledon showing lactiferous duct 

 on abaxial face of phloem ; B, transection of portion of stele and endodermis of primary root 

 of one-day-old seedling in which primary phloem and lactiferous ducts are formed and pro- 

 toxylem has just begun to differentiate; tangential divisions of endodermis are shown; 

 C, longisection through secondary phloem of old root showing lactiferous ducts with branches 

 and cross-anastomoses, variety New York : en, endodermis ; Ix d, lactiferous duct ; pel, peri- 

 cycle; ^/[> I, primary phloem; /)A 1, secondary phloem ; pAr, protoxylem. 



formed. This occurs slightly in advance of the differentiation of 

 the primary xylem, which is initiated on radii at right angles to 

 the phloem. Abutting the pericycle, annular and spiral pro- 

 toxylem vessels develop, establishing the two points of the diarch 

 xylem strand. The maturation of the primary xylem elements 

 progresses centripetally, and the last ones to be differentiated are 

 the reticulate vessels that occupy the center of the axis and complete 

 the primary strand. (Fig. 335.) Port has noted the occurrence 

 of lactiferous ducts in the pericycle and primary phloem. These 



