5M THE STRUCTURE OF ECONOMIC PLANTS 



—CO 



--phi 



quently be triarch or pentarch. (Fig. 177.) In all cases, the 

 development of the primary xylem is centripetal and the arrange- 

 ment of the primary phloem radial. 



Secondary Thickening of the Root. — At approximately the 

 time that the primary xylem is mature, cambial act vity is initiated 

 in the parenchymatous zone between the xylem and the phloem, 

 the first divisions occurring just outside the metaxylem portion 

 of the strand. (Fig. 178.) From these loci there is a lateral 

 extension of the cambial activity until it involves the pericyclic 



cells that subtend the protoxylem 

 points. In this manner, a com- 

 plete cambial cylinder is formed 

 which produces secondary xylem 

 elements centripetally and second- 

 ary phloem centrifugally. The 

 cambium is relatively inactive and 

 none of the roots of the primary 

 or adventitious root systems attain 

 large size. 



The secondary xylem consists 

 of vessels of two sizes, fibers, 

 and parenchyma, while typical 



The large 



initiation of secondary thickening: ca, j^ ^^^ arranged in approxi- 



cambium; co, cortex; ^«, endodermis ; mx, o rr 



metaxylem; pel, pericycle; ph i, primary mate radial rOWS, SUrroUnded by 



piiioem; px, protoxylem; xy x, secondary parenchyma and vessels of smaller 



xylem. 



caliber. The tracheae are pitted, 

 and the larger ones frequently become filled with tyloses from 

 the adjacent parenchymatous cells. The few elongated fibers 

 occur as scattered elements between the vessels. 



As a result of secondary thickening, the primary phloem cannot 

 be recognized in a transection of the mature root because of crush- 

 ing and partial digestion. The secondary phloem consists of 

 sieve tubes, companion cells, and parenchyma. No phloem fibers 

 are differentiated. The relative inactivity of the cambium and 

 the consequent slowness of secondary thickening result in a per- 

 sistence of the cortex through a considerable period of the later 

 ontogeny of the root. The parenchymatous cells of the cortex 

 compensate for the increasing size of the stele by enlarging tan- 

 gentially, but they do not divide to any great extent. In the 



^ ca 



Fig. 178. Young primary root after ttacheids are absent. 



