300 



THE STRUCTURE OF ECONOMIC PLANTS 



differentiated and two broad parenchymatous rays are formed 

 which may be regarded as either pericyclic or secondary xylem 

 rays. (Fig. 151, C) 



A periderm arises in the persistent multi-layered pericycle form- 

 ing the outer surface of the radish. (Fig. 151, £.) The phellogen 



-ph2 



Fig. 150. Transection of young root axis in which secondary thickening has caused split- 

 ting of cortical zone : ca, cambium ; co, remains of ruptured cortex which forms persistent 

 flap in cotyledonary plane; eti, endodermis; mx, metaxylem ; pel, pericycle; ph z, second- 

 ary phloem ; px, protoxylem ; ra, ray ; xy x, secondary xylem. 



which produces it is not continuous but consists of plates of tissue 

 that are more or less overlapping. As the axis increases in size, 

 there are frequently flaky plates of cork cells which are shed and 

 replaced by new phellem. In red radishes the pericyclic cells 

 contain anthocyanin. 



The secondary xylem vessels are arranged in approximate radial 

 rows separated by parenchyma, but they may be somewhat dis- 



t 



