THE ANGIOSPERMAE : ROOTS 



803 



the protoxylem, and because they are external and the later-formed elements 

 develop successively in a centripetal direction, the root stele is said to be 

 exarch. 



The difference between the protoxylem and the later metaxylem is hard 



XYLEM POLES 



PHLOEM POLES 



PERICYCLE 



MATURE 

 XYLEM ELEMENT 



CORTEX 



- EXODERMIS 



ENDODERMIS 



IMMATURE 

 XYLEM ELEMENT 



MATURE 

 PHLOEM 

 ELEMENT 



MMATURE 

 PHLOEM 

 ELEMENT 



ROOT CAP 



p,G 707 —Diagram of the course of differentiation in a root apex. 



(After Esau.) 



