■ ^5 



-er 



THE f-XnOFI ASMIC RKTICII.UM I5I 



J 



.cw 



■J %^ 



^^y 





.:^ — er 



.;■ -' ■ C 



> 



" s cw 



\\- . - 

 I 



■^: H^ 



.-v- 



\ 



"^ ^^ X 



-er 



N^ 



I v 



\ 



xw 



\ 



-er 



^^ 



V. 



FiG. 13. This micrograph shows a longitudinal section through a protoxylem 

 cell of the onion root tip. This cell, in the final stages of its differentiation, shows 

 secondary ring thickenings in the wall characteristic of early xylem units. Cross 

 sections of these thickenings are marked civ. The point of interest here is that the 

 ER is patterned with respect to these thickenings ; the evident profiles are marked er. 



