SPERMATOPHYTES 



249 



Fig. 555. — Partly diagrammatic transverse section of vascular cylinder of root of 

 Ranunculus : the two outermost layers belong to the cortex, the inner one being the 

 endodermis; the next layer (outermost one of the stele) is the pericycle; in the center is 

 the group of xylem vessels, extending in four rays to the pericycle; the outermost xylem 

 vessels of each ray are protoxylem, the imtaxylem having developed towards the center 

 in four converging lines; between the xylem rays are the four groups of phloem (shaded). 



Figs. 556, 557. — Diagrams to show secondary thickening in root, 556 being the 

 primary condition, 557 the secondary; x, primary xylem; />, primary phloem; c, cam- 

 bium; nx, secondary xylem; »/>, secondary phloem. — After COULTER. 



