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 metaxylem having developed towards the center 

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



556 



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

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

 bium; nx, secondary xylem; np, secondary phloem. After COULTER. 



