CUCURBITA 



597 



There is also a change in the spatial relationship of the proto- 

 and metaxylem, so that the primary xylem matures centrifugally 

 to form endarch strands in the upper limits of the transition, in 



'ap mer 



Fig. 311. Transections of portions of primary root showing origin of secondary roots: 

 A, first radial divisions in cortex and tangential divisions in endodermal and outermost 

 pericyclic layers (nucleated cells in region of primordium); B and C, further divisions in 

 cortex and pericycle ; D, formation of central cylinder and origin of apical meristem ; E, 

 secondary root which has nearly penetrated cortex of primary root with its stele and apical 

 meristem established : ap mer, apical meristem ; c cl, central cylinder ; co, cortex ; en, endo- 

 dermis; pel, pericycle; pph, protophloem; px, protoxylem; stl, stele. (After Whiting.) 



