70 VARIATIONS IN ROOT-STRUCTURE 



Wallflower diarch, and that of most Monocotyledons poly arch. 

 A pith composed of parenchyma, which is sometimes thick- 

 walled, is frequent in the roots of herbaceous Dicotyledons 

 (Fig. 30) and in Monocotyledons (Fig. 29), but in woody 

 Dicotyledons and Conifers the xylem-groups often meet at the 

 centre. Roots also vary in the manner of thickening of the 

 mature cells of the endodermis. 



EJ.S. 



FIG. 29. Transverse section of the central part of the root of the Iris. 

 C., cortex ; en., endodermis ; m.xy., metaxylem ; pa., passage cells ; 

 PC., pericycle ; ph., phloem ; p.xy., protoxylem. 



A transverse section of the root of the Iris illustrates these 

 features. It exhibits the typical Monocotyiedonous structure, 

 viz. a central pith and numerous alternating groups of xylem 

 and phloem (Fig. 29). The endodermis (en.) is conspicuous owing 

 to the marked thickening of all but the outer walls of most of 

 its cells. Opposite the protoxylem groups (p.xy.), however, the 

 endodermal cells are often thin-walled, and such passage-cells (pa.) 



