70 



VARIATIONS IN ROOT-STRUCTURE 



Wallflower diarch, and that of most Monocotyledons polyarch. 

 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. 



I'"iG, 2y. — Transverse section of the central part of the root of the Ins. 

 C, cortex ; en., endodermis ; m.xy., metaxylcm ; 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 Monocotyledonous 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 

 cndodermal cells are often thin-walled, and such. passagc-cdls [pa.) 



