MORPHOLOGY OF HIGHER PLANTS. 311 



dodermis frequently become thickened, either all around, or only 

 on the inner or radial walls, so that we might speak of an O- 

 endodermis as in Honduras sarsaparilla or an U-endodermis as 

 in Mexican sarsaparilla, according to the manner of thickening. 



FIG. 171. Cimicifuga. Transverse section of the central part of a mature root in 

 which the secondary changes are completed: a, parenchyma of primary cortex; b, endo- 

 dermis; c, cambium zone; d, tracheae in secondary xylem; e, broad, wedge-shaped medullary 

 ray; f, outer portion of one of the primary xylem bundles; g, pericycle-parenchyma beneath 

 the endodermis; h, inter-fascicular cambium. After Bastin. 



This is especially the case in the monocotyledons where the walls 

 of the endodermal cells become completely suberized and im- 

 permeable to water. In some roots the cells of the endodermis 

 may be uniformly thick-walled throughout, while in others some 



