34.] 



CHAPTER II. THE TISSUES. 



187 



responding with them in position. But to this rule there are some 

 exceptions ; for instance, when, as in the Grasses and Cyperacese, 

 the pericycle is wanting opposite the xylem-bundles, the lateral 

 roots are developed, not opposite to the xylem-bundles, but opposite 

 to the phloem-bundles ; again, when there are only two xylem- 

 bundles in the parent root, four rows of lateral roots are produced 

 each root being developed on one side of a xylem- bundle of the 

 parent root. A similar displacement occurs in Umbel 1 if erce, 

 Araliaceae and Pittosporeas, where the pericycle is interrupted 

 opposite to each xylem-bundle by an oil-duct (see p. 168). 



re 



ep-- 



FIG. 142. Illustrating the development of a secondary root in a Phanerogam. A trans- 

 verse, B longitudinal, section ; ep epiblema ; en endodermis ; pe pericycle ; w protoxylem 

 and b phloem of the parent-root ; re root cap ; c periblem, and pi plerome, of the de- 

 veloping lateral secondary root. (Teesdalia nudicaulie ; x about 300 ; after van Tieghetn). 



In most Vascular Cryptogams (except Lycopodium and Isoetes, 

 where secondary roots are produced only by dichotomy), the apical 

 cell of a secondary root is formed from one of a row of large 

 endodermic cells, the rliizogenic cells, lying just externally to each 

 xylem-bundle of the parent root. In Equisetum, where the endo- 

 dermis consists of two layers (see p. 165), the secondary roots are 

 developed from cells belonging exclusively to the inner layer, 

 which are adjacent to the xylem-bundles. 



