34.] 



CHAPTER 11. THE TISSUES. 



187 



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

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

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

 roots are developed, not opposite to the xvlem-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 Umbelliferoe, 

 Araliaceae % and Pittosporaceee, where the pericycle is interrupted 

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



re 



FIG. 142. Illustrating the development of a secondary root in a Phanerogam. -1 trans- 

 verse, B longitudinal, section ; ep epibleraa ; en endodermis; pe pericycle; w protox.vlem 

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

 veloping lateral secondary root. (Teesdalia nudicaulis ; x about i>00 ; after van Tieghem). 



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 ro\v of large 

 endodermal cells, the rlizogenic 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. 



