134 



PART II. ANATOMY AND HISTOLOGY. 



[34 



roots; the old root-cap is exfoliated, and each growing-point forms 

 a new one for itself. The successive dichotomies take place in 

 planes at right angles to each other. 



b. By lateral outgrowth. It has been already stated (p. 9) that 

 the lateral development of secondary members does not take place 

 at the growing-point of the root, but at a considerable distance 

 behind it, where the tissues have already assumed their permanent 

 differentiation. The lateral roots are developed endogenously from 

 a layer of this tissue which remains embryonic longer than the 



FIG. 107. Illustrating the development of a secondary root in a Phanerogam. A trans- 

 verse, D longitudinal, section ; ep epiblem ; en endodermis ; pe pericycle ; w protoxylem 

 and b phloem of the parent-root ; re root cap ; c periblem, and 2^ plerome, of the de- 

 veloping lateral secondary root. (Teesdalia nudicaulis ; x about ZOO ; after van Tieghem). 



adjacent tissues. This layer may be either the pericycle, as in 

 Phanerogams, or the endodermis, as in most Vascular Cryptogams. 

 In_the Phanerogams (Fig. 107), the growing- point of a lateral 

 root is formed by the growth and division of a group of pericycle- 

 cells, lying usually just externally to the outer end of a xylem- 

 bundle ; hence there are as many longitudinal rows of lateral roots 

 produced as there are xylem-bundles in the parent root, and cor- 

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

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



