THE STBUCTUEE OF THE ROOT 



381 



phelloderm, which behaves as did the first. Later phellugens 

 are successively formed still more internally, till the last- 

 formed one may be only 

 separated from the 

 oambimn ring by the 

 youngest bast. In this 

 way a bark is formed 

 in the root very simi- 

 lar in composition to 

 the same structure in 

 the stem. Like that 

 of the stem it often 

 becomes irregularly 

 cracked and fissured ; 

 sometimes it is shed 

 annually after the man- 

 ner of scale bark. 



The branches of the 

 root, Kke the main root 

 itself, arise endoge- 

 nously. In all flower- 

 ing plants they are 

 developed from the 

 pericycle, and bore then- 

 way outwards through 

 the cortex. 



The number of rows 

 of branches produced 

 from a root corresponds 

 to the number of pri- 

 mary xylem bundles, 

 one being usually 

 formed outside each. 

 At a very early stage 

 in the development of 

 the primary root a group 

 of cells can be distin- 

 guished in the peri- 

 cycle which elongate 

 in the direction of the 

 circumference (fig. 768). They divide then by tangential walls, 

 forming a layer two cells deep. The inner one of these is the 

 layer which gives rise to the plerome of the new root. The 



Fig. 768. Transverse section of root of HesperU, to 

 show a rootlet at two stages of development, rh. 

 Koot hairs, ec. Cortex, rf. Cells in process of ab- 

 sorption, f/;. Endodermis. j'^- Pericycle. c-o. 

 Conjunctive tissue, ^fi. Phloem. ^.Cambium. ./. 

 Xylem. c. Dermatogen of rootlet ; p. its periblem ; 

 pi. its plerome. After Van Teighem. x 250. 



