312 A TEXT-BOOK OF BOTANY. 



of the cells may remain thin-walled, and these cells, the so-called 

 " transition cells " or " passage cells," form channels of com- 

 munication between the cortical parenchyma and the vessels of 

 the stele (Fig. 169) ; they are therefore located just outside the 

 peripheral vessels of each ray of the xylem (or hadrome). 



Inside the emlodermis is the STELE, formerly called the central- 

 cylinder. In this the peripheral stratum, sometimes composed of 

 two or three layers of cells, represents the PERICAMBIUM (or 

 PERICYCLE). The cells are generally thin-walled, and in Dicotyle- 

 dons and Gymnosperms are able by cell-division to form cork and 



RB 



FIG. 172. A transverse section through the root of a germinating pea-plant (Pisum) 

 about 40 mm. from the tip, showing the origin of a root-branch (RB); E, epidermis; C, pri- 

 mary cortex; X, hadrome (vessels); P, leptome (sieve); EN, endodermis. 



secondary cortex, but in all vascular plants it is capable of giving 

 rise to "lateral branches" or "lateral roots" (Figs. 161, 172), 

 hence it is frequently referred to as the " RHIZOGENOUS LAYER." 



Inside the pericambium (by some authors compared with the 

 pericycle.of the stem) we find strands of phloem (or leptome) 

 (P) alternating radially with a corresponding number of strands 

 of xylem (or hadrome) (X). The number of these strands vary 

 in the different groups of plants (Figs. 169-174), being highest in 

 the monocotyledons where a pith is developed, as in sarsaparilla, 

 several grasses, palms, etc. This peculiar arrangement of the 



