166 STRUCTURAL BOTANY 



are formed later, are pitted. Sometimes two of the 

 xylem bundles converge inwards, and meet in a single 

 vessel. The small phloem groups have the same 

 structure as those in the stem. Xylem and phloem 

 are separated from each other by a layer of thin- 

 walled tissue. In the root figured there is also some 

 parenchyma in the middle of the cylinder, forming a 

 small pith, but sometimes the xylem groups meet in 

 the middle, so as to leave no room for a pith. 



FIG. 65. Transverse section of tissues at the extreme outside 

 of the same root, r.h, root-hair; ex, exodermis, form- 

 ing outermost layer of cortex. Magnified 330. (R. S. ) 



The outermost layer of the cylinder, surrounding 

 the whole ring of wood and bast, is a very definite 

 thin-walled pericycle (Fig. 64, pc). Outside this, 

 again, we come to the innermost layer of the cortex, 

 the very conspicuous endodermis (e in Fig. 64), the 

 cells of which fit closely together, and have those 

 walls cuticularised by which they are in contact 

 with each other. In the older roots the endodermal 

 cell-walls become thickened, especially on the radial 



