THE ROOT 198 
as previously explained, that the stress which roots have to 
withstand is largely longitudinal tension. : 
The center of the stele may be occupied by thin-walled pith 
cells, by thick-walled sclerenchymatous cells, or by one or more 
xylem vessels. Around this central portion the xylem, as seen 
in cross section, is arranged in rays which are usually widest 
near the center and taper 
toward the outside (Figs. 
188, .189). The phloém 
occurs in groups between 
the xylem regions, and 
not exterior to the xylem, 
asin stems. The alternate 
arrangement of the xylem 
and phloém is apparently 
connected with the absorb- 
ing function of roots. As 
the phloém is not at the 
outside of the xylem, water 
may pass straight from the 
epidermis to the xylem Fic. 190. Cross section of a portion of root 
of Wandering Jew (Commelina), showing 
endodermis and passage cell 

without going through the 
phloém. 
Pericycle The outer- be endodermis is a row of thick-walled cells 
Pater ay: running across the drawing ; the passage cell 
most part of the stele 1s, is the thin-walled cell in this row. Note the 
as in stems, the pericycle. xylem below the passage cell. (x 800) 
In roots the  pericycle 
never contains sclerenchyma cells but is composed altogether 
of parenchyma cells and usually consists of only a single layer 
(Pigs. 188, 192). 
Cortex. The cortex lies between the stele and the epidermis. 
The innermost layer of the cortex in the stem is known as the 
starch sheath. In roots this layer is called the endodermis. ‘The 
walls of the endodermis, particularly the radial ones, are par- 
tially cutinized. The walls may remain thin (Fig. 192) or they 
may be thickened (Figs. 187, 188). In the latter case all the 

