194 A TEXTBOOK OF GENERAL BOTANY 
walls may be equally thickened, but usually the radial and in- 
ner walls are thicker than the outer walls. 
the endodermis near the ends of the xylem 
OO ence 

56 i 2 Ai , 
PTS AE SOBRE 
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4 
Fic. 191. Longitudinal section of a portion of 
an onion root, showing internal origin of branch 
root. (x 175) 
tents. In old parts of roots the cortex 
Certain cells found in 
rays (Figs. 188, 190) 
remain thin-walled as 
long as that part of — 
the root is absorb- 
ing water through its 
root hairs. Such thin- 
walled cells are called 
passage cells and ap- 
parently serve as pas- 
sageways for water 
going from the cortex 
into the stele. An en- 
dodermis containing 
thin-walled cells and 
passage cells would 
appear to serve as a 
means of directing the 
movement of water 
so that as it enters 
the stele it will pass 
directly to the xylem 
and not through the | 
phloém. The water 
thus enters the xylem 
and is conducted up- 
ward without getting 
into the sieve tubes 
and diluting their con- 
frequently disappears 
and the endodermis functions as an epidermis (Fig. 194). 
The endodermal cells fit close together, so that there are no air 
spaces between them. As there are air spaces between the other 
_ cells of the cortex, the lack of them in the endodermis would 
appear to decrease the diffusion of air into the vascular tissues. 
