192 A TEXTBOOK OF GENERAL BOTANY 
concern us here; for the sake of simplicity we may regard the 
outermost layer of the root as an epidermis. 
Epidermis. The epidermis of a root is, as we have seen, 
very different in structure and function from that of stems 
and leaves, but agrees with that of the latter in bemg a 
single layer of cells. 
Roots, that grow 
underground are not 
exposed to high rates 
of evaporation, and 
so they do not need 
to have  cutinized 
walls to protect them 
from transpiration. 
Moreover, if their 
walls were cutinized 
or. thickened, this 
would interfere with 
the absorption of 
water. The walls 
are thin, soft, cellu- 
Fig. 189. Cross section of young root of a dicoty- lose membranes. The 
ledonous plant, mungo bean (Phaseolus radiatus) 

principal peculiarity | 
In the stele there are four rows of xylem vessels, of these cells con- 
the protoxylem, one of which is labeled x. Alter- 
nating with the xylem is the phloém, ph. (x 70) sists in the Set 
of the long root hairs, 
the structure and function of which have already been discussed. 
Stele. ‘The general structure of monocotyledonous and dicoty- 
ledonous roots is very similar (Figs. 188, 189). This applies to 
the arrangement of vascular bundles as well as to other features, 
and is strikingly in contrast with the great difference in arrange- 
ment of bundles in monocotyledonous and dicotyledonous stems. 
The stele of roots is usually much smaller in comparison with 
the cortex than is the case with stems (Figs. 188, 189). This 
is due to the greater centralization of the thick-walled ele- 
ments in roots than in stems, which is connected with the fact, 
