CELLULAR ANATOMY OF THE ROOT TIP 145 
regions of the root increase in diameter, but almost all elongation 
takes place in the growth zone, as shown in Figure 129. The 
meristematic zone is thus so situated that the new cells formed 
by it may be added both to the root cap, the thickness of which 
is thereby maintained in spite of its being rapidly worn away on 
its outer surface, and to the growth zone, the older portions of 
which are constantly taking on the fea- 
tures of the absorptive zone just behind. 
The growth zone merges imperceptibly 
into the absorptive zone where the fol- 
lowing tissues become quite well defined: 
(1) a surface layer of cells constituting 
the epidermis which has most to do with 
absorption, the special absorptive agents 
being the root hairs, which, as the section 
shows, are merely projections of the epi- 
dermal cells; (2) a broad band of cells 
just beneath the epidermis and constitut- 
ing the corter; and (8) a group of con- 
ductive tissues forming a central cylinder, y 
known as the vascular cylinder. Fic. 129.— The radi- 
It is to be noted that the epidermis of 
roots, unlike that of leaves and stems, 
has no cutinized walls and contains no 
stomata or other openings for the entrance 
of air, although so many active cells re- 
quire much oxygen for respiration. How- 
ever, openings are not necessary, for the 
uncutinized walls offer practically no re- 
sistance to the passage of water, which 
usually carries in solution oxygen enough 
cle of a Corn seedling 
marked to show the re- 
gions of clongation. A, 
radicle just after being 
divided into spaces of 
about 35 of an inch in 
width. B, radicle sev- 
eral hours after mark- 
ing, showing the region 
where elongation is tak- 
ing place. Modified from 
Andrews. 
to support quite active respiration. 
Through the development of the root hairs the absorptive 
surface of the root system is much increased, and may be thereby 
increased from five to six times in Corn, about twelve times in 
Barley and as much as eighteen times in some other plants. All 
root hairs are able to absorb regardless of their size, which ranges 
from a slight bulge near the growth zone of the root to often 
more than an inch farther back. They live only a few days, 
but, as they die off behind, new ones form ahead, and in this 
