74 
A TEXT-BOOK OF BOTANY 
36. Root-hairs.—A short distance behind the root-cap 
the surface of the root becomes covered by a more or less 
dense growth of hairs, known as root-hairs (Fig. 71). 
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Fic. 71.—Root-tips of corn, showing 
root-hairs and their position in ref- 
erence to the growing tip: 4, grown 
in soil (higher up the hairs become 
much more abundant and longer); 
B, grown in moist air. 
These 
hairs are outgrowths, some- 
times very long ones, from the 
superficial cells, a single cell 
producing a single root-hair. 
In fact the root-hair is only an 
extended part of the superfi- 
cial cell. The root absorbs 
water and materials dissolved 
in it from the soil, and the 
root-hairs enormously increase 
the absorbing surface. Gen- 
erally root-hairs do not last 
very long; but new hairs are 
being put out by the elonga- 
ting root as the old ones behind 
die, so that there is always a 
zone of active root-hairs near 
the tip, but none on the older 
parts of the root. 
37. Internal structure.—A cross-section of a young root 
shows two prominent regions (Fig. 73). 
In the center is a 
solid vascular cylinder, often called the central azis. It 
will be remembered that in the stems of Dicotyledons and 
Gymnosperms (§ 24) the vascular cylinder is hollow, en- 
closing pith. Investing the solid vascular evlinder of the 
root is the cortex, which often can be stripped from the 
central axis like a spongy bark. If the section has passed 
through the zone of root-hairs, they can be seen coming 
from the superficial cells. A longitudinal section of a root- 
tip, in which these regions are very young, is shown in 
Fig. 70. 
The wood (xylem) and the bast (phloem) of the vascular 
