10 INTRODUCTION TO BOTANY 
and terminal rootlets have no bark, and from their surfaces 
there grow very fine tubular threads known as root hairs 
(figs. 5 and 6). Root hairs have extremely thin walls, through 
which water from the soil can pass into the interior; thence it 
passes upward through rootlets and larger roots, through the 
stem and into the leaves. Substances that are in solution in 
the soil water may be transferred into the plant through the 
delicate walls of the root hairs and 
their lining. Root hairs do not 
grow over all the surface of the 
terminal rootlets, but develop a 
little way from the root tip. They 
do not live very long, the older 
ones constantly dying. Thus, as 
the root tip grows forward through 
the soil the actual number of root 
hairs on a rootlet may remain prac- 
tically constant during the growing 
season, on account of the dying of 
older root hairs and the develop- 
: ment of new ones near the root tip 
Fic. 5. A mustard seedling on the new growth of the rootlet. 
grown ina band of flterpaper’ Td is evident that the ares, of root 
inside a drinking glass, so as to : Se ate 
show the root Hairs hairs advances, although no indi- 
Note the differenceinlengthand Vidual root hairs move forward 
condition of the root hairsonthe through the soil. Although ex- 
i tl i ae tremely delicate in structure, root 
y delicate ucture, ro 
hairs grow between and arcund the hard particles of soil 
(fig. 6). It is easy to count the root hairs on a small portion 
of the root. Corn rootlets grown in ee air have been found 
to bear 425 root hairs on an area j, inch square. The large 
number of rootlets and the enormous number of root hairs 
serve to make a network which completely permeates the soil 
in the region of the rootlets. The root hairs are the chief 
organs by which water and substances in solution in water 
are absorbed from the soil. 
