312 
PHYSIOLOGY 
tern, however, only the younger parts are permeable to water, since with 
age the surface cells become altered, or usually are underlaid and finally 
replaced by corky or cutinized 
tissues, whose walls are nearly 
waterproof. But as the roots are 
growing at the tips and branching, 
there are always young and per- 
meable parts. 
Root hairs. The surface cells 
of the young root in most land 
plants, at a short distance behind 
the growing apex, branch, sending 
out tubular extensions, the root 
hairs (fig. 625), which push their 
way among the soil particles, dis- 
placing some and being deformed 
by crowding against others, to 
which they often adhere strongly. 
These root hairs increase 5-1 2-fold 
the permeable area of the root, and 
by their size and radial position 
come ^ immediate contact with 
a cylinder of soil 3-8 mm. in di- 
ameter (fig. 626). They anchor the young root 
thoroughly, since they adhere so firmly to the soil 
particles that they tear away from the root when 
that is pulled out of even the loosest soil; and if 
by chance they come away, they bring with them 
ihe adherent grains. The root hairs are transient, 
not living through even one growing season. They 
die away on the older parts of the roots, from 
which the hair-bearing cells usually slough off; 
but new hairs are being formed continually, during 
the growth of the root in length, just behind the 
advancing apex. (See Part III, p. 495, for varia- 
tion of root hairs and for kindred topics.) 
Soil. The soil, into which roots clothed with 
root hairs spread, consists of particles of weathered 
or comminuted rock of various kinds, usually 
FIG. 625. Root hairs of lettuce, with 
adherent soil grains (s). From PART III. 
b a 
FIG. 626. Seedling 
of mustard ; a, grown 
in moist air ; b, grown 
in sand and withdrawn 
to show mass of soil 
grains clinging to root 
hairs. After SACHS. 
