122 HISTOLOGY OF MEDICINAL PLANTS 
the surface of the root, caused by the development of side 
walls by the growing tip of the root hair until a tube-like struc- 
ture, root hair, is produced. The root hair is then a modified 
epidermal cell. The protoplast lines the cell, and the central 
part of the root hair consists of a large vacuole filled with cell 
sap. The wall of the root hair is composed of cellulose, and 
the outermost part is frequently mucilaginous. As the root 
hairs develop, they become bent, twisted, and of unequal diam- 
eter, as a result of growing through narrow, winding soil 
passages. During their growth, the root hairs become firmly 
attached to the soil particles. The walls of root hairs give an 
acid reaction caused by the solution of the carbon dioxide ex- 
creted by the root hair. The acid character of the wall attracts 
moisture, and in addition has a solvent action on the insoluble 
compounds contained in the soil. It will thus be seen that the 
method of growth, structure, composition, and reaction of the 
wall of the root hair is perfectly suited to carry on the work 
of absorbing the enormous quantities of water needed by the 
growing plant. It is a well-known fact that when two solutions 
of unequal density are separated by a permeable membrane, 
the less dense liquid will pass through the membrane to the 
denser liquid. The wall of the root hair acts like an osmatic 
membrane. The less dense watery solution outside the root 
hair passes through its wall and into the denser cell sap solution. 
As the solution is absorbed, it passes from the root hair into 
the adjoining cortical parenchyma cells. 
It is a fact that root hairs are seldom found in abundance 
on medicinal roots. This is due to the fact that root hairs 
occur only on the smaller branches of the root, and that when 
the root is pulled from the ground the smaller roots with their 
root hairs are broken off and left in the soil. For this reason 
a knowledge of the structure of root hairs is of minor importance 
in the study of powdered drugs. An occasional root hair is 
found, however, in most powdered roots, but root hairs have 
little or no diagnostic value, except in false unicorn root and 
sarsaparilla. When false unicorn root is collected, most of the 
root hairs remain attached to the numerous small fibrous roots, 
owing to the fact that these roots are easily removed from the 
sandy soil in which the plants grow. The root hairs of false 
