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. Tliis 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 



