A root hair is formed by the extension of the peripheral wall of an epidermal cell. At first this wall is only slightly papillate, but gradually the end wall is extended farther and farther from the surface of the root, caused by the development of side walls by the growing tip of the root hair until a tube-like structure, 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 diameter, 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 excreted 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 osmotic 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 unicorn are so abundant and so large that they form dense mats, which are readily seen without magnification. These hairs are, therefore, macroscopically as well as microscopically diagnostic. The root hairs of false unicorn (Plate 39, Fig. 2) have white, wavy, often decidedly indented walls. The terminal, or end wall, is rounded and much thicker than the side walls. [Illustration: PLATE 38