CROSS-SECTION OF SARSAPARILLA ROOT (_Smilax officinalis_, Kunth) 1. Epidermal cell developing into a root hair. 2. Developing root hair. 3. Nearly mature root hair. 4. Hypodermal cells.] [Illustration: PLATE 39 ROOT HAIRS (Fragments) 1. Sarsaparilla root (_Smilax officinalis_, Kunth). 2. False unicorn root (_Helonias bullata_, L.).] In sarsaparilla (Plate 39, Fig. 1) the root hairs are curved and twisted. The end wall is thicker than the side walls. In some hairs the walls are as thick as the walls of the thin-walled bast fibres. This accounts for the fact that the root hairs are persistent on even the older portions of sarsaparilla root, and it serves also to explain why these root hairs remain on the root even after being pulled from the firmly packed earth in which the root grows. WATER ABSORPTION BY LEAVES In many xerophytic terrestrial plants, the trichomes occurring on leaves act as a water-absorbing tissue. In such plants the walls of the hairs are composed largely of cellulose. It is obvious that these hairs absorb the water of condensation caused by dew and light rains--water which could not reach the plant except by such means. There is no special tissue set aside for the absorption of gases from the air. Carbon dioxide, which contributes the element carbon to the starch formed by photosynthesis, enters the leaf by way of the stoma and lenticels. The structure and the chief functions of these will be considered under aërating tissue. CHAPTER V CONDUCTING TISSUE All cells of which the primary or secondary function is that of conduction are included under conducting tissue. It will be understood how important the conducting tissue is when the enormous quantity of water absorbed by a plant during a growing season is considered. It will then be realized that the conducting system must be highly developed in order to transport this water from one organ to another, and, in fact, to all the cells of the plant. Special attention must be given to the occurrence, the structure, the direction of conduction, and to the nature of the conducted material. The cells or cell groups comprising the conducting tissue are vessels and tracheids, sieve tubes, medullary ray cells, latex tubes, and parenchyma. VESSELS