=Vessels= and =tracheids= form the principal upward conducting tissue of plants. They receive the soil water expressed from the cortical parenchyma cells located in the region of the root, immediately back of the root hair zone. This soil water, with dissolved crude inorganic and organic food materials, after entering the vessels and tracheids passes up the stem. The cells needing water at the different heights absorb it from the vessels, the excess finally reaching the leaves. When the stem branches, the water passes into the vessels of the branches and finally to the leaves of the branch. In certain special cases the vessels conduct upward soluble food material. In spring sugary sap flows upward through the vessels of the sugar maple. Vessels are tubes, often of great length, formed from a number of superimposed cells, in which the end walls have become absorbed. The vessels therefore offer little resistance to the transference of water from the roots to the leaves of a plant. The combined length of the vessels is about equal to the height of the plant in which they occur. The length of the individual vessels varies from a fraction of a meter up to several meters. ANNULAR VESSELS The =annular vessels= are thickened at intervals in the form of rings (Plate 40, Fig. 1), which extend outward from and around the inner wall of the vessel. In fact, it is the inner wall which is thickened in all the different types of vessels. The ring-like thickening usually separates from the wall when the drug is powdered. Such separated rings occur frequently in powdered digitalis, belladonna, and stramonium leaves. Annular vessels are not, however, of diagnostic importance, because more characteristic cells are found in the plants in which they occur. Not infrequently a vessel will have annular thickenings at one end and spiral thickenings at the other. Such vessels are found in the pumpkin stem (Plate 40, Fig. 1). Vessels are distinguished from other cells by their arrangement, by their large size when seen in cross-section, and by the thickening of the wall when seen in longitudinal sections of the plant or in powders. The side walls of vessels are thickened in a number of striking yet uniform ways. The chief types of thickening of the wall, beginning with one that is the least thickened, are annular, spiral, sclariform, pitted, and pitted with bordered pores. SPIRAL VESSELS