In the =spiral vessel= the thickening occurs in the form of a spiral, which is readily separated from the side walls. This is particularly the case in powdered drugs, where the spiral thickening so frequently separates from the cell wall. There are three types of spiral vessels: those with one (Plate 41, Fig. 1), those with two, and those with three spirals. Single spirals occur in most leaves; double spirals occur in many plants (Plate 41, Fig. 2), but they are particularly striking in powdered squills. Triple spirals are characteristic of the eucalyptus leaf (Plate 41, Fig. 3); in fact, they form a diagnostic feature of the powder. Frequently a spirally thickened wall indicates a developmental stage of the vessel. Many such vessels are spirally thickened at first, but later, when mature, an increased amount of thickening occurs and the vessel becomes a reticulate or pitted vessel. Many mature vessels, however, are spirally thickened as indicated above. In herbaceous stems and in certain roots and leaves spiral vessels are associated with the sclariform reticulate and pitted type. In certain cases a single spiral band will branch as the vessel matures. There is a great variation in the amount of spiral thickening occurring in a vessel. In leaves, particularly, the spiral appears loosely coiled; while in squills and other rhizomes and roots the spiral appears as a series of rings. When viewed by high power only half of each spiral band is visible. At either side of the cell the exact size and form of the thickening appear in two parallel rows of dark circles or projections from the walls. This thickening of the wall is rendered visible from the fact that the light is retarded as it passes through that portion of the spiral extending from the upper to the under side of the spiral; while the light readily traverses the upper and lower cross bands of the vessel. It should be remembered that, when the upper part of the spiral vessel is in focus, the bands appear to bend in a direction away from the eye; while when the under side of the bands are in focus, the bands appear to bend toward the eye. These facts will show that it is necessary to focus on both the upper and lower walls in studying spiral vessels. In double spiral vessels the spirals are frequently coiled in opposite directions; therefore the bands appear to cross one another. In eucalyptus leaf the three bands are coiled in the same direction. In all cases the thickening occurs on all sides of the wall. Its appearance will, therefore, be the same no matter at what angle the vessel is viewed. SCLARIFORM VESSELS