The =relation of the stoma to surrounding cells= is best shown in cross-sections of the leaf. In powders the relationship of the stoma to the surrounding cells is, however, readily ascertained. If the guard cells come in focus first, they are above the surface; if the guard cells and the surrounding cells come in focus at the same time, the stomata are even with the surface; if the stomata come in focus after the surrounding cells, they are below the surface of the leaf. The relationship of the stoma to the surrounding cells should always be ascertained, not only in cross-sections of the leaf, but also in powders. There is the greatest possible variation in the size of guard cells. This fact must always be kept in mind when studying leaves. This variation in the size of the guard cells is clearly illustrated by coca, senna, and by deer’s-tongue. In coca the stomata are very small; in senna they are larger; while in deer’s-tongue the stomata are very large. The width and length of the stoma or opening between the guard cells are of a character which must not be overlooked. Generally speaking, those leaves which have large guard cells will have correspondingly large stomata. The guard cells usually contain chloroplasts showing various stages of decomposition. In bay-rum leaf the guard cells are of a bright reddish-brown color, but in most leaves the guard cells are colorless. LENTICELS =Lenticels= are small openings occurring in the bark of plants. The lenticels bear the same relationship to the stem that the stomata do to the leaves. Lenticels, like stomata, have a threefold function--namely, exchange of gases in photosynthesis, in respiration, and the giving off of water. Lenticels are macroscopically as well as microscopically important. When unmagnified the lenticels are circular, lens-shaped, or irregular in outline. They are arranged in parallel longitudinal lines or parallel transverse lines, or they are irregularly scattered. The latter is the usual arrangement. In most cases they are elevated slightly above the surface of the bark. In root barks particularly the lenticels stand out prominently from the surface of the bark and in many cases appear stalked. The color of the lenticels differs greatly in the different plants. In acer spicatium they are brown; in witch-hazel they are gray; in xanthoxylium they are yellowish; and lastly, the number of lenticels occurring in a given surface of the bark should always be considered. On cross-sections the lenticel (Plate 57, Fig. 2) is seen to have a central depressed portion made up of loosely arranged cells. Bordering the cavity are typical cork cells. The cork cells immediately surrounding the lenticels are usually darker in color, and many of the cells are partly broken down.