The =arrangement of the surrounding cells= of the stoma is one of the most important characteristics of the different leaves. As a rule the number of surrounding cells about a stoma is constant for a given species. In senna leaves (Plate 54, Fig. 2) there are normally two surrounding cells about each guard cell, while in coca there are four (Plate 55, Fig. 1). In senna the long diameter of the surrounding cells is parallel to the long diameter of the guard cells; but in coca the long diameter of two surrounding cells is at right angles to the long diameter of the guard cells, while two cells are parallel to the long diameter of the guard cells. In most leaves there are more than two cells around the guard cells. The form and size of the surrounding cells must always be considered. In most leaves they are variable in size and form. Guard cells occur first, even with the surface of the leaf (Plate 56, Fig. A); secondly, above the surface of the leaf (Plate 56, Fig. B); and, thirdly, below the surface of the leaf. (Plate 56, Fig. C). Only one of the above types occurs in a given species of plant. That is, plants with stomata above the surface of the leaf do not have stomata on a level with or below the leaf surface. The number of stomata on a given surface of a different leaf varies considerably. In many of the medicinal leaves stomata occur only on the under surface of the leaf. In other leaves stomata occur on both surfaces of the leaf; but in such cases there are a greater number on the under surface. In certain leaves the long diameter of the guard cells is parallel to the length of the leaf; in other cases the long diameter of the stoma is arranged at right angles to the length of the leaf. In other leaves the arrangement is still more irregular, the guard cells assuming all sorts of positions in relation to the length of the leaf. [Illustration: PLATE 55 LEAF EPIDERMI WITH STOMA 1. Under epidermis of coca leaf (_Erythroxylon coca_, Lam.) with stoma on a level with the surface. 2. Under epidermis of false buchu (_Marrubium peregrinum_, L.) with stoma below the level of the surface. 3. Upper epidermis of deer tongue (_Trilisa odoratissima_, [Walt.] Cass.) with stoma above the leaf surface.] [Illustration: PLATE 56 _A._ Cross-section of belladonna leaf (_Atropa belladonna_, L.). 1, Epidermal cells; 2, Guard cells even with the leaf surface; 3, Surrounding cells; 4, Air space below the guard cells; 5, Palisade cells; 6, Mesophyll cells. _B._ Cross-section of deer tongue leaf, 1. Epidermal cells; 2, Guard cells above the surface of the leaf; 3, Surrounding cells; 4, Air space below the guard cells; 5, Hypodermal cells. _C._ Cross-section of white pine leaf (_Pinus strobus_, L.). 1, Epidermal and hypodermal cells; 2, Guard cells below the leaf surface; 3, Surrounding cells; 4, Air space below the guard cells; 5, Parenchyma cells with projecting inner walls.]