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.
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.
