The chief external openings of the epidermis of leaves, of herbs, and of young wood stems are known as stomata. Surrounding the stoma are two cells known as guard cells.
Guard cells differ greatly in form, in size, in arrangement, in occurrence, in association, in abundance (Plates 53, 54, and 55), and in color. The guard cells surrounding the stoma vary in form from circular to lens-shaped. In most leaves the outline of the guard cells is rounded or has a curved outline; but in a few cases the guard cells have angled outlines.
PLATE 53
1. Stoma and surrounding cells of aconite stem (Aconitum napellus, L.).
2. Stoma and angled striated walled surrounding cells of peppermint stem (Mentha piperita, L.).
3. Stoma and elongated surrounding cells of lobelia stem (Lobelia inflata, L.).
PLATE 54
Types of Stoma
1. Under epidermis of short buchu (Barosma betulina, [Berg.] Bartling and Wendl., f.) showing stoma and deposits of hesperidin.
2. Under epidermis of Alexandria senna (Cassia acutifolia, Delile) showing stoma and thick-angled walled surrounding cells.
3. Upper epidermis of eucalyptus leaf (Eucalyptus globulus, Labill.) showing sunken stoma and slightly beaded walled surrounding cells.
4. Under epidermis of belladonna leaf (Atropa belladonna, L.) showing stoma and wavy, striated, walled epidermal cells.
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.