The =aerating tissue= of the plant performs a threefold function: first, it permits the exchange of gases during photosynthesis; secondly, it permits the entrance of oxygen and the exit of carbon dioxide during respiration; and, thirdly, it permits the exit of the excess of water absorbed by the plant. The above functions are carried on by the stomata, the water-pores, the lenticels, and the intercellular spaces of the plant. The stoma functions as the chief channel for the passage of CO₂-laden air into the leaf and of oxygen-laden air from the leaf to the atmosphere. The stoma also functions as an organ of transpiration, since through the stoma a large part of the excess water of the plant passes off into the air. WATER-PORES In certain plants the primary epidermis is provided with openings resembling stomata, but unlike stomata the orifice remains open, and instead of being located on the upper or lower surface of the leaf, they are located on the margin of leaves immediately outward from the veins. Water is given off to the atmosphere from these openings. Such an opening is usually designated as a water-pore. STOMATA 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. [Illustration: 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.).] [Illustration: 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.]