=Stomates= (sometimes called =breathing-pores=) _are small openings or pores_ in the epidermis of leaves and soft stems that allow the passage of air and other gases and vapours (_stomate_ or _stoma_, singular; _stomates_ or _stomata_, plural). They are _placed near the large intercellular spaces_ of the mesophyll, usually in positions least affected by direct sunlight. Fig. 114 shows the structure. There are =two guard-cells= at the mouth of each stomate, which may in most cases open or close the passage as the conditions of the atmosphere may require. The guard-cells contain chlorophyll. In Fig. 115 is shown a case in which there are compound guard-cells, that of ivy. On the margins of certain leaves, as of fuchsia, impatiens, cabbage, are openings known as =water-pores=. [Illustration: FIG. 114.—DIAGRAM OF STOMATE OF IRIS (Osterhout).] [Illustration: FIG. 115.—STOMATE OF IVY, showing compound guard-cells.] _Stomates are very numerous_, as will be seen from the numbers showing the pores to each square inch of leaf surface: Lower surface Upper surface Peony 13,790 None Holly 63,600 None Lilac 160,000 None Mistletoe 200 200 Tradescantia 2,000 2,000 Garden Flag (iris) 11,572 11,572 [Illustration: FIG. 116.—STOMATES OF GERANIUM LEAF.] The arrangement of stomates on the leaf _differs with each kind of plant_. Fig. 116 shows stomates and also the outlines of contiguous epidermal cells. [Illustration: FIG. 117.—LENTICELS ON YOUNG SHOOT OF RED OSIER (CORNUS).] The function or work of the stomates is to _regulate the passage of gases_ into and out of the plant. The directly active organs or parts are guard-cells, on either side the opening. One method of opening is as follows: The thicker walls of the guard-cells (Fig. 114) absorb water from adjacent cells, these thick walls buckle or bend and part from one another at their middles on either side the opening, causing the stomate to open, when the air gases may be taken in and the leaf gases may pass out. When moisture is reduced in the leaf tissue, the guard-cells part with some of their contents, the thick walls straighten, and the faces of the two opposite ones come together, thus closing the stomate and preventing any water vapour from passing out. _When a leaf is actively at work making new organic compounds, the stomates are usually open; when unfavourable conditions arise, they are usually closed._ They also commonly close at night, when growth (or the utilizing of the new materials) is most likely to be active. It is sometimes safer to fumigate greenhouses and window gardens at night, for the noxious vapours are less likely to enter the leaf. Dust may clog or cover the stomates. Rains benefit plants by washing the leaves as well as by providing moisture to the roots.