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 found with only one. Such plants as iris and compass plant, which have both surfaces of the leaf equally exposed to sunlight, usually have a palisade layer beneath each epidermis.

Epidermis.—The outer or epidermal cells of leaves do not bear chlorophyll, but are usually so transparent that the green mesophyll can be seen through them. They often become very thick-walled, and are in most plants devoid of all protoplasm except a thin layer lining the walls, the cavities being filled with cell sap. This sap is sometimes colored, as in the under epidermis of begonia leaves. It is not common to find more than one layer of epidermal cells forming each surface of a leaf. The epidermis serves to retain moisture in the leaf and as a general protective covering. In desert plants the epidermis, as a rule, is very thick and has a dense cuticle, thereby preventing loss of water.

There are various outgrowths of the epidermis. Hairs are the chief of these. They may be (1) simple, as on primula, geranium, nægelia; (2) once branched, as on wall-*flower; (3) compound, as on verbascum or mullein; (4) disk-like, as on shepherdia; (5) stellate, or star-shaped, as in certain crucifers. In some cases the hairs are glandular, as in Chinese primrose of the greenhouses (Primula Sinensis) and certain hairs of pumpkin flowers. The hairs often protect the breathing pores, or stomates, from dust and water.

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 vapors (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