=Mesophyll.=--The mesophyll is _not all alike or homogeneous_. The upper layer is composed of elongated cells placed perpendicular to the surface of the leaf. These are called =palisade cells=. These cells are usually filled with green bodies called =chlorophyll grains=. The grain contains a great number of chlorophyll drops imbedded in the protoplasm. Below the palisade cells is the spongy =parenchyma=, composed of cells more or less spherical in shape, irregularly arranged, and provided with many intercellular air cavities (Fig. 113). In leaves of some plants exposed to strong light there may be more than one layer of palisade cells, as in the India-rubber plant and oleander. Ivy when grown in bright light will develop two such layers of cells, but in shaded places it may be 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 wallflower; (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.