LEAVES— STRUCTURE OR ANATOMY 87 



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 epi- 

 dermis 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 pre- 

 venting loss of water. 



There are various outgrowths of the epidermis. Hairs 

 are the chief of these. They may be (i) simple, as on 

 primula, geranium, naegelia ; (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 



