LEAF STRUCTURE 187 



in the lower epidermis in the majority of ieaves, and, 

 in a few instances, also occur in the upper epidermis. 

 On surface view the epidermal cells of different plants 

 show definite characteristics as regards form of cells, 

 character of their walls, location of stomata and 

 occurrence of trichomes. On surface view the epi- 

 dermal cells may be rectangular, polygonal or irregular 

 in form (Plate 33). Their walls may be thin, irregu- 

 larly thickened or beaded, and may show striations 

 extending parallel with the border of the cell. The 

 stomata may be located on the same level as the 

 epidermal cells, or may be above or below the latter. 

 Trichomes, both glandular and non-glandular, are 

 outgrowths of the epidermal cells, and are exceedingly 

 important in the identification of leaves in powdered 

 condition. The more common forms of trichomes 

 are described in connection with the covering tissues 

 in Chapter VII. In powdered materials the epi- 

 dermal cells are usually apparent on surface view, 

 although they may be so adherent to the deeper 

 tissues that details of cellular structure can only be 

 seen with difficulty. 



Palisade Cells. — These cells are located immedi- 

 ately below the epidermal or subepidermal tissues 

 and are arranged in one or more layers (Plates 62, 

 63). Palisade layers may occur on both surfaces of 

 the leaf or may be present only on the upper surface. 

 Unifacial leaves are those possessing palisade tissues 

 on both surfaces, while bifacial leaves possess only 

 an upper palisade layer. Occasionally palisade tissues 

 are lacking on both surfaces of the leaf. The indi- 

 vidual palisade cells appear as narrow rectangles with 

 the short side toward the epidermis. The walls are 



