44 BOTANY OF CROP PLANTS 



prominent veins from which arise numerous minor veins, 

 thus forming quite a conspicuous. network. 



Leaves are often classed as simple or compound. The apple 

 leaf is an example of a simple leaf (Fig. 159). In this there 

 is an undivided blade. The bean, pea, carrot or parsnip leaf 

 is compound (Fig. 19). The blade is divided into a number 

 of segments, or leaflets. 



We may classify leaves as to their arrangement on the 

 stem. Leaf arrangement is the same as bud arrangement, 

 for ordinarily a bud arises in the axil of each leaf. They 

 may be alternate, opposite or whorled (see page 25). 



Leaves vary widely in size, shape, character of margin, and 

 base, texture, thickness, nature of epidermal coverings, etc. 

 Some of these variations will be mentioned throughout the 

 pages that follow. 



Structure of Leaves. — The structure of a leaf is best shown 

 in a cross-section (Fig. 20). The upper epidermis, usually 

 consists of a single row of cells. Below it, is the palisade 

 layer, composed of one or more rows of cells the long axes 

 of which are perpendicular to the leaf surface. Below the 

 palisade cells is the spongy parenchyma, varying in thick- 

 ness, and composed of rather irregularly shaped cells that 

 fit together loosely, thus leaving intercellular spaces (air 

 spaces) . The lower epidermis is seldom more than one layer 

 of cells thick. Chloroplastids are abundant in palisade and 

 spongy parenchyma cells, but absent from all epidermal cells 

 except the guard cells of stomata. 



The outer wall of epidermal cells is normally thicker than 

 radial or inner walls. Cutin, a fatty substance, highly im- 

 pervious to water, is deposited on the outer wall, to form a 

 layer called the cuticle. A thick cuticle is a common char- 

 acteristic of leaves growing in arid situations. The same 

 variety will develop a thicker cuticle under arid conditions 



