LEAVES AND LEAF STRUCTURE 



87 



GENERAL STRUCTURE OF LEAVES 



A leaf usually consists of a thin flattened portion (the blade) 

 stiffened by a framework of veins which are really bundles of 

 ducts connecting with those in the stem. Usually the blade is 

 attached to the stem and held out into the light by a stalk (the 

 petiole). Its point of at- 

 tachment is called the 

 node of the stem, above 

 which all branch buds 

 originate. The veins may 

 form a network throughout 

 the leaf or may be almost 

 parallel (grass). There 

 may be one large midvein 

 with branches like a feather 

 (elm), or several veins of 

 equal size may spread from 

 the petiole like the fingers 

 of your hand (maple), 

 but whatever the arrange- 

 ment, their function is 

 to support the blade and 

 transfer the liquids concerned in the various leaf processes. 



Leaf Forms. The outline of a leaf depends largely upon the 

 arrangement of its veins. If netted veined the leaves are usually 

 broad, notched, or lobed; while if the veins are parallel they are 

 usually long and slender. The forms of the leaves are almost as 

 various as the kinds of plants; some having regular or entire edges 

 (lily), others notched, lobed, or finely divided (elm, maple, carrot), 

 while still others are composed of separate leaflets (pea, horse 

 chestnut), and so are called compound. 



ADAPTATIONS FOR EXPOSURE 



Form. These different-shaped leaves are developed with but 

 one end in view the complete exposure of the leaf tissues to 



FIG. 21. Structure of leaf exterior. 



