CHAPTER V 



LEAVES 



The leaves constitute the foliage of the plant. They are the 

 expanded part and have a very definite relation to sunlight and 

 air. They are naturally the most proniinent part of the plant 

 and together with the flowers and fruits attract the greatest 

 amount of attention. 



Parts of the Leaf.— The parts of the leaf (Fig. 49) are the 

 blade or lamina, which is the expanded part and is supported 

 by a mid-rib and numerous secondary ribs and veins. The leaf 

 is usually attached to the stem by means of a petiole which is 

 prolonged into the mid-rib. Some leaves do not have petioles 

 and are called sessile (Fig. 49). At the base of the petiole there 

 are often two small more or less leaf -like structures known as 

 stipules; they may be permanent or may fall very soon after 

 the leaf unfolds. 



The stipules are extremely variable in character and are very 

 important in writing descriptions of plants. The free type is 

 separate from the petiole and may be persistent or may fall 

 early; the adnate type is attached to the petiole and is charac- 

 teristic of the clovers. The ligule is the short, thin, projecting 

 membrane at the top of the sheath of the leaves of the grass 

 plants (Fig. 50). 



Types of Leaves. — There are a gi-eat many types of leaves, 



dependent on character of venation, form, thickness and other 



characters. These variations in type are usiially correlated with 



the relationship of the leaf to air and sunlight or to some special 

 70 



