ILLINOIS NATURAL HISTORY SURVEY MANUAL 1 



attached directly to the stem, in which case the leaf is said to 

 be sessile. In other cases where the petiole is absent, as in the 

 grasses and grasslike plants, a lower portion of the leaf clasps 

 or encloses the stem, and either splits down one side or forms a 

 complete tube. In either case the clasping segment is called a 

 kqf sheath. 



SIMPLE AND COMPOUND LEAVES 



A simple leaf is one in which the leaf blade is a single piece, 

 fig. I a. 



A compound leaf is one in which the blade is divided into 

 leafhke parts, called leaflets. If the leaflets are arranged like 

 pinnae of a feather the leaf is pinnately compound^ fig. i b; ar- 

 ranged like fingers of the hand, the leaf is palmately compound^ 

 fig. I c. Sometimes the leaflets are divided, so that the leaf is 

 said to be twice compound, and subdivision of these latter parts 

 makes the leaf three times compound. A leaf whose petiole has 

 three main divisions is ternately compound; each of these ■, 

 divisions may have its little petiole, or petiolule^ divided three 

 times to make the leaf twice ternately compound. The leaf 

 which simply has three leaflets is called trifoliate, and the term 

 ternate is usually reserved for leaves two or three times ternately 

 compound. 



VENATION AND SHAPE OF LEAVES 



Pinnate and palmate are terms used also to describe arrange- 

 ment of the principal veins of the leaf. If there is a midvein 

 and the other large veins branch from it like pinnae, the leaf is 

 pinnately veined, fig. 2 a; but if all the main veins start from the 

 base of the blade, the leaf is palmately veined, fig. 2 b. 



In many leaves numerous small veins connect the larger 

 ones, to form a network. Such a leaf is said to Be netted veined, 

 fig. 2 c. In other cases, such as the leaves of orchids, lilies and 

 grasses, there are none of these small cross-veins; all veins run 

 in approximately the same direction and are nearly parallel, 

 fig. 2 d. Such leaves are said to be parallel veined. 



The commonest shapes of leaves are shown in fig. 3. Some- 

 times a lanceolate, a, or ovate, b, leaf may have the broadest 

 part near the tip instead of the base, in which case the terms 

 oblanceolate and- obovate are used. The heart-shaped leat, c, 

 is another common form. The terms linear, d, and oblong, e, 

 describe leaves such as those of the larger blue flag and common 



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