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 
leaf 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 
leaflike parts, called /eaffets. If the leaflets are arranged like 
pinnae of a feather the leaf is pinnately compound, fig. 1 b; ar- 
ranged like fingers of the hand, the leaf 1s palmately compound, 
fig. 1 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 petio/ule, 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 pa/mately 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 Janceolate, 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 leaf, c¢, 
is another common form. The terms /inear, d, and oblong, e, 
describe leaves such as those of the larger blue flag and common 
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