106 A TEXT-BOOK OF GRASSES 
grass leaf at the junction of the sheath and blade. The 
usual form of the ligule is a membrane extending across 
the top of the sheath at first tightly clasping the culm, the 
membrane being longest in the middle portion. The shape 
and appearance of the ligule are characteristic of each 
species and are often used for taxonomic purposes. 
The ligule may be truncate or even somewhat concave on the 
upper margin. It is nerveless but may be 1-keeled (Dactylis glom- 
erata) or apparently 2-keeled by the extension upward of the base 
of the blade on each side. Not infrequently the ligular tissue extends 
down the margins of the sheath. Sometimes the ligule is lacerate or 
ciliate, or may appear as a row of hairs (Panicum Lindheimeri Nash 
and its allies). The ligule is obsolete in some species (Panicum sphero- 
carpon Ell.) and entirely absent in others (Echinochloa crus- 
galli (L.) Beauv.) In Anthochloa colusana (Davy) Scribn. there is 
no differentiation into sheath and blade, hence no collar or ligule. 
The exact morphology of these leaves has not been investigated. It 
may be the sheath that is obsolete. 
136. The blade is the usually flat part of the leaf 
beyond the sheath and is the chief foliage organ of the 
plant. In most grasses the blade is many times longer 
than wide but in many tropical species and in a few of the 
temperate regions it is short and broad, from oval to 
lanceolate in outline (see Fig. 29). Grasses with this kind 
of blade are mostly confined to the damp forest regions 
of the tropics. 
In such grasses the shoots are often strongly dorsiventral, the 
blades being turned into the plane of the culm, and the shoots 
usually prostrate, procumbent or ascending. The genera Oplismenus, 
Ichnanthus, Senites, Pharus and many species of Panicum (as P. 
trichoides) furnish familiar examples of species with broad short 
blades that are found on the floor of tropical forests. These blades 
often resemble those of other families of plants, especially Commely- 
nacee. The bamboos and the bamboo-panicums (Lasiacis) usually 
