Poaceae and Cyperaceae
The ligule is part of the leaf, and is found at the junction of the blade and sheath of the leaf. It may take several forms but is commonly some form of translucent membrane or a fringe of hairs. The membranous ligule can be very short 1–2 mm (Kentucky Bluegrass, Poa pratensis) to very long 10–20 mm (Johnson grass, Sorghum halepense), it can also be smooth on the edge or very ragged. Some grasses do not have a ligule, for example barnyardgrass (Echinochloa crus-galli).
A ligule can also be defined as a membrane-like tissue or row of delicate hairs typically found in grasses at the junction of the leaf sheath and blade. The ligule appears to be a continuation of the leaf sheath and encircles or clasps the stem as does the leaf sheath. There are three basic types of ligules: membranous, a fringe of hairs (ciliate), and absent or lacking. Most grasses have ligules, and the shape, length, and appearance of the ligule margin provide consistent characters for separating genera and some species of grasses.
In grass-like plants such as sedges (Cyperaceae) and rushes (Juncaceae), ligules are usually absent or poorly developed.
Read more about this topic: Ligule