LIGULE: an extension of the leaf sheath, continuing above the point where the petiole joins the sheath.
Maas (and others) use the length and shape of the ligule as a characteristic for identification of Costus species. The Costus Identifier Tool is set up to classify species as having long or short ligules, with 10 mm (1 cm) as the dividing point. Measure from the base of the leaf petiole (where it meets the stem to the top of the ligule. Costus species with ligules always longer than 10 mm are marked "Y". Costus species with ligules always shorter than 10 mm are marked "N". If this characteristic is unknown, or if the ligule length can be either longer or shorter than 10 mm, it is entered in the GingersRus Plant Database as "U".

EXAMPLES OF COSTUS LIGULES - These are pictures of Costus ligules, showing long and short ligules, truncate or lobed.
Examples of ligules, showing where to measure