Ryegrass Leaf with Sheath
Lolium perenne. A sheath, in botany is "the part of an expanded organ that is rolled around a stem or other body, forming a tube, as in the lower part of the leaves of grasses..." —Whitney, 1889
This illustration show a single leaf of ryegrass in its sheath.
William Dwight Whitney, PhD, LLD The Century Dictionary: An Encyclopedic Lexicon of the English Language (New York, NY: The Century Co., 1895) 5558