ocrea

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A sheath formed at the node of a stem by the fusion of two stipules, found in many plants of the family Polygonaceae, such as rhubarb.

from The Century Dictionary.

• noun In botany, a sheathing stipule, or a pair of stipules united into a sheath around the stem, like a legging or the leg of a boot; also sometimes, in mosses, the thin sheath around the seta, terminating the vaginula.
• noun In zoology, a sheath; an investing part like or likened to an ocrea of a plant. Also, erroneously, ochrea.

from the GNU version of the Collaborative International Dictionary of English.

• noun See ochrea.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun botany A sheath around a plant stem forming from the stipule of a leaf and extending above the point of insertion of the leaf.