What is this sheath around the stem on a Polygonaceae called?

The sheath around the stem, as shown in this diagram, is a whitish membranous sheath called an ocrea. The cosmopolitan family, Polygonaceae, can be recognized immediately by the presence of the ocrea. This family contains around 1000 herbs, shrubs, climbers and a few trees. Characteristic of each of these is that the base of each leaf forms a sheath around the stem at the stalk attachment point.

Some members of this family are popular and useful, like rhubarb. Some are pernicious and hated by gardeners for their resistance to removal and eradication like Japanese knotweed. This one might almost be called the criminal within the Polygonaceae genus.