Glyceria fluitans.

Botanical name: 

Glyceria fluitans R. Br. Gramineae. Float Grass. Manna Grass. Poland Manna.

Northern temperate regions. The seeds of this grass are collected on the continent and sold as manna seeds for making puddings and gruel.3 According to Von Heer,4 it is cultivated in Poland.

Sturtevant's Edible Plants of the World, 1919, was edited by U. P. Hedrick.