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1. A bract, usually one of two, at the base of a grass spikelet.
2. A bract in a sedge spikelet, usually subtending a floret.

[Latin glūma, husk; see gleubh- in Indo-European roots.]

glu·ma′ceous (glo͞o-mā′shəs) adj.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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Different types of bracts may be recognize in the inflorescence: (1) foliaceous, with a well-developed blade and a scarcely or not developed sheath; (2) setiform, without a sheath, the leaf blade enlarges at the base but the mid and upper portions are narrow and stretched (setalike), sometimes acuminate; and (3) glumaceous, similar to a glume, but with a short setiform appendix.
Because of this, the main axis may show, in acropetal succession, all three types of bracts (foliaceous-setiform-glumaceous), or two types (foliaceous-glumaceous, setiform-glumaceous) or only the glumaceous type.
The primary bracts are often described as involucral bracts, the lowermost bracts are usually foliaceous (sheathing or not) or setiform, and the uppermost bracts are very small and glumaceous; however, not rarely, primary bracts can be all foliaceous (Cyperus imbricatus Retz.) or all glumaceous (Cyperus papyrus).