What is a perigynium and how does it surround a Carex fruit?

In the plant family Cyperaceae, a perigynium is a specialized, modified tubular bract that surrounds the female flower and, subsequently, the fruit of sedges in the genus Carex.12

Regarding its function and structure:

• Development: The perigynium is derived from the partly or wholly closed first leaf, known as a prophyll, of the spikelet axis.2
• Enclosure: In Carex, the perigynium acts as a sac that invests the achene (the sedge's fruit).31
• Variability: Depending on the species, this envelope may tightly envelop the achene, become inflated like a bladder, appear flattened and scalelike, or even develop into a fleshy, edible structure.1
• Dispersal: The perigynium plays a critical role in seed dispersal. Some woodland species possess food bodies called elaiosomes at the base of the perigynium, which attract ants that then disperse the fruits.1