A single closed spore case in fungi, especially in ascomycetes, where asci (spore sacs) develop completely enclosed within a protective covering.
From cleisto- (closed) + theca (case) + -ium (singular diminutive suffix). Singular form of cleistothecia, created in mycology to describe individual fungal fruiting structures that remain completely sealed.
Under the microscope, a cleistothecium looks like a tiny fortress—completely walled off, it's the ultimate privacy for fungal reproduction, protecting delicate spore development from contamination, predators, and drought.
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