The external opening of a tiny channel or passage, especially the outer opening of the micropyle in a seed.
From Greek 'exo-' (outside) + 'stoma' (mouth/opening). Botanical term from the 19th century describing the external pore structure in seed coats and bryophytes.
Seeds have this incredibly tiny hole called the exostome that water uses to enter during germination—it's like a seed's front door, and without it, the seed can't wake up and grow!
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