In botanical terms, having a simple or unadorned peristome (the structure around a moss spore opening).
From Greek 'aplos' (simple) + 'peristoma' (the mouth region) + '-ous' (having the quality of). A highly specialized botanical term for moss morphology.
When bryologists see 'aploperistomatous' moss, it tells them they're looking at a primitive lineage—the simplicity of the structure is actually a clue that they've found an ancient type of moss!
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