Cleistocarpous mosses, those lacking a differentiated operculum and having the capsule dehisce irregularly, are generally thought to have evolved via reduction from stegocarpous ancestors. Bruchia (Bruchiaceae), a cleistocarpous genus of approximately 15 species, shares gametophytic similarities with the genus Trematodon, which has a functional annulus and dehiscent capsule. The sequence of cell divisions characterizing the sporophyte of Bruchia flexuosa shows that development is typically haplolepideous up to the stage in which the peristomial formula is 4:2:2. Whereas other haplolepideous species, including Trematodon longicollis, complete development with a 4:2:3 peristomial formula, the last set of anticlinal divisions in the Inner Peristomial Layer do not occur in B. flexuosa, which therefore appears to mature at what is an immature stage in related stegocarpous taxa. This pattern of development is consistent with an origin of the cleistocarpous capsules of Bruchia through paedomorphosis from a Trematodon-like (haplopideous) ancestor.
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Vol. 103 • No. 1