| Summary: | The genus Mannia (Aytoniaceae, Marchantiales) was first described by Opiz in 1829 and it is still not clear how many species it comprises worldwide. The generic and subgeneric division of Mannia has long been confused and several attempts have been made to subdivide it into smaller genera. Morphologically it is typically characterised by a small, rather narrow thallus, cup-shaped involucres and the lack of a pseudoperianth. It includes both xeromorphic and mesomorphic species, which are all drought- tolerant. Mannia occurs almost exclusively in the northern hemisphere with exceptions in Africa and South America. It is found on rock, rocky soil or rock crevices in arctic - alpine and Mediterranean climates. A worldwide revision of the genus has not been previously attempted, and earlier treatments focused on thallus morphology and limited geographic regions. This study comprises the first worldwide revision of Mannia and about 1600 specimens were studied, including all available type specimens, from both dried and fresh material collected on several field trips. The monographic work is based on reviewed morphological and anatomical characters using dissecting, compound and scanning electron microscopy. Surface ornamentation of spores has been studied and with their relatively large spores reaching up to about 90 μm diameter, the species within Mannia show striking spore ornamentation patterns, which are found to be species -specific and have proved to be a very valuable tool in identification and synonymy. Most spore characters are constant with only size showing large variations though this can nevertheless be diagnostic. The species status of M. sibirica is confirmed and its close relationship to M califo-rnica is contrary to former assumptions. A total of seven species are recognised from 22 formerly described names: (1) M. androgyna (L. ) A. Evans, (2) Mannia sihirica (Milll. Frib) Frye & L. Clark, (3) Mannia califarnica (Gottsche ex Underw. ) L. C. Wheeler, (4) Mannia fragrans (Balbis) Frye & L. ...
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