The liverwort genera (Bryophyta: Hepaticae and Anthocerotae) of Britain and Ireland

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L. Watson and M. J. Dallwitz

Preissia Corda in Opiz

Gametophyte. Plants thalloid, not leafy (in thin pale green patches, the thallus ligulate, nearly flat above with thin, wavy, reddish-brown margins and two rows of ventral scales). Acrogynous (in that the stalked, more or less hemispherical, shallowly 3–4-lobed female receptacles are apical). Rhizoids of two kinds, smooth and tuberculate.

The cells of the gametophyte with numerous small chloroplasts. The thallus branching dichotomously; without a conspicuous conducting strand (and with no dark line associated with there midrib); with ventral scales (these imbricate, purplish, with a row on either side of the midrib, ephemeral); without gemma-receptacles; with conspicuous dorsal pores. The pores complex (small, barrel-shaped, comprising 4–5 rings each of 4–6 cells, superimposed to appear as two rings from above). The thallus containing conspicuous cavities. The air chambers in a single layer in section. The ventral surface green, or purplish black.

The plants dioecious, or bisexual (then monoecious).

The gametangia and capsules borne externally. The archegonia and sporophytes with a pseudoperianth. The gametangia borne on special ‘receptacles’. The female receptacles arising terminally, stalked (the peduncles to 5 cm long, reddish below); shallowly 3–4 lobed on stalks to 5 cm. The male receptacles arising terminally stalked (more or less circular on the 1–2 cm stalk, the latter with two rhizoid furrows).

Sporophyte. The sporophyte elevated by elongation of the seta, with no intercalary meristem. The capsule wall 1 layered (with numerous annular thickenings). The capsule dehiscing irregularly. The spores reddish-brown, corsely reticulate, unicellular when shed. Elaters present; bispirally thickened; free. Ecology. Occurring in basic habitats.

British representation. 1 species (P. quadrata); England, Wales, Scotland, and Ireland.

Classification. Class/Division Hepaticae. Subclass/Class Marchantiidae. Order Marchantiales. Family Marchantiaceae.

Illustrations. • P. quadrata: Jameson, in Macvicar (1926). • P. quadrata (as commutata): Pearson fig. CCXI (1902). • P. quadrata (as commutata): Pearson fig. CCXI legend.

To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2005 onwards. The liverwort genera (Bryophyta: Hepaticae and Anthocerotae) of Britain and Ireland. Version: 24th February 2016.’.