Preissia quadrata (Scop.) Nees   

Kingdom: Plantae
Phylum: Bryophyta
Class: Hepaticopsida – liverworts
Order: Marchantiales

British distribution: Throughout Britain though rare in southeastern and central England.
World distribution: Europe, Asia, N.Africa, N.America.

Preissia quadrata is one of our more frequent, large, 'thallose' (i.e. not leafy) liverworts. It is confined to lime-rich areas, naturally (as shown here) in shady places amongst limestone rocks, but also not uncommon in crevices of old walls, growing on the mortar, and in bare, shady places on riverbanks where there is lime in the alluvium. The rather dull or greyish green colour with purple edges to the thalli is characteristic.

As is typical of the Marchantiales, the gametophyte thallus is dichotomously branched and many cells thick, with tissue differentiated into the chlorenchyma - an upper region of small cells containing chloroplasts, and a lower parenchyma of larger cells containing starch grains. There are well developed epidermises and there are purple or dark-red scales on the ventral surface. Numerous air pores (somewhat analagous to stomata in the sporophyte generation of vascular plants) occur in the upper epidermis, the pores being compound in this species. The thallus is typically up to 10 or 12 mm across.

The vegetative thallus is shown here, but young, developing receptacles (male?) are also visible at tips and joints in the thalli. Both the male receptacles (containing the antheridia) and the female receptacles (containing the archegonia) are shortly stalked when mature, like tiny umbrellas, the female distinctly lobed. Preissia can be monoecious (male and female gametes produced from the same plant) or dioecious (male and female gametes produced on separate plants) and a number of small sporophytes may develop on each fertilised female conceptacle.

Asexual reproduction is by regeneration from broken pieces of thallus, the thallus being brittle and no doubt easily transported by water and other agents. Gemmae are not produced.

This is the only species in the genus.

Photographs: Perthshire, Scotland, April 2003.

© A.J. Silverside
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