Marchantia polymorpha subsp. ruderalis

Liverworts are non-vascular plants which, along with the mosses and hornworts, make up the bryophyte group. Of the different bryophytes, there are 763 species of moss, ~300 species of liverwort and just four species of hornwort in Britain and Ireland. Liverworts are split into two main groups based on their structure: leafy liverworts, which have leaves and a stem, and thallose liverworts, which don't have leaves or a distinct stem, but have flattened, lobed, leaf-like bodies called thalli.

Hornworts are superficially very similar to the thallose liverworts, and are often regarded as 'honorary liverworts', however they differ slightly to liverworts in their reproductive methods and structures, whereby asexual reproduction occurs primarily via fragmentation as lobes separate off from the main part of the thallus. In hornwort sexual reproduction, archegonia (female part) and antheridia (male part) are produced in rows just beneath the upper surfaces of the gametophytes and the sporophytes grow into long, horn-like structures, with the sporangium located at the tip. These sporophyte 'horns' are very long lived (unlike liverwort sporophytes) and can grow quite long. Hornworts are also distinguished from liverworts by the presence of stomata and cuticle, which is also seen in the mosses.

The hornwort Anthoceros agrestis © Štephán Koval (2010) - https://bit.ly/2r7UuzU

In liverworts and hornworts, reproduction occurs via the alternation of generations where the gametophyte stage is dominant (unlike other plants such as clubmosses and ferns which have dominant sporophyte stages) reproduction can occur sexually or asexually. The way in which liverworts reproduce also depends on whether they are leafy or thallose. The sexual reproduction of liverworts occurs predominantly during the winter months when conditions are wetter. This is because their spore dispersal is water dependent.

Leafy liverworts always have two rows of partially overlapping 'leaves' which don't have midribs, but do often have folds and lobes (unlike the 'leaves' of mosses). Rhizoids are present on the underneath of some underleaves, which help anchor the plants. The male and female reproductive structures (archegonia and antheridia) of leafy liverworts are inconspicuous, and are produced in cup-like structures made out of some modified leaves on separate plant bodies. At maturity, the sporophyte capsule is pushed out from among the leaves as the seta (stalk) elongates.

Sporophyte development in leafy liverworts.
Illustration © Mosses & Liverworts of Britain & Ireland/British Bryological Society

Some of the best-known species of thallose liverworts are in the Marchantia genus. Asexual reproduction in thallose liverworts occurs via a rather cool system, in which structures called gemma cups form on the top of the liverwort gametophyte (thallus). These gemma cups colonially produce identical 'leaflets' (gemmae) within them, which are then dispersed into the wider environment via rain splashes - raindrops may splash the gemmae up to 1m away! While the gemmae are in the gemma cups, lunularic acid inhibits their further development, but each gemma remains capable of growing into a new thallus as soon as it leaves the cup.

Asexual and sexual reproduction in thallose liverworts
(© Plant and Microbe Diversity by Harriet Jones and Anthony Davey)

In the sexual reproduction of thallose liverworts, there are two types of gametophores (umbrella-like structures on stalks which arise from the thallus): the antheridiophore (male gametophore) and the archegoniophore (female gametophore). The two gametophores are shaped differently, with the male 'umbrellas' being disc shaped and the female receptacles shaped like miniature palm trees.

Reproductive structures of thallose liverworts.
Illustration © Mosses & Liverworts of Britain & Ireland/British Bryological Society

Fertilisation causes the development of sporophytes anchored to the underside of the archegoniophores. Meiosis occurs in certain cells within the sporophyte, creating haploid spores, but the rest of the cells do not undergo meiosis, instead remaining diploid and developing into long, pointed elaters with spiral thickenings. The elaters are hygroscopic and therefore change shape in the presence of water. This causes the elaters of Marchantia to rapidly twist and untwist, causing the  spores to be shaken out of the sporophyte, ready to be dispersed by water.

Marchantia polymorpha is a large, thallose liverwort that is widely distributed around the world, and is also found on the UEA campus. It is very variable in appearance and has several subspecies, 3 of which are found in Britain and Ireland. The subspecies I found at UEA is Marchantia polymorpha subsp. ruderalis (aka the Common Liverwort). As with all liverworts, M. p. ruderalis has a horizontal growth structure and no vascular network to transport water or nutrients, instead relying on its large surface area and preference for damp conditions. Due to the changeability of environmental conditions, liverworts are well adapted to tolerate desiccation; their flat structure maximises photosynthetic abilities and they minimise water loss by only having pores (for respiration and photosynthesis) on their upper side.

Marchantia polymorpha subsp. ruderalis with numerous
gemma cups present on the upper-side of thallus.
Found on the UEA campus 03/02/18.

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