Lower plants as simple systems for engineering
Liverworts are sister clade to flowering plants, and believed to have diverged up to 500 million years ago. They include about 9,000 species that are divided into three groups, simple and complex thalloid and leafy liverworts. The dominant forms of the plants are haploid, with a short-lived sporophyte stage (diploid). The thalloid liverworts have relatively simple morphologies, being derived from spores or vegetative propagules that germinate to produce flattened tissues with meristematic apices that undergo dichotomous branching. The dorsal and ventral surface are distinct, with single-cell rhizoids produced on the lower surface, and photosynthetic tissues found on the upper. Complex thalloid liverworts can form specialised dorsal air chambers with pores for gas exchange. Sexual reproduction requires formation male and female gametes via specialised organs, and leads to spore development and dispersion.
The morphological simplicity of the liverworts sets them apart from other land plants. Speculations about the form of the earliest terrestrial plants (an example is shown above) invoke thalloid-like features with aerial spore-bearing structures. Relatively uncomplicated patterns of cellular growth and differentiation have been harnessed to generate morphological diversity across the liverworts. Below are shown examples from within the thalloid liverworts. The plants open form of development, ease of culture, microscopic analysis, bare and stripped down genome architecture makes them unique models for understanding the fundamentals of plant growth - and as test-beds for reprogramming plant form and function.
Examples of thalloid liverwort plant species
A conspicuous thallose liverwort that often forms mats. D. hirsuta is somewhat aromatic, with broad, flat, semi-translucent, dichotomously branching thalli up to 2 cm wide. The thallus is dull and dark green, sometimes yellow-green. It lacks air pores (except sometimes a few indistinct pores near the tip) and has no network of lines on its upper surface. The thallus margins and undersides bear scattered, stiff bristles. The male receptacles are bristly and borne on a very short stalk. Female plants have long-stalked, bristly receptacles borne at the thallus tip; each receptacle is flat-topped with 6 to 12 short, spreading lobes.
T. hypophylla has long, narrow, leathery, strongly aromatic thalli, 2–5 mm wide, dark green or slightly bluish-green above with dark purple margins. The upper surface is flat, with weak reticulations and conspicuous dots (air pores). When dry, the thallus rolls up to form a black, worm-like tube. Male receptacles are sessile on very short branches on the underside of the thallus. Unique, shiny, black, mussel-like, 2-valved involucres in which the capsules are enclosed are borne beneath the tips of the thalli
This is one of our largest Riccia species, forming persistent rosettes or mats up to 2.5 cm across. Branches are oblong to slightly curved, widest above the middle, up to 2.5 mm wide, usually shiny, greyish-green above, and frequently tinged reddish on the margins. The median groove is narrow at the extreme tip, but broad and flat- bottomed behind. Margins are swollen and rounded in section, and may have very short, colourless hairs near the tip. Old parts of the thallus become channelled and often bear capsules.
P. neesiana is vegetatively very similar to P. epiphylla with thalli about 1 cm wide, although it has a greater tendency to develop reddish or purple tints and has a sharp, aromatic smell. It is dioicous, with separate male and female individuals. Females develop a short, vertical tube of tissue around the sex organ. This tube is not closely toothed at its mouth.
L. cruciata forms large, dichotomously branching thalli up to 12 mm wide. Its shiny, faintly lined, pale green surface is dotted with tiny, though relatively conspicuous
air pores. L. cruciata is the only thallose liverwort with crescent-shaped receptacles which contain green, disc-like gemmae. When dry, thalli can become yellowish, the margins inroll, and the reticulations almost disappear. Capsules are very rare, and are borne on a distinctive, cross-like, stalked female receptacle.
P. endiviifolia usually has green or even blackish-green thalli up to about 1 cm wide, without reddish tinges. In autumn and early winter they develop numerous, narrow (to about 6 mm wide) branches at the tips which are sometimes so abundant that they obscure the broader thalli on which they have developed. Plants are dioicous. A vertical tube surrounds the female organ. This tube has a closely toothed mouth.
C. selebrosum has large (up to 12 mm wide), very conspicuous thalli that often form extensive mats. The thalli are strongly aromatic, flat, dichotomously branching. The surface is dull in appearance. C. salebrosum has thalli with conspicuous grooves defining the lines on the surface. Male plants have sessile, terminal cushions. Fruiting female plants bear terminal, stalked, conical receptacles with short descending lobes.
This medium-sized thallose liverwort has dichotomously branching, flat, leathery thalli up to 8 mm wide, often forming loose mats. Plants are weakly aromatic, and the upper surface of the thallus is pale or glaucous green, contrasting with the red or purplish margins. The surface is smooth, with a faint network of lines and inconspicuous air pores. The male receptacles form sessile, often purplish cushions. Female receptacles are borne on short, terminal stalks and are green and hemispherical with 4 to 7 spreading or deflexed lobes. Air pores are quite conspicuous on the receptacles.
This medium-sized thallose liverwort has dichotomously branched, flat, leathery thalli up to 1 cm wide, often growing in mats. Plants are not aromatic, although the thallus has a distinctively hot taste when nibbled on the tip of the tongue. The upper surface of the thallus is usually dark green, and conspicuously dotted with minute air pores which are raised above the surface. The margins are tinged dark purple. Male and female receptacles are stalked and may be borne on the same or separate plants; male receptacles are flattened, disc-like and unlobed; female receptacles are hemispherical with 4 broad ridges, each ending in a short lobe, and the surface is dotted with air pores.
Marchantia polymorpha subsp. ruderalis
This common liverwort has thalli that are relatively thick and leathery, up to 20 mm broad, with a median black line which is irregularly developed and sometimes interrupted. It has dichotomously branching, prostrate or ascending flat thalli often growing in dense mats. The upper surface lacks a black midrib, is covered with conspicuous dots (air pores), and bears scattered, cup-shaped gemma receptacles. Male plants have distinctive, stalked, flat-topped, disc-like receptacles, the margins with short, rounded lobes. Female receptacles are similar, but the lobes are finger-like and spreading.
There are three subspecies in the UK, montivagans and polymorpha typically grow in natural habitats. Subsp. ruderalis is the most abundant, and is almost always found in man-made or disturbed habitats, particularly gardens, glasshouses and plant nurseries where it can be a serious weed of plant pots; it is also common on waste ground, between paving stones, on footpaths, on brickwork, and on old bonfire sites.
Identified by its dull, velvety and glaucous thalli with simple, very inconspicuous pores and by its reddish pink or purple scales. Thallus 2.3-5 mm wide, usually bluish green, sometimes whitish or partly tinged with purple, with narrow purplish margins; apical adventitious branches rare. Epidermis covered with a hydrophobic deposit; epidermal cells 31-40 × 20-24 µm. Epidermal pores (90-) 130-280 per mm2, not raised above epidermis, 5-6 µm diam., bounded by a single ring of 4-6 cells, often irregular in shape, radial walls ± thickened. Ventral scales purplish, sometimes borders lighter in color, of cells of the same size and shape.
Notes: Images of liverwort plants were taken using extended focus macrophotography techniques by Jim Haseloff, with access to the collection of liverwort species held at the Cambridge Botanic Gardens, maintained and curated by Caroline Sinnett-Smith, Alex Summers, Sam Brockington and colleagues (https://www.botanic.cam.ac.uk).
More images: (https://www.flickr.com/photos/haseloff/collections/72157706090662481/) - Flickr galleries with collections of liverwort images.
Technical details: (https://haseloff.plantsci.cam.ac.uk/synbotany/macrophotography/index.html) - for more information about extended focus image capture and processing.
(http://www.britishbryologicalsociety.org.uk) - for details of the Britsh Bryological Society Field Guide to the Mosses and Liverworts of Britain and Ireland. The site provides PDF extracts and allows purchase of the book - which is a user-friendly guide to identifying British and Irish bryophytes in the field. It contains colour photographs and black and white drawings showing what species look like, together with notes on how to identify and distinguish similar species, and habitat notes and distribution maps showing where they occur. An excellent field guide.