Flowering plants are the insects of the plant kingdom. They dominate every terrestrial environment except the northern coniferous forests, and they make up almost 90% of all plant species. It’s no wonder then that most people think of little else when they think of plants.
But their older, less prominent cousins, an example being the liverworts, are worthy subjects for interest too and by contrast give us clues to the successes of flowering plants (and their first cousins the gymnosperms). I recently found a trove of liverworts while searching for a rather lovely flowering plant, the snowdrop bush. I believe my liverworts are Marchantia polymorpha, which is probably a good guess because this liverwort is the most widely distributed one on Earth, occurring from the tropics to the arctic. Liverworts get their common name from the Anglo-Saxon lifenwyrt; the root lifer means “liver” and wyrt means “wort” or plant. The name has been commonly used for these plants because their “leaves” resemble the shape of the animal organ, liver; traditionally liverworts have been used by herbalists to treat problems of the liver.
“Snowdrop bush, Styrax redivivus”
Finding these ones was a little lucky, not because they’re rare but because they’re so small and (apparently) unremarkable that they’re easily overlooked. They are located on a hillside that faces due north on a mineral soil that’s quite moist this time of year and that 18 months earlier experienced a wild fire. A trail runs for 1000 feet along this hillside, which does not vary much in its slope or orientation to the Sun or, as far as I can see, in its flora, soil or moisture. The liverworts are scattered along the trail in moist, sheltered nooks generally in association with small mosses.
“Misty trail in morning”
“Redheart or greenbark ceanothus, Ceanothus spinosus and dead California bay, Umbellularia californica”
Liverworts are primitive plants. There are somewhere in the neighborhood of 6000 to 8000 species (compared with around 260,000 flowering plant—angiosperm—species). Most of them live in the tropical zones. All are found in moist environments; almost all of these are terrestrial, though one genus is aquatic. Liverworts will colonize areas disturbed by natural occurrences such as fire, often as a confluent mat, and so they aid in reducing soil erosion. But they can be a nuisance when they invade gardens and lawns. They are related to mosses and hornworts; all three have traditionally been considered members of the same plant division (Bryophyta), but recent taxonomic changes have relegated the three to their own divisions.
In the annals of life on Earth, liverworts are probably one of the first plants to have moved from the oceans to colonize the land, not quite 500 million years ago. As plants, they long ago evolved the ability to harvest the Sun’s energy and convert it to simple sugars and then to whatever biochemical building blocks their metabolisms require. But life on land has different demands than life in the sea. Land plants must cope with variations in temperature, light level, and moisture; and they need to be able to anchor themselves to the soil and extract water and minerals from it.
Liverworts, unlike higher plants, have no leaves or stems and no true roots. Their most prominent feature is an above-ground, leaf-shaped structure called a “thallus” that has on its upper surface a waxy cuticle layer that protects it from drying out. It also has epidermal pores that are analogous to the stomata found in higher plants that absorb carbon dioxide and release oxygen during photosynthesis. The undersurface of the liverwort thallus has root-like structures called “rhizoids” that help anchor the plant to the soil. But liverworts have no vascular system (no xylem, no phloem) and so no means of actively absorbing water and minerals and no means of translocating the products of photosynthesis to other parts of the plant.
“Liverworts, Marchantia polymorpha”
Liverworts produce no flower, seed or fruit. Their life cycle is an alternation between the vegetative (thallus) stage and a (sexual) reproductive stage. The sexual stage is relatively short lived and occurs after specialized male and female structures that produce sperm and egg grow on the thallus. The sperm is motile and must swim to the egg to fertilize it. When fertilized, the egg ripens to form many spores with potentially novel traits. These spores are then released to the wind and deposited near or far from their origin, and liverwort life then begins anew as the spores germinate to generate new plants. The thallus can also produce “gemmae” cups that contain cells that develop into new plants that are exact clones of the mother plant. The parasol-shaped bodies in the photos are the female reproductive structures (archegonia); male reproductive structures (antheridia) and gemmae cups are not present.
“Archegonium emerging from liverwort thallus”
“Group of archegonia”
Liverworts were not a stepping stone in the evolution of flowering plants. We know this from chloroplasts, the light absorbing bodies where photosynthesis occurs. All plants have chloroplasts, and all chloroplasts have the same genes, and these genes control the same essential functions. But in the liverwort chloroplast, the physical order of some genes is different from that of flowering plants (and all the other groups of plants). Additional molecular evidence also points to the conclusion that liverworts are an ancient, lone group that represents an evolutionary deadend.
But from an evolutionary standpoint, liverworts are a pretty fair first attempt. They still exist today and do everything they need to do to survive on land; they just don’t do it very well. Their lack of roots and a vascular system places critical limitations on them. To acquire sufficient water, they must wait for it to seep into them (by diffusion and capillary action). This essentially dictates that they remain forever small and inhabitants of moist areas only, restricting their ability to colonize new, dryer habitats as well as compete with more genetically plastic plants. Vascular plants, in contrast, in nearly every shape and size imaginable, have colonized nearly every terrestrial habitat on Earth. And the liverwort life cycle that requires the spore to swim to the object of its desire is anything but efficient or elegant. Consider the apple flower and fruit. Could there be a better scheme? Motivated flying animals pollinate the flower and others harvest the fruit and sow the seed. If the liverworts made it to first base, the apple (and by extension, the flowering plants as a whole) made a home run.
“Liverworts in the undergrowth (Stelaria media?).”