By Guru V Radhakrishnan (@guru_vighnesh)
The umbrella liverwort (Marchantia polymorpha) is a noxious weed. It grows in most gardens, paths and glasshouses, and is found in almost all parts of the world – from tropical to arctic climates. There are several subspecies of this plant that are found in damp habitats at varying altitudes. The plants themselves are of little economic importance, but in ancient times, liverworts were believed to be a cure for liver diseases. This ancient idea was part of the “doctrine of signatures” . This doctrine suggested that plants resembling a certain body part could be used to treat ailments of that body part. Although an interesting idea, there is no scientific evidence to support this claim.
The genus name Marchantia comes from the name of a French botanist Nicholas Marchant. And the species name polymorpha is Greek for many forms (polymorph). This name comes from the ability of these plants to take up different sizes, shapes and shades of green depending on the conditions they are grown in.
These liverworts, along with hornworts and mosses, form a group of plants known as the bryophytes. Of all the groups of plants that are alive today, the bryophytes are suggested to be the most ancient. The earliest ancestor of the bryophytes is thought to be ancestral to all other land plants . Because of the evolutionary position of the liverworts, scientists are using M. polymorpha to understand how early land plants evolved.
Like all other land plants, liverworts also have two alternating stages in their life cycle, known as the sporophyte and the gametophyte. In ferns, seed plants and other higher plants, the structure we would recognise as being “the plant” is the sporophyte, and the gametophyte stage is short-lived. For example, in seed plants the gametophytes are the egg and sperm cells within the flowers or cones on the sporophyte. However, in bryophytes the gametophyte stage as the dominant part of their life cycle. In M. polymorpha, the gametophyte structure is known as the thallus – which is the green, flat plant body (see photo). The small sporophyte forms within the female sexual structures of the gametophyte, which is an umbrella-shaped structure called the archegonial head (see figure below).
M.polymorpha is dioecious, which means that there are separate male and female plants. The male plants only produce male sexual structures, which are known as antheridial heads, and female plants produce only female structures. When water falls onto the male plant, sperm can be released from the antheridial head and swim through the water to reach the archegonial head on a nearby female plant. Here, the sperm cells can fertilize egg cells leading to the production of millions of spores on the archegonial head. These spores are minute and can be transported by wind. Once the spores settle on a suitable part of the ground, they germinate into individual gametophytes.
In addition to sexual reproduction, there is also asexual reproduction in liverworts. Here, specific cells separate themselves from the plant and start forming vegetative buds called gemmae. These gemmae originate from single cells and undergo multiple cell divisions. They accumulate in a specialised structure called a splash cup (or a gemma cup; see figure). These cups contain about a hundred gemmae. When water splashes onto these cups, the gemmae are catapulted out of the splash cups and fall on to the ground. They then continue to grow and become new plants.
These vigorous and efficient methods of reproduction help these plants to quickly colonise a patch of land after a fire. This is also why it is so hard to get rid of these plants from our gardens!
About the author: Guru is a PhD student at the John Innes Centre, UK studying how plants evolved the ability to interact with microbes. Follow him on twitter (@).
The umbrella liverwort is the Organism of the Month.