Most people have at least a passing acquaintance with mosses and lichens, but what about liverworts. What is a liverwort anyway? Liverworts are a group of primitive, nonvascular plants containing only one or a few layers of cells, and tend to grow in thin, flat sheets. Liverworts are found only in moist environments due to their lack of vascular tissue (i.e., xylem and phloem) and their inability to store water. This means if you’re interested in finding one, you should look along the banks of freshwater streams and ponds, on rocks regularly splashed by spray from waterfalls or rivers, or in places where the precipitation regime is such that surfaces rarely dry out. 

But what’s with the name? The term “wort” is actually just an old English word for plant, and has been retained in many modern plant names (e.g., St. John’s wort, glasswort, and mugwort). The “liver” part of the name originated from early herbalists who thought the shape of one of the liverworts had some resemblance to a liver, and might therefore have some use as medicine for liver ailments. 

The liverwort pictured here is common liverwort, and can be found worldwide in a variety of climates, from tropical to arctic. All liverworts are divided into two groups: leafy and thalloid. Common liverwort (Marchantia polymorpha) is a type of thalloid liverwort. Leafy liverworts have only a single layer of cells, while thalloid liverworts are a little more complex, with tissue several layers thick, and can form large colonies over the surfaces on which they grow. This multi-layered sheet of tissue is referred to as a thallus. The upper side of the thallus is green and photosynthesizes, while the underside has hair-like cells called rhizoids that absorb water and anchor the thallus to the ground (liverworts lack true roots). The upper surface of the thallus is also covered with tiny pores, which allow for gas exchange in the absence of stomata, which are found on the leaves of vascular plants but which liverworts lack.

Like mosses, liverworts reproduce using spores rather than seeds, and in order for reproduction to be successful, the plants need to be covered in a thin layer of water to allow the sperm to travel between plants. Occasionally, small parasol shaped structures can be seen growing from the thallus of a common liverwort. Those that are palm tree shaped are generated by a female plant and will produce female gametes, while those that are only slightly segmented (I’ve seen this referred to as “doily” shaped) are generated by male plants and will produce male gametes. Common liverwort can also reproduce asexually. Small saucer-shaped cups on the surface of the thallus contain small rounded objects called a gemmae; each gemma is capable of growing into a new independent thallus if conditions are correct. Gemmae can be dispersed by rain, insects or small mammals. Marchantia is the only native genus of thalloid liverworts that produces gemmae cups.