Bryant is concluding his stint as Botany Photo of the Day work-learn student with a series on liverworts. He is the author of today's entry, and writes:
Today's photographs are courtesy of Robert Klips (aka Orthotrichum@Flickr) and BlueRidgeKitties @Flickr respectively (image 1, image 2). The two photographs are of Marchantia polymorpha, a near-cosmopolitan distributed member of the Marchantiaceae. It is frequently encountered, hence one of its common names: "common liverwort". Another name is umbrella liverwort. Marchantia polymorpha is a dioecious (or heterothallic) thallose liverwort, most often found in moist to wet areas near streams, bogs, and ponds. The thalli of Marchantia polymorpha can grow to around 10cm long and roughly 2cm across, and often form a flattened mat or rosette.
This being the first post in the series, I thought a little background on the natural history of liverworts might be of interest. There is no formal consensus on the classification of liverworts above the family level, however they are most often categorized as members of the division Marchantiophyta. Liverworts are considered to be some of the earliest true plants to colonize the land--the earliest fossil records of liverwort spores date back roughly 470 million years! By comparison, the earliest flowering plants are currently dated to 140 mya. The rather unfortunate common name of "liverwort" stems from the resemblance of cell structure in the leaves/thallus of some liverworts to those found in animal livers. This resemblance also caused liverworts to be used in early medicine to treat liver ailments, in accordance with the doctrine of signatures. Another distinguishing feature of liverworts is the single-celled root-like structures, known as rhizoids, which represent an early stage in the evolutionary development of roots in land plants. The life cycles of members of the Marchantiophyta vary greatly-- liverworts can be monoecious or dioecious (i.e., either both male and female in one individual, or each individual either male or female).
In some species (like Marchantia polymorpha), individuals may reproduce both sexually and asexually. The first image shows the female gametophytes, which produce the palm tree-shaped structures containing archegonia on the underside of each archegonial head. Sperm produced by the antheridia, located on the male gametophyte, fertilizes the ovum within the archegonia resulting in the production of a sporophyte. This may be better understood by looking at this useful diagram of the life cycle of some liverworts.
The second image shows a close up of the gemma cups containing gemmae, which are vegetative clones of the mother plant that are largely dispersed by wind or rain (allowing asexual reproduction to occur). In the image above you can see some of the gemmae have splashed out of the cups and on to the thallus!