5.2: Liverworts

Liverworts, Phylum Marchantiophyta

Gametophyte Morphology

There are two distinct type of liverworts: leafy liverworts and thalloid liverworts. Leafy liverworts have leaves arranged in a flat plane with a set of smaller underleaves.

Figure \(\PageIndex{1}\): The leafy liverwort Porella has larger leaves running opposite each other down either side of the stem, making the liverwort look flat. The photo on the right shows a microscopic view of the leaves. Notice that there is no central rib on these leaves, which helps distinguish leafy liverworts from mosses. Photo on the left by Maria Morrow CC-BY-NC. Photo on the right by Rafael Medina, CC-BY-NC.

Figure \(\PageIndex{2}\): Another distinguishing feature of leafy liverworts are their underleaves. These are smaller, scale-like leaves that run along the center of the thallus on the underside. This image shows the underside of a Porella navicularis thallus covered with smaller underleaves. Photo by Howard Bruner, CC-BY-NC.

Thalloid liverworts have no leaves and their gametophytes look more similar to hornwort gametophytes. Another similarity to hornworts is the presence of simple pores for gas exchange (no guard cells, meaning pores are permanently open). Unlike hornworts, liverwort cells have multiple chloroplasts. Many liverwort gametophytes produce asexual clones called gemmae.

Figure \(\PageIndex{3}\): The thalloid liverwort Lunularia has crescent-shaped gemmae cups. Several of these are present in the image above. The small green pebble-looking structures within them are asexual clones of the thallus called gemmae. Liverwort gametophytes do not have stomata. Instead, the thallus is covered in raised bumps where simple pores are located. Photo by Maria Morrow CC-BY-NC.

Figure \(\PageIndex{4}\): The thalloid liverwort Marchantia has many complex structures. A group of gemmae cups are circled in the lower left corner. There are several brown palm tree-like structures circled and labeled as archegoniophores. These are the part of the gametophyte where archegonia are produced. When fertilized, the sporophyte will grow within the archegonium and emerge on the underside of the archegoniophore. Photo by Maria Morrow CC-BY-NC.

Figure \(\PageIndex{5}\): This image shows structures from both gametophytes. The antheridiophore is on the left, with a flat surface that water can splash onto. This carries sperm from the embedded antheridia to other sufaces (hopefully, to an awaiting egg). On the right, there is a fertilized archegoniophore. The palmtree-like branches have lifted upward. Beneath them, yellow sporangia emerge at the tips of transparent setae. Photo by Felix Riegel, CC-BY-NC.

Figure \(\PageIndex{6}\): A micrograph showing a cross section through the top of an antheridiophore. An antheridium has been indicated with a black box. Inside the antheridium, sperm will be produced (in the darkened area), though none are visible here. Photo by Maria Morrow CC-BY-NC.

Figure \(\PageIndex{7}\): A close up of an archegonium (outlined in orange) on the underside of an archegoniophore. The egg within the archegonium has been indicated by an arrow. Photo by Maria Morrow CC-BY-NC.

Sporophyte Morphology

Liverworts produce a single sporangium (capsule) at the end of a seta, which is often fragile and transparent. The sporangium dehisces, splitting into four pieces. In thalloid liverworts, the sporophyte may emerge directly from the thallus or, as is the case with the genus Marchantia, from complex structures called antheridiophores and archegoniophores. In this case, the sporophytes emerge from the underside of the archegoniophore.

Figure \(\PageIndex{8}\): Liverwort sporophytes are comprised of a stalk called the seta and, at the top of the seta, a single capsule where spores are produced. In this liverwort (perhaps a Fossombronia), the sporophytes are short and stout with globose capsules. Photo by Maria Morrow CC-BY-NC.

Figure \(\PageIndex{9}\): Some liverwort sporophytes have long, transparent setae, making them look almost like molds. The capsules of the sporophytes in the image are also elongated. They will dry out and dehisce, pealing back into four triangular pieces and releasing the spores. Photo by Maria Morrow CC-BY-NC.

Figure \(\PageIndex{10}\): A sporophyte produced on the underside of an archegoniophore. It emerges upside-down and will eventually become more horizontal as the branches of the archegoniophore raise upward. The sporophyte attaches to the gametophyte at a region called the foot. It grows within the archegonium, now labeled as the calyptra. From the foot a short seta emerges, topped by a large sporangium. The sporangium is filled with rows of haploid spores produce by meiosis. Between these rows of spores, long elaters await the dehiscence of the sporangium to aid in spore dispersal. Photo by Maria Morrow CC-BY-NC.

Marchantia Life Cycle

Figure \(\PageIndex{11}\): The Marchantia polymorpha life cycle. Starting with meiosis (left, center): Meiosis produces haploid spores which will grow by mitosis into either male or female gametophytes. Either of these gametophytes can produce gemmae cups--structures that hold asexually produced clones of the gametophyte (gemmae). Male gametophytes produce tall structures with flat tops called antheriodiophores. Many antheridia are embedded in the flat top of the antheridiophore. Antheridia produce biflagellate sperm by mitosis. Female gametophytes produce palm tree-like structures called archegoniophores. These archegoniophores produce archegonia on the underside of the 'branches', each with a single egg that was produced by mitosis. When water hits the flat top of the antheridiophore, it can splash sperm onto a female gametophyte. If the sperm are able to swim through the water to an egg, fertilization occurs producing a diploid zygote. The zygote grows into a sporophyte from within the archegonium. The remaining archegonial tissue is called the calyptra. A mature sporophyte will have a sterile stalk called a seta and a sporangium containing cells that will undergo meiosis to make haploid spores. Within the sporangium (not pictured), long twisted structures called elaters help disperse the spores aerially. Drawing by Nikki Harris CC-BY-NC with colors and labels added by Maria Morrow.

Attribution

Content by Maria Morrow, CC-BY-NC.