2.5.2.2: Marchantiophyta

Learning Objectives

• Use morphological traits and cellular components to distinguish between liverworts and other bryophytes.
• Identify structures and phases in the Marchantia life cycle; know their ploidy.

Liverworts, Phylum Marchantiophyta

The liverworts, formerly the Hepatophyta, got their name from their thalloid gametophytes being compared to the shape of a liver. However, many liverworts produce leafy gametophytes. This group is often presented as a basal lineage of bryophytes due to the lack of stomata present in either stage of the life cycle (among other traits). However, recent genetic evidence does not support this and instead places mosses and liverworts as sister taxa. Liverworts have a global distribution and can be found in many habitats, including a few desert and even arctic species. There are around 5,000 described species of liverworts, though estimates put this at about half of the actual number of species. The type genus for this group, Marchantia, is a common invader of greenhouses and potted plants.

Gametophyte Morphology

There are two distinct type of liverwort gametophytes: leafy liverworts and thalloid liverworts. Leafy liverworts have leaves (though not true leaves, as they lack vascular tissue) without a central costa, which you will see in mosses. The leaves are arranged in a flat plane (Figure \(\PageIndex{1}\)) with a set of smaller underleaves (Figure \(\PageIndex{2}\)).

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 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. Rhizoids in this group are unicellular. Asexual clones, called gemmae (sing. gemma), are sometimes produced in structures called gemmae cups (Figure \(\PageIndex{3}\)). These are haploid and genetically identical to the parent thallus.

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.

The thalloid liverwort Marchantia has complex reproductive structures. Palm tree-like structures called archegoniophores are formed from the haploid gametophyte tissue. Archegonia are produced on the underside of the extending arms. When fertilized, the sporophyte will grow within the archegonium and emerge on the underside of the archegoniophore (see the right side of Figure \(\PageIndex{4}\)). The antheridia are produced in a separate stalked structure with a flat top called an antheridiophore (see the left side of Figure \(\PageIndex{4}\)). Water droplets splash onto the flat top, dispersing flagellated spores from the embedded antheridia.

Figure \(\PageIndex{4}\): This image shows gametangia producing 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.

Sporophyte Morphology

Liverworts produce a single sporangium (also called a capsule) at the end of a seta, which is often fragile and transparent (Figure \(\PageIndex{5-6}\)). The seta does not elongate until after the sporangium has formed. The sporangium dehisces, splitting into four valves (Figure \(\PageIndex{7}\)). In thalloid liverworts, the sporophyte may emerge directly from the thallus or, as is the case with the genus Marchantia, from under archegoniophores. In leafy liverworts, the sporophyte often emerges laterally from the thallus. Within the sporangium, there are elaters (true unicellular elaters, Figure \(\PageIndex{8}\)) that aid in spore dispersal. Unlike the hornworts, there is no columella. Unlike the mosses, the capsule lacks an operculum.

Figure \(\PageIndex{5}\): 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 \(\PageIndex{6}\): 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.

Figure \(\PageIndex{7}\): Noteroclada confluens capsule dehiscence. The capsule has split into four valves. Many elaters, as well as a few green spores, remain. Photo by George Shepherd, CC BY-NC-SA.

Figure \(\PageIndex{8}\): Noteroclada confluens elaters and spores. The elaters in the capsule are specialized elongated cells with a spiral thickening in their walls. In this case they are hygroscopic and twist and actively move as they dry out, helping to liberate the spores gradually. Photos and caption text by George Shepherd, CC BY-NC-SA.

Marchantia Life Cycle

Marchantia polymorpha is a thalloid liverwort with a complex life cycle (Figure \(\PageIndex{9}\)). Asexual reproduction is accomplished through the production of haploid gemmae from the gametophyte thallus. Sexual reproduction occurs from dioecious gametophytes: archegoniophores and antheridiophores are produced on separate gametophytes. Sperm splashed from the antheridial head swim through the water with their dual flagella to reach an egg at the base of an archegonium. These archegonia are situated on the underside of the archegonial head. The diploid zygote grows within the archegonium, surrounded by its remaining tissue (the calyptra). As the sporangium develops, meiosis occurs simultaneously to produce haploid spores. A short seta extends to push the developed sporangium outward, lifting the arms of the archegoniophore. The sporangium dehisces into four valves, exposing the elaters to the external environment where they rapidly twist, flinging the haploid spores into the air. These haploid spores can germinate and grow into male or female gametophytes.

Figure \(\PageIndex{9}\): 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.

Content by Maria Morrow, CC-BY-NC

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