Bryophytes: Liverworts

General Features of Bryophytes

We have now officially moved into Kingdom Plantae by just about anyone's taxonomy. We have also moved into terrestrial plants.

We first focus on the lowest forms of plants...the Bryophytes (phylums Bryophyta, Anthocerophyta, Hepatophyta...or classes Muscopsida, Anthocerotopsida, and Hepaticopsida within phylum Bryophyta...are you a lumper or a splitter?). All bryophytes are eukaryotic, have chlorophyll a and b as well as xanthophylls and carotenoid pigments for photosynthesis, store starch, have pectin-cellulose walls, mitosis is open (nuclear membrane disappears in mitosis), and cytokinesis is by formation of a cell plate along a phragmoplast. These characteristics are common for ALL members of Kingdom Plantae. All members have a sporic life cycle that is oogamous. In some ways, then, these bryophytes are very similar to each other and fundamentally similar to flowering plants.

The dominant phase of the bryophyte life cycle is the gametophyte (haploid). The small green plant you find as a moss or a liverwort is the haploid gametophyte. The gametophyte plant body is called a thallus, which means that it lacks xylem and phloem and therefore does not have true stem, true leaf or true root. The plant body may have conducting tissue and some of this has a familiar look (hydroids are similar and functionally very similar to xylem, and leptoids are similar and functionally very similar to phloem). Some gametophytes are erect and some are prostrate; but bryophytes lack lignin biosynthesis capacity, so they are never tall. The thallus is anchored to the substrate by rhizoids. A rhizoid is a hair-like structure that may be only one cell wide, though it can be more than one cell in length. There is no root in a bryophyte! The epidermal layers may posess cutin to prevent desiccation in the terrestrial environment, if so, the thallus has pores to allow for gas exchange.

For reproduction, the gametophyte produces gametangia. Both the antheridium and archegonium have a sterile jacket of cells, which better protects the gametes against desiccation in the terrestrial environment. The antheridium sterile jacket has a cap cell which disintegrates when turgor pressure rises, for example when the gametophyte is wetted with hypotonic rain water, allowing for sperm release into the surrounding water. The flagellated sperm are chemotactic and swim through free-water up a concentration gradient of the chemotactic agent to find the open archegonium. The archegonium sterile jacket also has a cap cell which disintegrates when turgor pressure rises (when surrounded by rain water for example). Likewise the neck- and ventral-canal cells disintegrate and ooze out the end opened by the cap cell. The diffusion of the breakdown products of these cells allows for chemotaxis by the sperm to swim to the egg. The egg is sessile and remains in the venter of the archegonium.

The egg and sperm unite in syngamy. The resulting zygote and developing diploid sporophyte is not photosynthetic and so is parasitic (dependent!) on the gametophyte for its nutrition. Bryophyte sporophytes consist of the haustorial foot (nutrition exchange area), a supporting seta (stalk, maybe with hydroids and leptoids), and sporangium (capsule_. The sporangium has a sterile jacket of cells surrounding the sporocytes. The sporocytes divide meiotically to produce haploid spores. The spores develop a wall impregnated with sporopollenin (a water-proofing agent) and are distributed in the terrestrial environment by wind. Spores landing in a suitable environment germinate into new gametophytes.

The sporophyte generally has no method for asexual reproduction. The gametophyte may reproduce asexually via gemmae in gemmae cups, bulbils budding from gametophyte surfaces, or fragmentation of the branching protonema.

The Liverwort Gametophyte

Liverworts come in two types: thallose and leafy. Most books detail the biology of thallose liverworts and ignore leafy liverworts. The leafy liverworts outnumber the thallose liverworts in the real world, so most books are not showing the most typical liverwort type to begin with. Moreover, most books use Marchantia as the example thallose liverwort. Among thallose liverworts, Marchantia is rather unique in having elongated and elaborated antheridiophores and archegoniophores.

The liverwort thallus is generally a flattened body. In thallose liverworts it is ribbon-shaped and often not vascularized; in leafy liverworts there is thickened axis with a prominent vascular (hydroid and leptoid) center with flattened leaf-like lobes on either side of the axis. These gametophytes are generally prostrate. The thallus may branch dichomously. In leafy liverworts, the leaf-like portions are a sheet of cells usually one-cell thick. Thallose liverwort gametophytes are many cells thick with chambers of filamentous parenchyma cells just under the upper epidermis, and a pad of cortical parenchyma cells beneath. In the upper epidermis there may be chimney cells surrounding pores. All liverworts are anchored by rhizoids and intertwining gametophyte thalli, if they are anchored at all. Some float as aquatics.

Liverwort gametophyte thalli may reproduce vegetatively by means of gemmae. These are produced in a more-or-less elaborated splash-cup on the upper epidermis. These give the appearance of a tiny nest. The "eggs" in the nest are the gemmae. Each gemma is a tiny gametophyte with two growing points from which new gametophyte tissue can arise mitotically. The splash cup works as expected. A drop of rain water falls into the cupule, some mucilage produced around the base of the gemmae swells with hydration and breaks off the connection between the thallus and the gemmae. The gemmae float freely into the drop of water in the cupule. The next drop of rain that falls into the cup will splash the gemmae out into the environment to either land in a suitable nearby environment, or be carried away in a film of flood water to a new location farther away.

In most liverwort gametophytes, another flap of tissue on the gametophyte encloses the gametangia. On a female thallus, the flap contains archegonia; on a male thallus, the flap contains antheridia. When rainwater collects in the flap, the cap cells burst and sperm are shed in the film of water on the gametophytes and the sperm must swim (or be carried en masse by bulk flow) to get to an open archegonium. Thus most liverwort gametophytes are dioecious (heterothallic) and are one gender or the other. The zygote which develops in the archegonium will only be found on the female thalli.

In Marchantia all of this sex happens almost "in the air." Rather than being produced in pouch on the gametophyte surface, the gametangia are attached to a fleshy receptacle and are lifted up on an elongated stalk from the surface of the gametophyte. On female Marchantia thalli, the archegonia hang on the underside of radiating fingers of the receptacle on the end of a long stalk called the archegoniophore. On male Marchantia thalli, the antheridia are embedded in the surface of a disc-like receptacle on the end of a long antheriophore. The first drop of rain lands on the male receptacle. The antheridium cap cells burst and sperm ooze into the water drop on the receptacle. The drop turns a milky white. A second drop of rain splashes the sperm cell into a nearby film of water, which may include an archegoniophore. The sperm swim to the egg in the venter of the archegonium hanging down underneath the receptacle. They are directed by chemotaxis--swimming up a concentration gradient of a chemotactic chemical agent.

Regardless of the location of the archegonia (on the thallus surface or up on an archegoniophore), the sperm finds the egg, syngamy is completed and the zygote is formed in the venter of the old archegonium. The zygote grows into a parasitic sporophyte with haustorial foot, a stalk, and a sporangium. The stalk may have hydroids and leptoids. The sporangium has a sterile jacket surrounding the sporocytes. The sporocytes undergo meiosis to produce spores. These are ejected from the sterile jacket when it cracks open by desiccation. Specialized cells in the sporangium contain wall thickenings which spiral open in dryness and help empty the sporangium of its spores. These thickenings are called elaters. These and the sporopollenin in the spore walls are adaptations for a terrestrial environment.

In a few species the spores divide mitotically inside their spore wall before being shed. In these cases, what is floating on the wind is not a spore, but an endosporic (but immature) gametophyte! Keep that thought in mind for later in this course...this foreshadows the pollen grain!

Would production of antheridiophores or archegoniophores be apomorphic for a species on a path to terrestrial evolution? Is there an advantage to wind as the carrier for spores; is that an adaptation for terrestrial life? What is the adaptive significance of hydroids and leptoids? Think about cutin, chimney cells, splash mechanisms, cap cell mechanisms and so on. Are these adaptations for terrestrial existence? How so?

The bulk of this presentation was viewing slides of the various stages of the life history of liverworts!