Station 3: Clade Bryophyta - Phylum Marchantiophyta (Liverworts)

Liverworts are a diverse group of non-vascular land plants within the phylum Marchantiophyta, closely related to mosses and hornworts. They are among the earliest lineages of plants to colonize land, with fossil evidence dating back more than 470 million years. Like other bryophytes, liverworts lack true roots, stems, and leaves, and they reproduce using spores rather than seeds.

A defining feature of liverworts is their gametophyte-dominant life cycle. The green plant body you see in nature is the haploid gametophyte, which produces gametes (sperm and eggs). Following fertilization, a small, short-lived diploid sporophyte grows directly from the gametophyte but remains nutritionally dependent on it. This contrasts with vascular plants, where the sporophyte is the dominant, independent stage.

Today, there are roughly 9,000 described species of liverworts distributed across nearly every habitat on Earth. Most species are tiny—typically only a few millimeters wide and less than 10 cm long—and are often overlooked. However, they play important ecological roles:

• Soil stabilization: Forming dense mats that reduce erosion.

• Moisture retention: Helping maintain humidity in microhabitats.

• Early succession: Among the first colonizers on bare soil, rock surfaces, or disturbed areas.

• Habitat formation: Providing shelter and food for small invertebrates.

Liverworts are commonly found in humid, shaded environments such as forest floors, stream banks, and the bases of trees. Some species thrive in extreme habitats, including deserts, alpine tundra, and even the Arctic. Others can become weedy invaders in moist greenhouses and gardens, where they grow on soil or potting media.

Morphologically, liverworts occur in two main forms:

1. Thalloid liverworts — Flat, ribbon-like bodies without distinct leaves or stems (e.g., Marchantia).

2. Leafy liverworts — Small, moss-like plants with tiny, flattened leaf-like structures, but with distinct differences from true mosses.

Because of their simple anatomy, dependence on external water for reproduction, and ancient evolutionary history, liverworts provide an important model for understanding the transition of plants from aquatic to terrestrial environments.

Some liverworts grow as flattened, leafless thalli, while others resemble tiny, flattened mosses. In this lab, you’ll examine Marchantia, a classic thalloid liverwort used to study both structure and reproduction.

Male and Female Gametophytes

Marchantia has separate male and female gametophytes, and their reproductive structures are strikingly different:

• Male gametophytes produce antheridiophores: stalked structures with a flat disc at the top where sperm-producing antheridia are located.

• Female gametophytes produce archegoniophores: stalked structures with umbrella-like tops that hold archegonia, each containing a single egg.

Compare the shapes of the male and female structures on your specimen or images provided. Notice how the flat-topped antheridiophores differ from the umbrella-like archegoniophores.

Microscopic Examination

Under the microscope, the differences between the sexes become clearer. In the antheridia, thousands of tiny flagellated sperm are produced. These require water to swim to the archegonia. In contrast, each archegonium produces only one large egg, which may be visible under higher magnification.

If you locate a good slide of an archegonium, try to identify the egg. If not, look for the shape of the chamber where it would normally sit.

Sporophyte Development

After fertilization, a diploid sporophyte begins developing within the archegonium, still attached to the female gametophyte. In Marchantia, the sporophyte grows downward beneath the archegoniophore disc. It consists of:

• A foot, which anchors it and absorbs nutrients from the gametophyte.

• A seta (stalk).

• A sporangium, where meiosis produces haploid spores.

On prepared slides, find a developing sporophyte. Identify the foot, seta, and sporangium, and note how it remains attached to and dependent on the gametophyte.

Life Cycle

Marchantia’s life cycle illustrates alternation of generations:

1. The haploid gametophyte is the dominant, photosynthetic stage.

2. Gametophytes produce eggs and sperm.

3. Fertilization occurs in the archegonium, forming a diploid zygote.

4. The sporophyte develops, remaining attached to the gametophyte.

5. The sporophyte produces haploid spores by meiosis, which disperse and grow into new gametophytes.

Use the provided diagram to trace where the gametophyte phase ends and the sporophyte phase begins.