Liverworts and mosses are a group of plants that is perhaps descended from early colonists of land. They are restricted to wet environments and depend on water for fertilization of gametes. They hug the ground, because they do not have specialized cells for transport of water and nutrients, and they lack strong supporting tissues.

LIVERWORTS: As you look at the materials of Marchantia draw a chart that shows all the relevant structures superimposed on a diagram of the life cycle. Hand this chart in at the end of lab. Liverwort and moss life cycles are nearly identical.

Marchantia thallus

Alternation of generations (sporophyte/gametophyte)

1. diploid sporophyte (2n) meiosis produces haploid spores (n)
2. haploid spores germinate (mitosis) into haploid thallus (n), the gametophyte, which can reproduce asexually by gemmae cups containing gemmae or sexually with gametangiophores.
3. fertilization of egg by sperm produces a diploid zygote (2n) which divides by mitosis into new sporophyte.

The male gametangiophore is called an antheridiophore; it grows up from the thallus and consists of a stalk with a flat-topped head. Close to the top surface, there are antheridia full of flagellated sperm; when it rains, the sperm are spilled out onto the surface and washed off onto the ground. The female gametangiophore is called an archegoniophore; it grows up from the thallus and consists of a stalk with an arched head and pendant (hanging) lobes or fingers. On the underside of the head are archegonia, each of which is like an inverted vase and holds a single haploid egg. Rain drops hit the ground and splash water carrying sperm up to the archegonia, which face downwards. The sperm swim up the tube of the archegonium and fertilize the egg. The zygote (fertilized egg) divides by mitosis, and a diploid sporophyte plant grows as a parasite on the tissue of the archegonium. A large, egg-shaped sporangium forms as a part of it. Meiosis occurs inside the sporangium and hundreds of haploid spores are produced.

Class Hepaticae—Liverworts, genus Marchantia. Slides:

Marchantia thallus x.s. The haploid thallus (n) grows flat on moist ground, and it is many-cells thick. Pores to permit gas exchange are in the top surface, and there are air spaces within the top zone of the thallus. Extending into the ground under the thallus are tapering chains of cells called scales, which anchor the plant, and twisting cells called rhizoids, which absorb water. It is easy to tell which way is up.

Marchantia thallus cross section

Marchantia antherids l.s. This is the male reproductive structure (antheridiophore) that grows up from the thallus. Identify the antheridia (singular, antheridium), chambers full of sperm, that open onto the flattened top surface of the head.

   
Marchantia antheridiophore and close up of 2 antheridia

Marchantia: archegones l.s. This is the female reproductive structure (archegoniophore) that grows up from the thallus. Identify the archegonia (singular, archegonium), which are vase shaped and hang down with their tubular openings facing the ground. Each archegonium contains a single egg.

Marchantia archegoniophore and close up of three archegonia with eggs

Marchantia: differentiating sporophyte. The diploid sporophyte plant (2n) occupies a former archegonium and is dependent on the haploid tissue of the archegoniophore for nutrients and water. It is dominated by a large egg-shaped chamber, the sporangium, in which meiosis occurs to produce haploid spores.

Marchantia with developing sporophytes

live specimen: Marchantia thallus (see also image above). Note the general shape and flatness. Look on the top surface for gemmae cups containing gemmae, tiny green discs of haploid cells.

Marchantia thallus with gemmae cups

Class Musci—Mosses. Slides:

Mnium archegone l.s. Archegonia are at the upper tip of the moss, and the tubular openings face up. When the egg is fertilized, the sporophyte will grow here.

Mnium antherids near-median l.s. Antheridia are at the top of broad, flat tips of the male moss stem. The tip is surrounded by leaf-like scales to form a cup. When it rains, the sperm empty into the cup and raindrops splash them out. Archegonia on female moss point up to receive the sperm-laden droplets.

live specimens: Mnium, a moss. Fruiting moss shows the brown stalk and sporangium of the mature diploid sporophyte. Note that it is growing right out of the tip of a female moss; as in liverworts the diploid sporophyte is parasitic on the haploid gametophyte.

wild moss at base of tree trunk

Return to index.