1 minute read

Liverwort

Evolution

There are only a few fossils of liverworts and mosses and there are no fossils of hornworts. This is because the soft tissue of these bryophytes does not fossilize well. The oldest known liverwort fossil is from the late Devonian period, about 350 million years ago. Most botanists believe that they originated long before this.

Some botanists have proposed that there are over 10,000 species of liverworts in the world. A more realistic estimate is about 6,000 species. The number of species may have been overestimated in the past because the morphology of many species is plastic, in that it differs in different environments. This makes identification Common liverwort (Marchantia polymorpha). JLM Visuals. Reproduced by permission. of liverwort species very difficult, typically more difficult than that of higher plants.

Interestingly, even though liverworts originated several hundred million years before the flowering plants, there are several hundred thousand species of flowering plants but only about 6,000 species of liverworts. The reason for this may be that liverworts rely upon the inefficient mechanism of water-transported sperm for sexual reproduction. Thus, it has been proposed that most species of liverworts rely upon asexual gemmae as a means of reproduction. Asexual reproduction tends to reduce genetic diversity. Since genetic diversity is needed for new species to evolve, the liverworts and other bryophytes may have evolved into a sort of evolutionary dead end.

See also Bryophyte.


Resources

Books

Greenaway, T. Mosses and Liverworts. Orlando: Raintree Steck-Vaughn, 1992.

Margulis, L., and K.V. Schwartz. Five Kingdoms. W.H. Free-man and Company, 1998.

Peter A. Ensminger

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Diploid

—Nucleus or cell containing two copies of each chromosome, generated by fusion of two haploid nuclei.

Elater

—Specialized tubular cells with helically oriented cell wall thickenings to which liverwort spores are attached.

Gametophyte

—The haploid, gamete-producing generation in a plant's life cycle.

Gemma

—Multicellular asexual reproductive structure of mosses and liverworts.

Haploid

—Nucleus or cell containing one copy of each chromosome.

Meiosis

—Division of the cell's nucleus in which the number of chromosomes is reduced by half, typically from the diploid to the haploid.

Sporophyte

—The diploid, spore-producing generation in a plant's life cycle.

Thallus

—A single plant body lacking distinct stem, leaves, and roots.

Additional topics

Science EncyclopediaScience & Philosophy: Linear expansivity to Macrocosm and microcosmLiverwort - General Characteristics, Life Cycle, Spore Dispersal, Evolution - Asexual reproduction