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Mosses and Liverworts (Bryophyta)

Mosses and liverworts are traditionally classified together in the Division Bryophyta on the basis of their sharing: Some 23,000 species of living mosses and liverworts have been identified. These are small, fairly simple, plants usually found in moist locations.

Neither mosses nor liverworts have any woody tissue so they never grow very large. They have neither xylem nor phloem for the transport of water and food through the plant.

The Gametophyte Generation

The leafy shoot of mosses is haploid and thus part of the gametophyte generation.

(Link to a general discussion of the alternation
of generations in all plants.

In the common haircap moss, Polytrichum commune (shown here), there are three kinds of shoots:

In early spring, raindrops splash sperm from male to female plants. These swim down the canal in the archegonium to the chamber containing the egg. The resulting zygote begins the sporophyte generation.

The Sporophyte Generation

Mitosis of the zygote produces an embryo that grows into the mature sporophyte generation. It consists of: The sporangium is

During the summer, each spore mother cell undergoes meiosis, producing four haploid spores — the start of the new gametophyte generation. Late in the summer, the calyptra and operculum become detached from the sporangium allowing the spores to be released.

These tiny spores are dispersed so effectively by the wind that many mosses are worldwide in their distribution.

If a spore reaches a suitable habitat, it germinates to form a filament of cells called a protonema. Soon buds appear and develop into the mature leafy shoots.


Asexual Reproduction in the Mosses

Many species of moss never go through an alternation of generations; that is, they have no sexual reproduction. Reproduction is accomplished by fragments of sterile gametophytes breaking loose from the parent. If they land in a suitable habitat, these fragments can develop into a clone of the parent.

Evolutionary Position of the Bryophytes

Evidence from the chloroplast genome

The chloroplasts of mosses and liverworts, like those of all photosynthetic eukaryotes, contain multiple copies of a small genome: circular DNA molecules encoding some — but not all — of the genes needed for

Chloroplast genomes have been sequenced from representatives of most of the plant groups. Although they all contain the same genes, they fall into two distinct groups with respect to the organization of their genes.

Evidence from the mitochondrial genome (mtDNA)

The mitochondrial DNA of plants further refines the evolutionary scenario. The mtDNA of all plants contain certain shared introns. This suggests that liverworts may have been the first group of plants to evolve from algal ancestors.

Link to a discussion of the possible evolutionary origin of chloroplasts and mitochondria and their genomes.
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1 September 2023