|Alternation of Generations
The life cycle of peat moss is comprised of two distinct generations, the gametophyte and the sporophyte. At the gametophyte stage, the plant bears leaves, synthesizes chlorophyll, and is able to reproduce sexually. The sporophyte generation consists mainly of a capsule that encloses the spores. It is attached to a thin stalk (pseudopodium) of the gametophyte, and lives on it in parasitism.
|The sporophyte is the diploid generation of peat moss that develops from a zygote as the result of sexual reproduction. Tissue derived from the base of the female organ protects and nourishes the sporophyte during its early stages of development. Upon maturation, large numbers of microscopic spores are contained in a capsule (sporogonium) that towers above the leafy plant to facilitate the spreading of spores. An overpressure of 4-6 atm. builds up insides the capsule. Under dry conditions its lid (operculum) bursts open with an audible sound. The spores are catapulted up to 20 centimeters, and born on the wind.
The haploid spores are derived from diploid sporocytes by meiosis: After replication of the genome, homologous pairs of chromosomes come into close proximity, and crossovers of their chromatids may occur. During metaphase I, the chromosome pairs line up across the equator of the spindle; in anaphase I, they separate and move to opposite ends of the spindle. Subsequently, comparable to mitosis, two new spindles separate the chromatides during metaphase II and anaphase II. Thus, meiosis of one diploid sporocyte yields four haploid spores.
Bottom: spore, showing impressions of the 3 other spores generated by meiosis
(images by scanning electronic microscope)
|The gametophyte, the leafy plant we generally associate with moss, is the haploid generation of peat moss. It develops from a bud in the protonema, a one cell layer-thick juvenile stage generated by germination of a spore.|
Gametophytes are capable of sexual reproduction. They develop male organs (antheridia) and female organs (archegonia) that always grow on different branches. Some sphagna produce both on the same plant (monoecious species), others only on different plants (dioecious species). Antheridium-bearing branches often show typical color changes to red, yellow or brown.
|Antheridia and Archegonia
|Antheridia are the male organs, archegonia the female organs of a gametes-producing gametophyte (gametophore). Antheridia are small spheres that grow in abundance on short side branches near the bases of leaves. Each of them produces thousands of motile sperm (antherozoids). When these male gametes are released upon maturation, they are splashed or swim to a stationary egg that attracts them chemically. Therefore, a film of water is required for procreation.
Archegonia, the flask-shaped female organs of the gametophore, grow individually or in small clusters of up to five on short, bud-like side branches. Each contains a single egg at the base of a central cell row consisting of one ventral canal cell and several neck canal cells. When the egg has matured, these canal cells disintegrate to mucus, making way for the antherozoids. During fertilization, a single sperm fuses with the egg to produce a diploid zygote that will develop into a sporophyte.
with branching stem
|Sphagna are also capable of vegetative reproduction. In pseudodichotous branching, a new stem develops from a branch. During the humification processes of peat formation, it becomes separated, and grows into an independent plant. New plants can also regenerate from shedded plant fragments. Vegetative reproduction can also take place at the protonematal stage. Additional buds develop via formation of secondary protonemata, thus increasing the number of buds derived from a single spore. Vegetative reproduction always results in genetically identical sphagna at the gametophyte generation.|
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