- Prothallium
A prothallium, or prothallus (from
Latin "pro" = forwards and Greek "θαλλος" ("thallos") = twig) is usually apteridophyte , i.e. spore-bearing plant, (almost exclusivelyfern s) ingametophyte stage in thealternation of generations cycle. Occasionally the term is also used to describe the young gametophyte of a liverwort or peat moss as well.The prothallium of a pteridophyte forms from a
spore that has fallen from thesporophyte stage and germinated. It is a short-lived and inconspicuous heart-shaped structure typically 2-5 millimeters wide, with a number ofrhizoid s (root-like hairs) growing underneath, and the sex organs:archegonium (female) andantheridium (male). Appearance varies quite a lot between species. Some are green and conductphotosynthesis while others are colorless and nourish themselves underground assaprotrophs .Alternation of generations
Spore-bearing plants, like all plants, go through a life-cycle of
alternation of generations . The fully grownsporophyte , what the layman refers to as thefern , produces genetically uniquespore s in thesori bymeiosis . Thehaploid spores fall from the sporophyte and germinate bymitosis , given the right conditions, into thegametophyte stage, the prothallus. The prothallus then develops independently for several weeks; it grows sex organs and produces ova and flagellated sperm. After rainfall, the sperm are able to swim to the ova forfertilization to form adiploid sporophyte cell. This cell divides by mitosis and grows out of thegametophyte into a new fern, which will produce new spores that will grow into new prothallia etc., thus completing the life cycle of theorganism .Advantages of alternation of generations
There are two important evolutionary advantages to the
alternation of generations plant life-cycle. Firstly, by forming a haploid gametophyte, there is only oneallele for any genetic trait. Thus, all alleles will be expressed because no allele may be masked by a dominant counterpart (there "is" no counterpart). The benefit of this is that any mutation that causes a lethal, or harmful, trait expression cannot be masked and will cause the gametophyte to die; thus, the trait cannot be passed on to future generations, preserving the strength of the gene pool. Also, crossing-over duringmeiosis in the formation of spores, andsexual reproduction in the gametophytes, allows for genetic diversity, which also inhibits harmful recessive genes from "surfacing" and being expressed.External links
* [http://www.anbg.gov.au/cryptogams/underworld/panel-9/images-large/6-11.jpgLiverwort Sporophyte]
* [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/FernLifeCycle.gifFern Life-Cycle]
Wikimedia Foundation. 2010.