A prothallus, or prothallium, (from Latin ''pro'' = forwards and Greek ''θαλλος'' (''thallos'') = twig) is usually the gametophyte stage in the life of a fern or other

pteridophyte A pteridophyte is a vascular plant (with xylem and phloem) that disperses spores. Because pteridophytes produce neither flowers nor seeds, they are sometimes referred to as "cryptogams", meaning that their means of reproduction is hidden. Ferns, ...

. Occasionally the term is also used to describe the young gametophyte of a liverwort or peat moss as well. In

lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.algae. The prothallus develops from a

germinating Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, fern ...

spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...

. It is a short-lived and inconspicuous heart-shaped structure typically 2–5 millimeters wide, with a number of rhizoids (root-like hairs) growing underneath, and the sex organs:

archegonium An archegonium (pl: archegonia), from the ancient Greek ''ἀρχή'' ("beginning") and ''γόνος'' ("offspring"), is a multicellular structure or organ of the gametophyte phase of certain plants, producing and containing the ovum or female ga ...

(female) and antheridium (male). Appearance varies quite a lot between species. Some are green and conduct photosynthesis while others are colorless and nourish themselves underground as

saprotrophs Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi (f ...

Alternation of generations

Spore-bearing plants, like all plants, go through a life-cycle of

alternation of generations Alternation of generations (also known as metagenesis or heterogenesis) is the predominant type of Biological life cycle, life cycle in plants and algae. It consists of a Multicellular organism, multicellular haploid sexual phase, the gametophy ...

. The fully grown

sporophyte A sporophyte () is the diploid multicellular stage in the life cycle of a plant or alga which produces asexual spores. This stage alternates with a multicellular haploid gametophyte phase. Life cycle The sporophyte develops from the zygote pr ...

, what is commonly referred to as the fern, produces genetically unique

s in the sori by meiosis. The

haploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...

spores fall from the sporophyte and germinate by

mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...

, given the right conditions, into the gametophyte stage, the prothallus. The prothallus develops independently for several weeks; it grows sex organs that produce ova (

archegonia An archegonium (pl: archegonia), from the ancient Greek ''ἀρχή'' ("beginning") and ''γόνος'' ("offspring"), is a multicellular structure or organ of the gametophyte phase of certain plants, producing and containing the ovum or female ga ...

) and flagellated sperm ( antheridia). The sperm are able to swim to the ova for fertilization to form a

diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...

zygote which divides by mitosis to form a multicellular sporophyte. In the early stages of growth, the sporophyte grows out of the prothallus, depending on it for water supply and nutrition, but develops into a new independent fern, which will produce new spores that will grow into new prothallia etc., thus completing the life cycle of the organism.

Theoretical advantages of alternation of generations

It has been argued that there is an important evolutionary advantages to the

plant life-cycle. By forming a multicellular haploid gametophyte rather than limiting the haploid stage to gametes, there is often only one allele for any genetic trait. Thus, alleles are not masked by a dominant counterpart (there ''is'' no counterpart). One benefit of this is that a mutation that causes a lethal, or harmful, trait expression will cause the gametophyte to die; thus, the trait cannot be passed on to future generations, preserving the strength of the gene pool. Furthermore, if individual cells of the gametophyte compete with one another, somatic mutations that reduce cell vigour may prevent a cell lineage from reproducing.

In lichens

The region of the thallus in lichens that is free of algae (the photobiont partner) and contains only fungus (the

mycobiont A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.crustose lichens, the prothallus is visible between

areoles In botany, areoles are small light- to dark-colored bumps on cactus, cacti out of which grow clusters of Thorns, spines, and prickles, spines. Areoles are important diagnostic features of cactus, cacti, and identify them as a family distinct fr ...

and on the growing thallus margin. In the large genus '' Cladonia'', the prothallus may provide a mode of vegetative reproduction, and it may have a role in stabilising the soil. In some genera, such as ''

Coenogonium ''Coenogonium'' is a genus of crustose lichens in the monotypic family Coenogoniaceae. It has about 90 species. Most species are leaf-dwelling or grow on bark, although a few are known to grow on rocks under certain conditions, and some are ...

'', the presence of absence of prothalli is an important

taxonomic Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...

character that is used to help classify species. The term prothallus was first used by German botanist Georg Meyer in 1825, who introduced it in a discussion of lichen growth.

References

{{Reflist, refs= {{cite journal , last=Hammer , first=Samuel , title=Prothallus structure in ''Cladonia'' , journal=The Bryologist , volume=99 , issue=2 , year=1996 , doi=10.2307/3244551 , jstor= 3244551 , pages=212–217 {{cite journal , last=Joshi , first=Y , title=A new species and a new record of the lichen genus ''Coenogonium'' (Ostropales: Coenogoniaceae) from South Korea, with a world-wide key to crustose ''Coenogonium'' having prothalli , journal=Mycosphere , volume=6 , issue=6 , year=2015 , doi=10.5943/mycosphere/6/6/3 , pages=667–672 {{cite journal , last=Mitchell , first=M.E. , year=2014 , title=De Bary's legacy: the emergence of differing perspectives on lichen symbiosis , journal=Huntia , volume=15 , issue=1 , pages=5–22 3, url=https://www.huntbotanical.org/admin/uploads/02hibd-huntia-15-1-pp05-22.pdf {{cite book , last1=Ulloa , first1=Miguel , last2=Halin , first2=Richard T. , title=Illustrated Dictionary of Mycology , edition=2nd , year=2012 , publisher=The American Phytopathological Society , location=St. Paul, Minnesota , isbn=978-0-89054-400-6 , page=507

External links

Liverwort SporophyteFern Life-Cycle
Ferns Fungal morphology and anatomy