Introduction to Prothallus

Prothallium which is more commonly known as the prothallus can be defined as the gametophyte of the fern and other pteridophytes species. The prothallium or prothallus is known to produce both male and female gametophytes of the plant. To understand the role of the prothallus it is important to understand what is prothallus, the structure of the prothallus. In this article, the discussion will be focused on the basic understanding to define prothallus, the reproductive phase of the fern or the sexual reproduction in the fern, and the significance of the fern prothallus in the reproductive cycle of the plant. Lastly, this article will also provide with some examples of plant that has prothallium.


It is very important to have a basic understanding of what is prothallus, to understand the role of the prothallium in the reproduction process of the plant. Prothallus is the gametophyte of the plant that is they are responsible for producing the gametes which are the male and female sex cells of the plants. These gametes fuse together to form the zygote. The male and female gamete is fused together allowing the cell to enter a diploid state, this process is known as fertilization. The prothallus or prothallium is produced during the sporophytic stages of the fern, that is they are produced by the spores of the plant. It is very important to note that the zygote formed after the fertilization is the sporophyte, this sporophyte grows and develops by uptaking the nutrition from the gametophytes of the plant, that is the prothallium generally assists in the growth of a spore.

The prothallium is generally a heart-shaped appendage, they are generally colorless, and have features like rhizoids which acts as the primary root, there is also the presence of the primary leaf, and the rudiment of a new stem. It is interesting to note that the plant prothallium provides nutrition to the sporophyte by an appendage named as the foot.  They are generally small, that is their diameter is only about 2-5 millimeters long. They are a very short lived phase of the plant, they generally can perform photosynthesis. Some of the prothallus reported are colorless while the others are generally of green color. 

It is very important to understand the alteration of generation in the fern and similar pteridophytes, it is the understanding of this that gives the proper answer to what is prothallus and the role of it in the life cycle of the fern prothallus. It is because of the alteration of a generation that structures like prothallium are evolutionarily developed.

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Alteration of Generations

Alteration of generation can be defined as the distinct haploid and diploid phases of the life cycle of the plant. It is because of the alteration of a generation that a prothallus structure is formed. There are two different phases one the haploid phase and the second the diploid phase. The gametophytes are produced during the haploid state by the spores of the plant, that spores give rise to prothallus and the prothallus further produces the gametophyte by the mitotic cell division. The gametes produced by are it are haploid when male and female gamete fertilize the plant enters into the diploid state. The initial nutrition is provided by the prothallium, after which the plant germinates and develops all its features. 

The common example of alteration of generation is ferns, liverwort, moss. The class of pteridophyte shows it, among them fern prothallus is the most widely studied example.

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Fundamentals of the Alteration of Generation

There are basic steps or the fundamentals of the alteration of generation seen in plants. Pants that produce sporophytes undergo alteration. The fundamental steps are as follows,

  1. The haploid stage of the life cycle is known as the sporophyte

  2. Sporophytes produce the spores

  3. Spores undergo the process of mitotic division to produce the gametophyte, these are known as prothallium. The common example is fern prothallus

  4. The prothallium essentially contains the gametophyte

  5. The gametophyte produces the sex cell, also known as gametes. They produce both the male and female gametes

  6. Male gametes are called as the antheridia, they are flagellated sperm

  7. The female gametes are known as ova. They are also referred to as the archegonia.

  8. Fertilization gives rise to the zygote, which is diploid.

  9. Zygotes undergo development and the nutrition for it is provided by the prothallus.

  10. Zygote germinates and undergoes mitotic division to produce sporophyte

  11. Sporophytes at maturity produce sporangia, they can produce a single or multiple sporangia. Single sporangia are known as sporangium.

  12. Sporangia further produce a diploid mother cell. These are also known as the sporocytes

  13. Sprocytes undergo meiotic division to produce the spores

This is the complete summary of the alteration of generation, and the role of prothallus in it. This cycle continues to main the population of the plant and to ensure its survival even during harsh conditions. Alteration of generation is also known by the name of metagenesis and heterogenesis.

Examples of Prothallus

There are the following examples of an organism that gas prothallium

  1. Fern

  2. Moss

  3. Liverwort

  4. Algae 

  5. club mosses

  6. Horsetails

  7. Psilopsida, 

  8. Lycopsida, 

  9. Sphenopsida 

  10. Pteropsida and all the trees that come under pteridophytes

Function of Prothallus

There is the following function of a prothallium

  1. Assist in reproduction

  2. Produce gametes, both the male and female gametes

  3. Provide nutrition to the zygote

  4. Protection of the zygote in adverse conditions and environmental stress

  5. Prothallus also serves as the food source of many herbivores

Importance of Alteration of Generations

After understanding the answer to the question about what is prothallus and the importance of the role of prothallium in sexual reproduction it is important to understand the need for evolution of this life cycle. Some of the points of importance of alteration of generation are mentioned as follows-

  1. Ensures the survival of plant even in stressful environmental condition

  2. Protection from a predator, prothallum sometimes serves as the food source for herbivores in the food chain, but a diploid structure formed due to alteration of generation are generally not consumed, thus providing the protection.

  3. They ensure the maintenance of the seed in the dormant stage in unfavorable condition

  4. It is because of alteration of generation a plant can maintain the balance and follow both the methods of reproduction that is asexual and sexual reproduction.

  5. It is due to the alteration of a generation that multicellular gametophytes are produced rather than haploid gametes, this evolutionary importance because it ensures that there is only a single allele for any genetic trait. It can be said that the alleles are not masked by a dominant counterpart.

  6. Alteration of generation also preserves the strength of the gene pool as in case of lethal mutation because the presence of the single gametophytes is eliminated in that generation itself without passing it to the progeny generations.

  7. It also allows genetic diversity to persist and develop. 

Difference Between Thallus and Prothallus

Often the term thallus and prothallus are used interchangeably but there are the certain difference between them some of them are mentioned in the table below:



It is a heart-shaped gametophyte generally present in pteridophytes

These are defined as the nondifferentiated plant body generally seen in lichens.

They are only involved in the sexual reproduction

They are involved in both sexual and asexual reproduction.

Antherdia is present in it

Antheredia is absent here


Archegonia is absent

They are always multicellular

They can be unicellular or multicellular

Example- pteridophytes, fern prothallus, 

Algae, fungi, liverworts and, lichens.

FAQs (Frequently Asked Questions)

Q.1 Define Prothallus.

Ans- Prothallus can be defined as a multicellular gametophyte, that produces male and female gametes. They are found in the group of pteridophytes.

Q.2 Prothallus is Formed During Which Stage in the Plant Life Cycle.

Ans- Prothallium is formed from the spores during the sporophyte stage of the life cycle. Spore is in turn developed from the meiotic division of the sprocytes.