Gametophyte

Sexual reproduction plants involve two types of forms named gametophytes and sporophytes. Gametophytes represent the sexual phase of the organism in the life cycle. Sporophytes represent the non-sexual phase in the life cycle of that plant. Transfer of gametophyte to sporophyte and vice versa represents the alternation of generation. Gametophytes represent the multicellular haploid phase of the organisms which means they have only one set of chromosomes. Gametophyte forms sex gametes, microspores, and megaspores by the process of mitosis. The structure of gametophytes is different in different cases. Some of the gametes are morphologically and anatomically equal, some of them are different and in some cases, male gametophytes are motile and female one is immobile in nature. They are known as isogamous, anisogamous and oogamous respectively. 

Definition

Gametophyte represents the sexual phase of the plants which is multicellular haploid in nature which means it has only one set of chromosomes. Gametophytes form the required gametes by the process of mitosis.

Types of gametophyte

Based on the morphological structures that the gametophyte produces, they are of three types-

Isogamous: When the two games are morphologically similar, they are known as isogamous. Iso means the same and gamous means gametes.

Anisogamous: When the two games are morphologically different, they are known as anisogamous. Aniso means different and gamous means gametes.

Oogamous: When the female gamete that is the ovum is large and non-motile and the male gamete is small but motile, it is known as oogamous. 

Algae

A wide range of multicellular algae undergoes sexual reproduction by the production of gametophytes. The gametophytes are morphologically and physiologically indistinguishable i.e. isogamous in nature.

Bryophytes

In the case of bryophytes, gametophytes are the most prominent phase in the life cycle of bryophytes. The mature stage of bryophytes is gametophytic in nature and is independent nutritionally. The mature gametophyte gives rise to a leafy stage known as the protonema. The sporophyte, in the case of bryophytes, is dependent on the gametophytic phase of bryophytes. The mature gametophytes give rise to gametes by the process of mitosis. The female gamete is the archegonia and the male gamete is the antheridia. In some special kinds of bryophytes, for instance liverworts of order Marchantiales, the gametes that are produced are present on the specialized structures known as gametophores. 

Pteridophytes

In the case of the vascular cryptogams, the sporophytic phase is dominant over the gametophytic phase. The gametophyte forms from the sporophytic phase by the process of meiosis. The gametes are formed from the gametophyte by the process of mitosis. The male gamete is known as antheridia and the female gamete is known as archegonia. The vascular cryptogams form only one type of gamete and therefore it is known as homosporous. The gametes are free-living and not dependent on the individuals and therefore known as exosporic gametes. Exosporic gametophyte is either bisexual that is can produce both male and female gametes in the same thallus or in the specialized separate thallus. The male gametes are transferred to the female gametes by the presence of water. 

Alternation of generation

Alternation of generation is the phenomenon in which the sexual phase that is the gametophytic phase is transformed into the sporophytic phase in the life cycle of the organisms. In the case of fungi, algae, and different plant organisms, this alternation of generation is quite distinct. The gametophytic phase produces the gametes by the process of mitosis. The male gamete is known as antheridia and the female gamete is known as archegonia. The antheridia and archegonia fuse together to form the zygote. The zygote rises to form the asexual phase in the life cycle which is the sporophytic phase. The sporophytic phase forms the gametophytic phase by the process of meiosis and the cycle goes on. 

Conclusion

Gametophytes represent the sexual phase of the organism in the life cycle. Sporophytes represent the non-sexual phase in the life cycle of that plant. Transfer of gametophyte to sporophyte and vice versa represents the alternation of generation. Gametophytes represent the multicellular haploid phase of the organisms which means they have only one set of chromosomes. Gametophyte forms sex gametes, microspores, and megaspores by the process of mitosis. The structure of gametophytes is different in different cases. Some of the gametes are morphologically and anatomically equal, some of them are different, and in some cases, male gametophytes are motile and female one is immobile in nature. They are known as isogamous, anisogamous, and oogamous respectively. In the case of fungi, algae, and different plant organisms, this alternation of generation is quite distinct.