ALTERNATION OF GENERATIONS IN PTERIDOPHYTES

In the life cycle of Pteridophytes, two distinct individuals can be observed i.e., there is a typical heteromorphic alternation of sporophytic and gametophytic generations. These two generations alternate with each other in regular succession one after the other, i.e. from sporophyte to gametophyte and from gametophyte to sporophyte. The sporophytic or asexual generation is diploid (2n), while the gametophytic or sexual generation is haploid (n).

The actual phenomenon responsible for bringing about alternation in generation is the “Periodic Reduction of Chromosomes” as a result of meiosis. Due to meiosis a reduction in the chromosome number takes place which leads to the formation of haploid individuals. The haploid individuals in turn by produce haploid gametes which proceed the process of fertilization to produce diploid individuals.

1. Gametophytic Generation:- The haploid individual bears sex organs, antheridia and archegonia. Antheridia produce male gamete i.e., antherozoids or sperms and archegonia produce female gamete oosphere or egg, which is concerned with the sexual reproduction. These individuals are named as gametophyte or prothallus and represents gametophytic generation. Both the haploid gametes (male and female) come together and unite (i.e., fertilization or syngamy) to produce a diploid (2n) zygote. Zygote is the pioneer structure of the sporophytic generation. It germinates to form embryo which develops into new sporophytic individual.

The gametophyte in the homosporous forms is short lived, independent and may be surface living and green (autophytic) in nature. They are always exosporic and thus are not enclosed by a spore wall, e.g., Lycopodium. In heterosporous forms, they have separate male and female prothalli. The male prothallus is extremely reduced and is represented only by a single prothallial cell. The female prothallus is well developed and larger in size. Both male and female prothalli are endosporic and are enclosed inside a spore wall, e.g., Selaginella.

2. Sporophytic Generation:- Zygote is the pioneer structure of the sporophyte. It develops into embryo which in turn develops into diploid individual. This diploid individual is known as sporophyte and the generation is termed as sporophytic generation. The sporophyte bears a specialized spore bearing structure known as strobilus. Inside the strobilus numerous haploid spores are formed after meiosis which are known as meiospores. There are two types of sporophytic individuals – homosporous and heterosporous. Homosporous individual produce only one type of spores, whereas heterosporous individuals produce two types of morphologically distinct spores i.e., microspores and megaspores. The spores are the pioneer structure of gametophytic generation. It germinates to form a new haploid gametophytic individual.

In pteridophytes the sporophytic individual is complicated, independent and dominant generation.  It is no doubt independent, but it has to depend upon the gametophyte during earlier stages of development. It achieves its complexity after establishing its independence.

Both the homosporous and heterosporous pteridophytes exhibit heteromorphic type of alternation of generations, because the sporophyte and gametophyte individual present marked morphological and anatomical differences.

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