Alternation of Generations (Gametophyte and Sporophyte)

Alternation of generation occurs in plants. Every plant body shows two types of life cycle in their life:

Gametophytic(haploid) and Sporophytic(diploid).

The concept is very simple-

During Sexual reproduction in plants, formation of gametes takes place. For a better understanding of this concept, let's suppose that we have two plants- A and B.

If A and B belongs to algae or bryophyta, then their dominant phase(in which they remain almost whole life) is haploid. 

If A and B belongs to Pteridophytes, Gymnosperms or Angiosperms then their dominant phase is diploid stage.

• In the case 1, during Sexual reproduction, both these plants A and B produce haploid gametes by mitosis. 

Both these haploid gametes fuse to form zygote(diploid).

     n+n→2n

Now, in this diploid zygote mitotic division take place and a diploid structure informed that is known and sporophyte. 

Now in this sporophyte, meiosis division takes place and haploid spores are produced. 

Now these haploid spores germinate and give rise to a new plant body that will be haploid(gametophytic).

In a nutshell, Gametophyte→Sporophyte→Gametophyte

That's why it is called alternation of Generations.

           Gametophytic life cycle

• In the second case both the plants bodies will produce diploid spores and in the diploid spores meiosis division take place and haploid spores are formed.

These haploid spores germinate and give rise to a haploid strucutre(gametophyte). 

This gametophyte develops and when it attains maturity, it produces haploid gametes (mitosis). 

The haploid gametes fuse to form a diploid zygote.

This zygote develops into a diploid plant body (sporophyte).

Sporophyte→Gametophyte→Sporophyte