The life cycle of gymnosperms and alternation of generation in gymnosperms

Firstly, we will discuss the alternation of generation:

Simply it is the existence of two different stages in the life cycle of any organism. These stages are the sporophyte stage and the gametophyte stage.

The Sporophyte stage is diploid and forms spores
The Gametophyte stage is haploid and forms gametes
Meiosis occurs at sporophyte stage
Mitosis occurs at the gametophyte stage
In gymnosperms, the main body is sporophyte.

Now, what is meant by the main body of Gymnosperms?

The simple meaning of this sentence is that main or long-time gymnosperms live as sporophytes in nature or diploids, or the body of gymnosperms produces spores.

This gymnosperm forms a gametophyte stage during its life cycle and the conversion of sporophyte to gametophyte or gametophyte to sporophyte is called alternation of generation.

In the case of gymnosperms, a unique structure is formed which is called a cone. Cone bears sporangia.

The cone can be divided as:

Male cone
Female cone

The leaves of the male cone can be said to be the microsporophyll. On microsporophyll, microsporangia are produced.
While the leaves of the female cone can be said to be megasporophyll. On megasporophyll, megasporangia are formed.
Megasporangia can also be termed OVULE.
Megasporangia forms female gametophytes while microsporangia form male gametophytes.
Megasporangia form two or more archegonium inside it.
Microsporangia form pollens, which fuse with the archegonium of megasporangia and form a zygote.
This zygote converts into a seed.

Now, look at the process that occurs in the male cone and female cone.

Male cone.

The leaf of the male cone can be said a microsporophyll.
Microsporangia is formed inside the microsporophyll.
Meiosis occurs in microsporangia to form spores.
These spores are called microspores.
Microspores are haploid.
A single microspore undergoes cell division and forms a multicellular structure.
This multicellular structure is called a pollen grain.
The pollen grain is the gametophytic body.
A pollen grain itself is not a gamete, however, it is a gametophyte stage.
So, we can say that the whole male gamete moves towards the female gametophyte.

Female cone.

The leaf of the female cone is called a megasporophyll.
Megasporangia is formed inside the megasporophyll.
An outer covering covers the megasporangia called an integument.
Megasporangia is diploid.
A nucellus is a diploid group of cells formed inside the megasporangia.
Any single nucellus undergoes meiosis and forms a megaspore mother cell.
This megaspore mother cell is also diploid.
A megaspore mother cell forms four haploid cells.
These cells are formed by meiosis.
Out of these four cells, three cells are degenerate and only one cell will remain as a megaspore.
This single cell undergoes cell division and forms a multicellular structure called a female gametophyte.
This female gametophyte depends on the parent’s body.
Two or more archegoniums are formed on this female gametophyte.
Pollen grain (male gametophyte) releases its gamete near the mouth of the archegonium.
The fusion of these two gametes results in the formation of a zygote.
The zygote is diploid.
The zygote converts into a seed.

A seed has the structures like:

Seed coat
Embryo
Endosperm

The seed coat is the protective layer around the seed.
The embryo is the sporophyte of the next generation.
And endosperm is the nutritive group of cells that are haploid.