Dr. MALIK| The life cycle of angiosperms and alternation of generation in angiosperms

 

The life cycle of angiosperms and alternation of generation in angiosperms

 

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 angiosperms, the main body is sporophyte. 

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

The simple meaning of this sentence is prominent or long-time angiosperms live as sporophytes in nature or diploid or the body of angiosperm produce spores.

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

The life cycle of angiosperms

In the case of angiosperms, flowers are present. Flowers have both male and female sex parts. The male sex part is called the stamen while the female sex part is called the pistil.

Stamen has two parts:

1. Anthers
2. Filament 

While the pistil has three parts:

1. Stigma
2. Style
3. Ovary 

Stamen.

• Stamen has two parts. One is anther while the other is filament.
• Inside the anther, microsporangia are present.
• Meiosis occurs in microsporangia to form spores.
• These spores are called microspores.
• Microspores are haploid in nature.
• A single microspore undergoes cell division and forms a multicellular structure called a pollen grain.
• Each pollen grain consists of two male gametes which are formed through the process of mitosis.
• The pollen grain is the gametophytic body of the stamen.
• A pollen grain itself is not a gamete, however, it is a gametophyte stage.

 

Pistil.

• 
  
  
• The pistil has three parts: 

1. Stigma
2. Style  
3. Ovary

• Megaspore is produced inside the ovary.
• An outer covering covers the megasporangia called an integument.
• The number of integuments is two in angiosperms.
• Megasporangia is diploid in nature.
• The nucellus is the 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 an embryo sac.
• The embryo sac is a seven-cell structure.
• Three cells of antipodal, two polar nuclei cells, synergid, and an egg cell.
• This embryo sac depends on the parent’s body.

 

• Pollen grain (a male gametophyte) forms a pollen tube.
• One gamete fuses with polar nuclei cells. 
• The other male gamete fertilizes with egg cells which form a zygote.
• The zygote is diploid in nature.
• The zygote converts into a seed.

 A seed has the structures like:

1. Seed coat
2. Embryo
3. 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.