The four whorls:
Calyx and corolla are the two outermost whorls of a flower. As they do not directly take part in reproduction, they are also known as non-essential or accessory whorls.
As androecium (stamen) and gynoecium (carpel) take part directly in reproduction, they are known as essential whorls.
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Androecium and gynoecium
It is the male part of the flower that is composed of stamens. Each stamen is made up of a filament and a small pouch-like structure at the tip called the anther.
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Structure of stamen

The pollen grains are formed in the pollen sac's anther.
Pollen grain:
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Structure of a pollen grain
Pollen grains are typically spherical in shape. They have a two layered wall.  Exine is the hard outermost layer. It has germpores, which are large openings. Intine refers to the thin inner layer. It's a cellulose and pectin-based thin and continuous layer. The vegetative and reproductive cells are the two cells  found in mature pollen grains.  The nucleus of a vegetative cell is a large one. The generative cell splits into two male gametes during mitosis.
The female part of the flower which is made up of carpels is known as gynoecium.
It consists of three parts:
  1. Ovary
  2. Style
  3. Stigma
The ovary is a structure in which ovules are present.
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Structure of gynoecium