Theory:

Definition in plants:
The fusion ofa pollen grain and an embryo sac resulting the seed formation is known as sexual reproduction in flowering plants.
787px-Sexual_cycle.svg.png
The cycle of sexual reproduction
 
The sexual reproduction of plants can be categorized into three different stages such as,
  1. Pre-fertilization events
  2. Fertilization
  3. Post-fertilization events
SexualReproductionEvents1xw1754.png
 
Before we start to learn about the process, we first discuss the reproductive organ of a plant.
Flower - the reproductive organ:
A flower is a modified shoot to carry out sexual reproduction. A typical flower consists of four whorls born on the thalamus.
flower.jpg
Parts of a flower
In flowers, there are four different types of whorls. All these four whorls are categorised as,
  • Essential whorls - Androecium & Gynoecium 
  • Non-essential whorls- Calyx & Corolla 
 
Types of flowers:
 
1.  Flowers that contain either pistil or stamens are called unisexual flowers or incomplete flowers.
Example:
Watermelon, papaya and cucumber
2.  Flowers that contain both stamens and pistil are called bisexual flowers or complete flowers.
Example:
Sunflower, lily, rose, hibiscus and mustard
Important!
Both male and female unisexual flowers may be present in the same plant or in different plants.
Reference:
https://commons.wikimedia.org/wiki/Template:Other_versions/Mature_flower_diagram#/media/File:Mature_flower_diagram.svg
https://commons.wikimedia.org/wiki/File:Sexual_cycle.svg