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.
Position of calyx and corolla
Calyx: It is the outermost whorl that consists of green leaf-like structures known as sepals. It serves as a protective covering to the inner whorls of a flower in bud stage.
Corolla: It is the second whorl of the flower that consists of brightly coloured and scented structures known as petals. It is as a attractive structure for the pollination by insects.
As androecium (stamen) and gynoecium (carpel) take part directly in reproduction, they are known as essential whorls.
Androecium and gynoecium
1. Pre-fertilization events:
- Gametogenesis - The formation of a pollen grain (Microsporogenesis) and an embryo sac (Megasporogenesis).
- Pollination - The process of transferring pollen grains to the stigma of the flower.
Microsporangium: The pollen grains are produced in the anther, a small-pouch-like structure present in the stamens.
Structure of anther with microspore mother cells
The formation of pollen grains - Microsporogenesis:
- The pollen grains (male gametes) are formed inside the anther or microsporangium.
- Inside the anther lobe, a few cells are differentiated into germ cells known as microspore mother cells (diploid cells).
- These diploid cells undergo meiotic divisions to form haploid cells known as microspores.
- The haploid microspores undergo mitotic cell divisions to produce haploid pollen grains.
Structure of a pollen grain
Pollen grains are spherical in shape. They have a two-layered wall. The exine is the hard outermost layer. It has germ pores, which are large openings. Intine refers to the thin inner layer made up of cellulose and pectin.
The vegetative or tube nucleus and reproductive or generative nucleus are the two nuclei found in mature pollen grains. The nucleus of a vegetative cell is a large one, and the generative cell splits into two male gametes during mitosis.
Characteristics of pollen grain:
- A pollen grain is an immature male gametophyte with a vegetative and generative nucleus.
- During this stage, the pollen grains are released by breaking the anther wall, known as dehiscence.
- Compared to the vegetative cell, the generative cell is small and undergoes mitosis to form two male gametes. This process is known as male gametogenesis.
- The tube cell elongates to form a pollen tube during fertilization.