• Sexual reproduction involves fusion of male and female gametes to form a zygote, which develops in to an embryo.
• In crop plants, male and female gametes are produced in specialized structures known as flowers.

1. Flower:

• A flower usually consists of sepals, petals (or their modifications), stamens and/or pistil.
• A flower containing both stamens and pistil is a perfect or hermaphrodite flower.
• If it contains stamens, but not pistil, it is known as staminate, while a pistillate flower contains pistil, but not stamens.
• Staminate and pistillate flowers occur on the same plant in a monoecious species, such as maize, Colocasia, castor (Ricinus communis), coconut, etc.
• But in dioecious species, staminate and pistillate flowers occur on different plants, e.g., papaya, date palm (Phoenix dactylifera), pistachio (Pistacia vera), etc.

2. Sporogenesis:

• Productions of microspores and megaspores is known as sporogenesis.
• Microspores are produced in anthers (microsporogenesis), while megaspores are produced in ovules (megasporogenesis).

a) Microsporogenesis:

• Each anther has four pollen sacs, which contain numerous pollen mother
• cells (PMCs).
• Each PMC undergoes meiosis to produce four haploid cells or microspores.
• This process is known as microsporogenesis.
• Microspores mature into pollen grains mainly by a thickening of their walls.

b) Megasporogenesis:

• Megasporogenesis occurs in ovules, which are present inside the ovary.
• A single cell in each ovule differentiates into a megaspore mother cell.
• The megaspore mother cell undergoes meiosis to produce four haploid megaspores.
• Three of the megaspores degenerates leaving one functional megaspore per ovule.
• This completes megasporogenesis.

3. Gametogenesis:

• The production of male and female gametes in the microspores and the megaspores, respectively, is known as gametogenesis.

a) Microgametogenesis:

• This refers to the production of male gamete or sperm.
• During the maturation of pollen, the microspore nucleus divides mitotically to produce a generative and a vegetative or tube nucleus.
• The pollen is generally released in this binucleate stage.
• When the pollen lands onto the stigma of a flower, it is known as pollination.
• Shortly after pollination, the pollen germinates, the pollen tube enters the stigma and grows through the style.
• The generative nucleus now undergoes a mitotic division to produce two male gametes or sperms.
• The pollen, along with the pollen tube, is known as microgametophyte.
• The pollen tube finally enters the ovule through a small pore, micropyle, and discharges the two sperms into the embryo sac.

b) Megagametogenesis:

• The nucleus of a functional megaspore divides mitotically to produce four or more nuclei.
• In most of the crop plants, megaspore nucleus undergoes three mitotic divisions to produce eight nuclei.
• Three of these nuclei move to one pole and produce a central egg cell and two synergid cells; one synergid is situated on either side of the egg cell.
• Another three nuclei migrate to the opposite pole to give rise to antipodal cells.
• The two nuclei remaining in the center, the polar nuclei, fuse to form a secondary nucleus.
• The megaspore thus develops into a mature megagametophyte or embryo sac.
• The development of embryo sac from a megaspore is known as megagametogenesis.
• The embryo sac generally contains one egg cell, two synergids, three antipodal cells (all haploid), and one diploid secondary nucleus.