Gametogenesis
Production of male and female gametes in micro and mega spores is known as Gametogenesis.
a) Microgametogenesis:
It refers to the production of male gametes or sperm cells. The pollen grain consists of two coating, in which outer is exine and inner is intine. After maturation of pollen grain nucleus divides mitotically into two nuclei known as generative nuclei and vegetative or tube nuclei. As soon as pollen reaches to stigma, they swell up by absorbing moisture and due to this exine burst and intine continue to grow in the form of tube called as pollen tube. The pollen tube penetrative into stigma and style and at the stage only generative nuclei undergoes a mitotic division to produce two male gametes. The pollen tube finally enters into the ovule through micro Pyle and discharge, the two male gamete into embryo sac and then fertilization take place. The pollen along with pollen tube and two sperms is called micro gametophyte.
b) Megagametogenesis:
Development of embryo sac from a megaspore is known as Megagametogenesis. The nucleus of functional megaspore divides mitotically to produce two nuclei, which again divide mitotically then to produce eight nuclei. Out of these two nuclei, moves toward one pole i.e micropyle pole ( produced one eggs cells and two synergids) another three nuclei migrate towards the opposite pole i.e chalazal pole ( produced these antipodal cells) and two nuclei migrate at the centre to form polar nuclei.
Finally, the megaspore is developed into the embryo sac which contains one eggs cell , two synargids, two polar nuclei and three antipodal cells.