B. Gamete Transfer: Once the gametes are formed inside the gametangia, the next step is to bring these gametes closer to facilitate their fusion or fertilization. In lower plants, the flagellated male gametes, produced inside the antheridia are liberated into the external medium of water. They reach to the female sex organ archegonia, to fuse with female gamete egg. A large number of male gametes failed to reach female gamete. In higher plants, the male gametes are produced inside the pollen grains. These pollen grains are transferred to the stigma (female reproductive organ) through a process known as pollination. The mediator used for pollination are air, water, animal and insects (Figure 20.4). Once the pollen reach to the suitable stigma, pollen tube germinate to produce a long tube, called pollen tube. Each pollen tube carries the male gametes to the female gamete (Egg) to complete the sexual fusion.   

 

 

Figure 20.4: Sexual reproduction keeps the chromosome number constant.

2. Fertilization: The fusion or fertilization of male and female gametes takes place in different locations. The different variants are as follows:

A. External Fertilization and External Development: This is more common in aquatic animals such as fishes, frog etc. In this mode, male and female gametes are released into the surrounding water, where fertilization ocuurs and offsprings develops. It is important for the successes of this mode of fertilization and development that both parents should release gametes at the same time.