After the formation of male and female gametes, they must be brought together for fertilization. In most of organisms male gamete is motile and the female gamete is non-motile. However, there are a few fungi and algae where both types of gametes are motile Fig.
A medium is needed through which male gametes move. In algae, bryophytes and pteriodophytes, water serves as the medium through which gamete transfer takes place. Since several male gametes fail to reach the female gametes, hence the male gametes are produced in large number, i. In flower bearing plants, pollen grains carry the male gametes which are produced in large number.
The pollen grains are transferred to the stigma of the female organ carpel through the process of pollination. Transfer of pollen grains from the anther to the stigma is called pollination. Pollination is of two types: Self pollination and cross pollination.
Self pollination is the transfer of the pollen grains from anther of a flower to the stigma of either the same flower or the stigma of another flower of the same plant or genetically similar plant. Cross-pollination is the transfer of pollen grains from anther of one flower to the stigma of a genetically different flower of other plant of the same species. In unisexual animals, male and female gametes are formed in different individuals, therefore, the organism must evolve a special mechanism for gamete transfer.
Many animals have copulatory organs to transfer the male gametes. Transfer of gametes and coming together of gametes is essential for fertilization in sexual reproduction. The fertilization is the complete and permanent fusion of two gametes from different parents or from the same parent to form a diploid zygote. This process is also called syngamy. Although the terms syngamy and fertilization are frequently used interchangeable.
If syngamy does not occur, there would be no variations in the offspring. Where does fertilization occur? Fertilization occurs either in external medium water or inside the body of the organism. Thus there are two types of gametic fusion namely external fertilization and internal fertilization. The external medium such as water is required for this type of fertilization. Thus in most aquatic organisms such as a majority of algae, fishes and amphibians, external fertilization occurs.
Organisms exhibiting external fertilization produce a large number of gametes in water to enhance the chances of fertilization. This happens in bony fishes and frogs where a large number of offspring are produced. A major disadvantage of this type of fertilization is that the offspring are not protected from the predators and their survival is threatened up to adulthood. When egg is formed inside the female body where it fuses with the male gamete, the process is called internal fertilization or internal syngamy.
Many terrestrial organisms belonging to fungi, higher animals such as reptiles, birds and mammals and majority of bryophytes, pteridophytes, gymnosperms and angiosperms are the examples where internal fertilization occurs.
Here male gamete is mobile and has to reach the egg in order to fuse with it. The number of sperms produced is very large but there is reduction in the number of eggs produced. However, in seed plants, the non-motile male gametes are carried to the female gamete by pollen tubes. Events in sexual reproduction after the fertilization formation of zygote are called post fertilization events. These events may be described under two headings: After fertilization a diploid zygote is formed in all sexually reproducing organisms.
In external fertilization, zygote is formed in the external medium usually water whereas in internal fertilization, zygote is formed inside the body of the organism. Further development of the zygote depends on the type of life cycle of the organism and environmental conditions. Zygospore undergoes a period of rest. It germinates during next growing season.
The zygospore undergoes meiosis to produce haploid individuals. It leads haploidic life cycle. It divides by mitosis first forming a diploid embryo and then the individual which is also diploid. It leads a diplontic life cycle. The sporophyte has sporangia where meiosis takes place to form haploid spores.
The latter produce haploid gametophytes. Gametes are produced in the gametophytes. It leads a diplohaplontic life cycle. The process of development of embryo from the zygote is called embryogenesis. During embryogenesis zygote undergoes mitotic cell division and cell differentiation. Cell division increases the number of cells in the developing embryo while cell differentiation helps to form specialised tissues and organs to form an organism.
The oviparous animals such as reptiles and birds lay eggs. Their fertilized eggs are covered by hard calcareous shell and are laid in a safe place in the environment. After incubation period, young ones hatch out. In viviparous animals such as majority of mammals including human beings, the zygote develops into a young one inside the body of the female individual. After a certain growth, the young ones are delivered by the female individual.
Due to proper care and protection, the chances of survival of young ones are more in viviparous individuals. In ovoviviparous animals, the female retains the eggs inside its body after fertilization and allows the development of the embryo inside the body without providing extra nourishment to the developing embryo as the placenta is absent. However, the female animals give birth to the young ones. Examples of ovoviviparous animals are sharks and rattle snakes. After fertilization, the sepals, petals and stamens of the flower become faded and fall off.
The sepals remain attached in Hibiscus. However, the pistal remains attached to the plant. Seed and Fruit Formation: Zygote forms the embryo. Endosperm provides food to the growing embryo. The fertilized ovules mature and convert into seeds. The wall of the ovary forms the pericarp fruit wall. The ripened ovary with pericarp and seeds is called fruit. The pericarp protects the young seeds. After dispersal the seeds germinate to form new plants.
Maintenance of Chromosome Number: The reproductive units in sexual reproduction are the male and female gametes that are produced by testes and ovaries respectively. The gametes are haploid with only N chromosomes. Consequently the zygote resulting from fusion of two such haploid gametes becomes diploid with 2N chromosomes. The offspring that develops from the zygote is also diploid.