Sexual reproduction, which is common in higher plants and animals, is a far more complicated and elaborate process than asexual reproduction. This form of reproduction essentially involves the fusion of male and female gametes produced by reproductive structures. Plants which possess both male and female reproductive organs are called bisexual or monoecious, on the other hand, male and female organs growing on different plants is known as a dioecious or unisexual.
We see unisexual and bisexual organisms in the animal kingdom. The earthworm, which has both testis and ovary, is bisexual, human beings, on the other hand, are unisexual organisms as each individual possesses either the male or female reproductive organ. Reproductive structures play an important role in sexual reproduction, a process that’s divided into three distinct stages – namely, the pre-fertilisation, fertilisation and post-fertilisation stage.
The pre-fertilisation stage is the first stage in sexual reproduction and includes gametogenesis and gamete transfer – events that occur prior to the fusion of gametes. Gametogenesis refers to the process of formation of the haploid male and female gametes due to cell division in the parent body. The type of cell division is decided by the nature of the parent body. Moss, monera, fungi and algae produce gametes by mitotic division. However, diploid organisms such as pteridophytes, gymnosperms, angiosperms as well as most animals including human beings possess meiocytes that undergo meiosis, forming haploid gametes. Moreover, while the male gamete is called the sperm, the female gamete is called the ovum.
The formation of gametes is followed by gamete transfer, a process during which male gametes are brought in proximity with female gametes. In the case of algae, bryophytes and pteridophytes, water acts as a medium for such gamete transfer. However, during this process, many male gametes either die or fail to reach their destination. To overcome this, most plants produce several thousand male gametes. Wind, bees and insects facilitate the transfer of gametes in higher plants including angisoperms. In these plants, the anther contains pollen grains, which contain the male gametes.
The female gamete, on the other hand, lies inside the ovule, a part of the ovary which together with the style and stigma comprises the pistil. For fertilisation, it is necessary for pollen grains to be deposited on the stigma so that they reach the embryo sac. Thus, whether in animals or plants, gamete transfer and gametogenesis are the two main events that occur in the pre-fertilisation stage of sexual reproduction.