Reproduction enables a species to live generation after generation.
Reproduction in organisms can be broadly classified into asexual and
sexual reproduction. Asexual reproduction does not involve the
formation or fusion of gametes. It is common in organisms that have a
relatively simple organisation such as the fungi, algae and some
invertebrate animals. The offspring formed by asexual reproduction
are identical and are referred to as clones. Zoospores, conidia, etc., are
the most common asexual structures formed in several algae and fungi.
Budding and gemmule formation are the common asexual methods
seen in animals.
Prokaryotes and unicellular organisms reproduce asexually by
cell division or binary fission of the parent cell. In several aquatic and
terrestrial species of angiosperms, structures such as runners,rhizomes, suckers, tubers, offsets, etc., are capable of giving rise to
new offspring. This method of asexual reproduction is generally
referred to as vegetative propagation.
Sexual reproduction involves the formation and fusion of gametes.
It is a complex and slower process as compared to asexual reproduction.
Most of the higher animals reproduce almost entirely by sexual method.
Events of sexual reproduction may be categorised into pre-fertilisation,
fertilisation and post-fertilisation events. Pre-fertilisation events include
gametogenesis and gamete transfer while post-fertilisation events
include the formation of zygote and embryogenesis.
Organisms may be bisexual or unisexual. Sexuality in plants is
varied, particularly in angiosperms, due to the production of diverse
types of flowers. Plants are defined as monoecious and dioecious.
Flowers may be bisexual or unisexual flowers.
Gametes are haploid in nature and usually a direct product of meiotic
division except in haploid organisms where gametes are formed by mitosis.
Transfer of male gametes is an essential event in sexual reproduction.
It is relatively easy in bisexual organisms. In unisexual animals it occurs
by copulation or simultaneous release. In angiosperms, a special process
called pollination ensures transfer of pollen grains which carry the pollen
grains to the stigma.
Syngamy (fertilisation) occurs between the male and female gametes.
Syngamy may occur either externally, outside the body of organisms or
internally, inside the body. Syngamy leads to formation of a specialised
cell called zygote.
The process of development of embryo from the zygote is called
embryogenesis. In animals, the zygote starts developing soon after its
formation. Animals may be either oviparous or viviparous. Embryonal
protection and care are better in viviparous organisms.
In flowering plants, after fertilisation, ovary develops into fruit and
ovules mature into seeds. Inside the mature seed is the progenitor of
the next generation, the embryo.
KAPIL PATSARIYA
