Sexual reproduction is a type of reproduction in which the two sexes, namely, male and female are involved. Male sexual unit is known as male gamete while the female sexual unit is termed as female gamete. Two major processes viz., formation and gametes and fusion of gametes constitute sexual reproduction. Male organisms produced male gamete or sperm and female organisms produced female gamete or ova. Male gametes are generally smaller and more active than the female gametes. Female gamete or ovum is often filled with reverse food and remains passive. If a organisms produced sperms alone, it is termed a male. If it is produces only ova or eggs, it is called female. Such individuals are called unisexual, e.g., dog, donkey, cat, horse, man etc.
They are also known as monosexual or dioecious. In some animals like earthworm and hydra, one individual can produced both male and female gametes. Such organisms are called bisexual, monoecious or hermaphrodite. But it is generally observed that a bisexual individual, at a given time, produces either male gametes or female gametes. The fusion of male and female gametes is called fertilization. Even in bisexual organisms, the fertilization is nearly always cross-fertilization, i.e., ovum from one individual is fertilized by the sperm from another individual. Generally, the ovum from one individual is not fertilized by the sperm from the same individual because of different timings of their maturation.
Gametes are formed after meiotic divisions; hence the chromosome number is gamete is just half of the original body of that individual. When two such gametes (male and female) unite, the original number of chromosome is maintained. The chromosome number in each species remains fixed and specific to that species. The product, formed after the fusion of male and female gamete, is called zygote. Zygote divides to form a new individual. Although sexual reproduction is found in unicellular plants like bacteria, algae, and in unicellular animals like paramecium, it is most common in multi-cellular organisms.
Sexual Reproduction in Plants
The productive part of a plant is the flower which develops from floral buds. They are considered to be modified shoots.
Flowers and its parts
Most flowers have both male and female reproductive organs but some bear single sex. A flower generally bears long or short axis. This axis has two parts-a stalk of a flower called pedicel and swollen apical portion called thalamus. All the floral parts are arranged on the thalamus.
A typical flower consists of four sets of floral parts: calyx (petals), androecium (stamens) and gynoecium (carpeis). The first two whorls (sets) doe not directly take part in reproduction and are, therefore, known as accessory whoris. The inner two are directly concerned with sexual reproduction. The androecium constitutes the male part of the flower and consists of stamens, while the gynoecium consists of pistils and is the female reproductive part. These two whorls are called reproductive or essential whorls. The arrangement of whorls is in a definite sequence. The different components are described below:
The individual units of calyx are sepals which constitute the outermost whorl. Calyx is usually green, sometimes coloured. It may be polysepalous (sepals free) or gamosepalous (sepals united). It is said to be caduceus if the calyx falls off as soon as the floral bud opens, e.g. poppy. It is deciduous if it falls off when the flower withers. Sometimes it is persistent as in cotton.
The individual units of corolla are petals which constitute the next inner whorl. It may be white or brightly coloured. It attracts insects towards flower and thus helps in pollination. Like the calyx, the corolla may be gamopetalous (petals united) or polypetalous (petals free).
Andreoecium (Andros means male):
it is composed of a number of stamens and constitutes the third inner whorl. Each stamen consists of filament, anther and a connective. Anther is a bilobed structure which is attached to the tip of a slender stalk like filament. Each of the two anther-lobes has two chambers or loculi, called pollen-sacs or micro-sporangia. Anthers possess numerous pollens which produce male gametes. In the formation of male gametes, reduction division takes place which reduces the chromosome number to half.
Gynocium or Pistil: (Gyne means female)
It is centrally-placed fourth whorl which bears the female reproductive organ called carpel. The pistil may be simple (made of one carpel) or compound (made of two or more carpels). In a compound pistil, the carpels may be free, known as apocarpous (as in lotus), or may be united, known as syncarpous. Each pistil consists of basal, swollen portion called ovary, a narrow stalk like portion called style and a one to many-lobed. Flattened disk-like structure at the top of style called stigma.
The ovary is surrounded by an outer wall; the lumen of the ovary may or may not be divided into chambers by the septa. Ovules (one to many) are found in the chambers which are attached to the placenta. Each ovule encloses a large, oval cell known as embryo-sac. The female gametes or eggs produced are also haploid.
Pollen grains are carried by wind, water, insects and other animals up to the stigma of a pistil.
The process of transfer of pollen grains from the anthers of a flower to the stigma of the same or of another flower is known as pollination
If the pollen is transferred to the stigma of the same flower, it is termed self-pollinat5ion or autogamy (as in pea and China rose). Cross-pollination or allogamy is the transfer of pollens from the anthers of the flower to the stigma of another flower of the same species. It is very common in the majority of flowering plants.
The pollen grains germinate on the stigma after pollination. The inner wall of the pollen grain (intine) grows into a pollen-tube which grows down through the style between the cells and finally reaches the ovule. Two male gametes are present in the pollen tube. On pushing through the tip of the ovule (micropyle), the pollen tube releases the two male gametes. One of the male gametes fuses with the egg cell and the second male gamete with the fused nuclei of two polar bodies, called definitive nucleus. This phenomenon is known as double fertilization. Thus, as a result of fertilization, a diploid zygotes or oospore is formed from the fertilized egg. From the fertilized definitive nucleus the triploid endosperm develops.
Post-fertilization Changes: Zygote divides further to form the embryo while the endosperm nucleus forms the endosperm of the seed. The embryo processes a tiny root (radical), a tiny shoot (phumule) and cotyledons to store the food. The sepals, petals, stamens, style and stigma degenerate and usually fall off. The ovary wall ripens and forms the pericarp of the fruit or the fruit-wall. The ovule develops into seed and the funicle (the stalk of the ovule) of the ovule grows into the stalk of the seed. The walls of the ovule grow and thicken to form the protective seed-coat (testa). Embryo present inside the seed may have single cotyledon (as in monocots) or two cotyledons (as in dicots) containing reserve food. Finally the seeds lose water and become hard and dry. Seeds can withstand drought and other adverse conditions in this stage.
The wall of the ovary grows to form the fruit; it may be succulent or fleshy (as in tomato and mango) or may be hard, forming a capsule as in Mexican poppy or pod as in pea. After maturity, fruits as a whole or their seeds get dispersed. The embryo lies dormant in the seed, but after getting favorable environmental conditions like moisture, it becomes active and germinates to a small seeding. The embryo grows by absorbing food material stored up in the cotyledons or in the endosperm. Germination is a process by which the dormant embryo resumes growth and grows out of the seed-coat to establish itself as a seeding.