Reproduction

Sexual Reproduction

When reproduction occurs as a consequence of fusion of two gametes, it is called sexual reproduction. Sexual reproduction involves two parents, each contributing a specialized gamete.

The two sexes that take part in this mode of reproduction are the male and female. Male produces male gametes (sperm) and female produces female gametes (ova). The union of these two gametes results in the formation of zygote.

Formation of gametes, both male and female, is called gametogenesis. When the two haploid gametes (egg and sperm) fuse together, a diploid zygote is formed. This process is called fetilisaton. Sexual reproduction occur most of the multicellular plants and animals.

Male and female sex organs remain present in the same individual. Such individuals are called bisexual or hermaphrodite. On the other hand, the individuals possessing only one type of sex organ (either male or female) are called Unisexual.

Characteristics of sexual reproduction

(a) Two parents are involved in the form of sexual reproduction.

(b) Gametes are formed during sexual reproduction.

(c) During gamete formation, meiosis takes place, resulting in the formation of haploid gametes.

(d) Fusion of gametes from different parents takes place during sexual reproduction

(e) Daughter organisms are genetically different from the parents.

(f) Multiplication rate duringsexual reproduction is slower than in asexual reproduction.

Why the sexual Mode of Reproduction?

During the formation of gametes in sexual reproduction, crossing over takes place. Crossing over is the exchange of genetic fragments between homologous chromosomes causing genetic recombination which ultimately leads to variation. These variations play an important role in the evolution.

During sexual reproduction, fertilization brings together the two haploid sets of chromosomes, one from each parent, to form a new combination of genes so that each individual of a species can maintain its own identity.

Sexual Reproduction in flowering plants

Sexual reproduction occurs mostly in flowering plants. The reproductive part of plants remain present inside the flower. In other words, we say that flowers are the reproductive organ of plants. In most of the flowering plants, male and female reproductive parts are present in the same flowers e.g. Hibiscus, mustard, they are called bisexual flowers. There are some plants which have a single sex organ, either male or female; this is present in the flower like Watermelon and papaya. These are called unisexual flowers.

Structure of a flower

Flower has an axis on which all the parts are situated. The axis may be short or long. The axis may be short or long. The axis has two parts a stalk or pedicel and a swollen upper portion called thalamus. It is on the thalamus that all flower parts are based. A typical flower consists of four main whorls, Calyx(sepals)Corolla (petals), androecium (stamens) and gynoecium(carpels).

Calyx

Calyx attached to the base of the thalamus and forms the outermost whorl of a flower. The whorl constitutes the individual units called sepal. The function of sepals is to protect the flower when it is in the form of a bud.

Corolla

The colourful part of a flower and the inner whorl of a flower lying inside the sepals is called corolla. The individual unit of corolla is called a petal. The petals are usually the colourful part of a flower. The function of petals is to attract insects through their colouration and scent. The insects in turn help in pollination. Corolla protects the reproductive organs which are present in the centre of the flower.

Androecium

Its individual unit called stamen. Stamens are the male reproductive organ of a plant, which form the third whorl of a flower. There may be several stamens in a flower. A single stamen is made up of two parts.

Stalk

It is a filamentous part of stamen .

Anther

It is a swollen, bilobed structure attached to the distal end of the stalk. Each anther bears four inner chambers called pollen sacs or microsporangia. Inside the microsporangia are present numerous pollens. Pollen produces male gametes which remain present in the form of yellow powder called pollen grains.

Gynoecium (Carpel)

Gynoecium or Carpel is the female reproductive organ of a plant. Carpel is situated in the centre of the flower as a flask– shaped structure. A carpel is made up of three parts Stigma, Style and Ovary. The distal part of a carpel is called stigma. Stigma is responsible for receiving pollen(male gamete) during pollination. Style is the middle part of the carpel. It is an elongated tubular structure which connects stigma with ovary. The basal swollen part of carpel is the ovary.

The ovary bears several ovules. The ovules remain attached to the ovary wall by a placenta. Insede each ovule develops a single female gamete called ovum or egg. The ovule bears an opening at its base called micropyle.

Pollination

The Process of transfer of pollen grains from the anther of a flower to the stigma of the same flower or another flower is known as pollination.

There are two types of pollination: Self-pollination and Cross- Pollination.

Self Pollination

If the Pollen of a flower is transferred to the stigma of the same flower it is called Self-Pollination.

Cross Pollination

Cross Pollination is the transfer of pollen from one flower to the stigma of another flower too, Cross- pollination occurs as male and female gametes may not mature simultaneously. The agents which help in cross-pollination are wind, water, insects, birds etc.

Fertilization

The fusion of male and female gametes (pollen and egg) to form Zygote is called fertilization.

Pollination brings the pollen grain to the stigma. A pollen grain contains two male gametes. The inner wall if pollen grain grows downwards into the style as the pollen tube till it reaches near the micropyle. A pollen tube contains two male gametes. It pushes the micropyle and releases the two male gametes into the embryo sac. One of the male gametes fuses with egg cell to from the diploid oospore or Zygote (fertilization). The other male gamete fuses with the secondary nucleus, Which is the fusion product of the polar nuclei leading to formation of the triploid endosperm. As the result of these the seed is formed. A seed is the final product of the fertilization. The seed contains the future plant or embryo which develops into a seedling under appropriate condition. This process is known as germination.