It is the process of production of offsprings (new individuals) by the fusion of two cells (gametes), one from the male parent (male gamete) and the other from female parent (female gamete) to form a diploid cell (zygote) which develops into a mature organism.
SEXUAL REPRODUCTION IN FLOWERING PLANTS
In the flowering plants, the steps of sexual reproduction occurs within specialized reproductive organ called the flower.
Flower: A flower is defined as a specialized condensed reproductive shoot of flowering plants on which the essential reproductive parts are arranged.
Reproductive parts of flower
Parts of flower
(a) Calyx: It is the outermost whorl and the floral leaves of this whorl are called sepals. It is green and occasionally brightly coloured and are protective in function.
(b) Corolla: It is the second whorl of floral leaves which are called petals. They are showy and brightly coloured to attract the insect pollinators.
(c) Androecium: It is the collection of stamens of the flower. Each stamen is composed of an anther and a filament. The filament is a long stalk bearing an anther at the top. The anther consists of pollen sacs where pollens are formed. The pollen ultimately gives rise to the two male gametes.
Gynoecium (pistil): It is the collection of carpels. Each carpel possess a swollen ovary containing ovules (bearing egg), a long style and a stigma on suitable position to receive pollen grains during pollination and support their growth.
Types of flower based on reproductive organs
(a) Unisexual: In such flower only one reproductive part is present, either male (stamen) or female (pistil) e.g. cucurbits, mulberry, papaya, watermelon, etc.
Bisexual: When stamens and carpel are found in the same flower, it is called hermaphrodite or bisexual, e.g. Hibiscus (chinarose), mustard, rose, pea, cotton, etc.
The transfer of pollen grains from the anther of the stamen to the receptive stigma of the carpel is called pollination.
Cross pollination is favoured by some external agents like wind, water and animals.
Fertilisation in plants
Fertilisation: The fusion of male gamete with the female gamete to form a diploid zygote within the embryo sac is called fertilization.
(a) Mechanism of fertilization:
- Due to pollination the related pollen grains are deposited over the receptive stigma of the carpel.
- These pollen grains absorb water, swell and then germinate to produce pollen tubes.
- The pollen tube grows into the stigma, passes through the style and then moves towards the ovarian cavity.
- Two male gametes are formed inside the tube during its growth through the style.
- After reaching the ovary, the pollen tube enters the ovule and the tip of the ovule pierces the egg apparatus.
- After penetration, the tip of the pollen tube ruptures releasing two male gametes into the embryo sac.
- One male gamete fertilizes the egg to form the diploid zygote by the process of syngamy.
- The other male gamete fuses with the two polar nuclei to form the triploid (3N), primary endosperm. This is known as triple fusion. The mechanism involving two acts of fertilization in an embryo sac is called double fertilization.
(b) Formation of fruits and seeds:
- The fertilized egg divides several times to form an embryo within the ovule.
- The ovule then develops a tough coat around it which gradually gets converted into a seed.
- All the fertilized eggs in the ovules present in an ovary grow to become seed.
- The ovary of the flower develops and becomes a fruit, which may be soft like mangoes; juicy like oranges; hard, dry and woody like peanuts and almonds.
- The fruit protects the seed. The seed is the reproductive unit of a plant. It contains the baby plant.
- The part of the baby plant that develops into shoot is called plumule and the part which develops into root is called radicle.
- The part of the seed which contains stored food for the baby plant is called cotyledon.
- The baby plant inside the seed germinates under suitable conditions like water, air temperature, etc to grow into a new plant.
Reproduction in Human Beings
Human beings are unisexual and human reproduction is highly evolved. There is a distinct sexual dimorphism.
Males visibly differ from females in physical standards, external genital organs and accessory sex characters.
Sex organs & sexual characters
The reproductive system of males and females consist of many organs which are distinguishable into primary and secondary sex organs.
Primary sex organs: The primary sex organs are gonads which produce gametes and secrete sex hormones.
The gonad of male is called testis which produces sperm and male sex hormone testosterone.
The gonad of female is called ovary which produces female gametes ova and female hormones estrogen and progesterone.
(b) Secondary sex organs: The secondary sex organs include genital ducts and glands which help in the transportation of gametes and assist in the reproductive process.
These organs do not produce gametes or secrete hormones.
Primary sexual characters: Primary sexual characters are those present at birth.
(d) Puberty: Puberty is the period at which reproductive organs become functional in both human males and females, gonads start producing gametes and sex hormones, and the boys and girls become sexually mature.
- Puberty in males: It is attained at the age of 13 – 14 years. It is triggered by the secretion of testosterone from the testes which brings about the development of secondary sexual characters during puberty and maintains throughout life.
- Puberty in females: It is attained at the age between 10 – 12 years. It is triggered by the hormone estrogen from the ovaries. This hormone causes growth, maturation of reproductive tracts and development of secondary sexual characters.
(e) Secondary sexual characters: Secondary sexual characters are those that develop at puberty.
(1) In human males:
- Enlargement of penis and scrotum.
- Broadening of shoulders.
- Growth of the body hair and facial hair.
- Deepening of the voice due to enlargement of larynx and thickening of vocal cords.
- Increase in the development of musculature and bones.
- Increase in height.
(2) In human females:
- Growth of breast and external genitalia.
- Growth of pubic hair and extra hair in the armpits.
- Broadening of pelvis.
- Initiation of menstruation and ovulation.
- Increase in fat particularly in thighs, shoulders, buttocks and face.
Male reproductive system
Human male reproductive system consists of a scrotum, a pair of testes, vas deferens, urethra and penis.
- Scrotum: The scrotum is a pouch of pigmented skin from the lower abdominal wall and hanging between the legs. It is divided internally into right and left scrotal sacs. The scrotum acts as a thermoregulator and maintains a temperature of 2°C less than the body and provides an optimal temperature for the formation of sperms. The life of the sperm is reduced if the temperature is higher.
- Testis (Pl. Testes): Testes are the primary sex organs lying in the scrotum outside the abdominal cavity. Testes are the sites where sperms are produced. It also produces sex hormone testosterone.
- Vas deferens (pl. vasa deferentia): This is a straight tube about 40 cm long which carries the sperms to seminal vesicles. It temporarily stores the sperms.
- Urethra: It is 20 cm long tube that arises from urinary bladder to carry urine. It passes through the penis and open to outside through the male genital pore. It carries sperms from vasa deferentia through the penis.
- Penis: It is a long and thick muscular organ made up of mostly erectile tissue. It opens outside the body. It passes the sperms from the man’s body into the vagina of the women’s body during mating.
Reproductive system (Male)
(f) Accessory sex glands:
- Seminal vesicles: These are elongated coiled sacs. They secrete seminal fluid that contains sugar fructose. It provides energy to the spermatozoa.
- Prostate gland: This gland surrounds the first portion of urethra. It secretes a fluid containing citrate and enzymes that nourish and activates the spermatozoa that swim.
Female reproductive system
It is more complex as compared to that of males. It consists of a pair of ovaries, a pair of fallopian tubes, uterus and vagina.
(a) Ovaries: They are the primary sex organs located in the lower part of the abdominal cavity near kidney.
Each ovary is connected by a ligament to the uterus. It produces gametes (eggs) and hormones like estrogen and progesterone.
Ovary is composed of ovarian follicles. Each follicle contains a large ovum (egg) surrounded by many layers of follicle cells.
The release of egg from the ovary is called ovulation. It is caused by increase in turgidity aided by contraction of unstriped muscle fibres around the follicles. The force of ejection carries the egg to the fallopian tube.
(b) Fallopian tube (oviducts): A fallopian tube is about 10 to 12 cm long muscular tube which carries egg from the ovary to the uterus and provides suitable environment for fertilization.
(c) Uterus: It is a large, elastic sac specialized for the development of embryo until birth. It is located between urinary bladder and rectum.
(d) Vagina: The vagina is a large, median elastic muscular tube. It is adapted to receive the male penis during copulation. The vagina is also called “birth canal”. It allows the passage of baby at the time of child birth.
Reproductive system (Female)
Fertilization is the fusion of sperm nucleus with the egg nucleus to form a diploid zygote.
It is internal in human being. The sperm penetrates through penis into the vagina during copulation.
The sperm moves to the uterus and then to the fallopian tube.
The fallopian tube contains an ovum produced during ovulation.
Only one sperm fuses with the ovum in the oviduct to form the zygote.
The zygote divides and forms a hollow ball of hundred of cells called embryo which gets embedded in the thick lining of uterus. This embedding is called implantation.
A disc-like special tissue called placenta develops between the uterus wall and the embryo. It exchanges oxygen, nutrients and waste products between the embryo and the mother.
The complete development of the embryo (foetus) from the initial stage of conception till the birth of young ones is called gestation.
It is also termed as pregnancy. It is completed in about 280 days (or 9 months) in humans.
During gestation, the embryo grows to become a baby. The act of giving birth is called parturition.
Sexual cycles in females
(a) Menstruation: When the egg released during ovulation is not fertilized by the sperm, the thick and soft uterus lining having lot of blood capillaries is not required.
The unfertilized ovum dies and the uterus lining breaks down which produces blood along with other tissues.
The blood and other tissues come out of vagina in the form of bleeding called menstruation.
Menstrual cycles: The cycle of events taking place in female reproductive organs (ovaries and uterus) under the control of sex hormones, in every 28 days and marked by bleeding or menstrual flow is called menstrual cycle.
Significance of sexual reproduction
Sexual reproduction is a better mode of variation.
The two different individuals in a population (males and females) with different patterns of accumulated variations produce offspring with new combinations of variations.