Sexual reproduction is the process involving the fusion of haploid nuclei to form a diploid zygote and the production of genetically dissimilar offspring.
There are two types of reproduction, sexual reproduction and asexual reproduction.
Asexual reproduction:This is the production of genetically identical offspring from one parent. The parent cell divides by mitosis producing copies of it as new individuals. Since the offspring produced in this way are identical to the parent they are known as clones.
Sexual reproduction:Sexual reproduction involves two parents. The parents give rise to reproductive cells called gametes by meiosis. The male and female gametes contain the male and female nuclei which fuse together in a process known as fertilization to give rise to a single cell called the zygote. Zygotes develop into offspring which are genetically different from each other and from their parent. Most of the well developed organisms as well as flowering plants reproduce by sexual reproduction.
Flowers are the most advanced and complex reproductive structures containing the reproductive organs in the plant kingdom. Their function is sexual reproduction, as a result of which they form fruits and seeds that give rise to the next generation. There is a great variety of shape and structure among the flowers, but all of them have certain features in common.
STRUCTURE OF A FLOWER
PARTS OF FLOWER & THEIR FUNCTION
The transfer of pollen grains from the anther to the stigma of a flower is called pollination. The anthers when mature split open exposing the microscopic pollen grains. The pollen grains are then carried away on the bodies of the insect or simply blown by wind and may land on the stigma of another flower.
TYPES OF POLLINATION
There are two types of pollination
Self- pollination:This is the transfer of pollen grains from the anthers to the stigma of the same flower or to another flower of the same plant.·
Cross-pollination:This is the transfer of pollen grains from the anther of one flower to the stigma in a different flower of another plant of the same species.
AGENTS OF POLLINATION
Insects and wind are the two main agents of pollination.
Nectar is produced by the nectary at the base of the filaments. When an insect visits a matured flower to collect nectar, pollen grains released by the ripe matured anthers stick to the tube like mouth parts of a butterfly or body of an insect such as a bee. When this insect visits another flower to collect nectar, it pushes its way into the flower to reach the nectarines, its body brushes onto the sticky surface of the stigma which picks up the pollen grains from its body. Pollination is complete when the pollen grain has landed on a stigma.
INSECT POLLINATION IN A LEGUMINOUS FLOWER, EXAMPLE: LUPIN
STRUCTURE OF LUPIN
- Five sepals :joined together forming a short tube
- standard petal : uppermost petal held vertically
: acts as a flag to attract bees for pollination
- wing petals : two petals on the side which are partly joined together act as a landing platform for insects.
- keel petals : two petals inside the wing petals which are joined to form the shape of a boat. acts as landing platform for bee
- single carpel :long, narrow, pod shaped with 10 ovules in the ovary.
:style: bears the stigma
:stigma: ends just inside the pointed end of the keel petal
- Stamen :10 stamens
In most leguminous plants out of 10 stamens, 9 filaments are fused together forming a sheath around the ovary and one filament is free.
POLLINATION OF LUPIN
· The bees are attracted by the brightly coloured petals.
· The bee lands on the wings and keel petals for food.
· The weight of the bee depresses the petals down.
· The mature anthers touch the hairy abdomen and the legs of the insect and transfer the pollen.
· When the bee visit another flower of the same species as the same way stigma touches the underside of the insect and get the pollen grains.
Grasses, cereals and many plants are not pollinated by insects but by wind currents. The flowers of these plants are quite small, dull coloured and leaf like, rather than petals. They produce no nectar. The stigma and anthers are not enclosed by the leaf like petals instead they are exposed to the air. The pollen grains are light and smooth. So they can be easily carried by the wind. Some of them will be trapped on the stigmas of other plants.
In grasses at first the feathery stigmas stick out from the flower and they trap pollen grains floating in the air. Later the anthers hang outside the flower, the pollen sac split and the wind blows the pollen away.
DIFFERENCES BETWEEN INSECT POLLINATED AND WIND POLLINATED FLOWERS
FERTILIZATION AND FRUIT FORMATION
Pollination is completed when pollen from an anther lands on a stigma. If the flower is to produce seeds, pollination has to be followed by a process called fertilization. In all living things fertilization happens when a male gamete fuses with a female gamete. The resulting single cell formed is called a zygote. In flowering plants the male gamete is in the pollen grain and the female egg cell is in the ovule. For fertilization to occur the nucleus of the male gamete has to fuse with the egg cell nucleus. Of thousands of pollen grains released from the anthers of a flower, only a tiny fraction reaches the stigmas of another flower of the same species. And even if the pollen grains become firmly attached to the stigma, the male gamete is still separated from the female gamete because of the wall of the pollen and the cells of the stigma and style. In order for both male and female nuclei to meet the following processes follows.
1. Soon after the pollination, the stigma produces sucrose solution which nourishes the pollen grain.
2. The pollen grain absorbs this fluid, exine burst and the intine grows down as the pollen tube.
3. The tip of the pollen tube releases enzymes which break down the cells of the stigma and style.
4. This tube grows down the style and into the ovary.
5. The growth of the pollen tube is directed towards the ovule by chemicals produced by the egg cell.
6. When the pollen tube arrives at the ovary, it enters the ovule though a small hole called micropyle.
7. Once the pollen tube is inside the ovule, the tube nucleus degenerates, and the generative nucleus divides to form two male nuclei, and the tip of the tube bursts releasing the two male nuclei into the embryo sac.
8. One male nucleus fuses with the egg cell nucleus to form zygote.
9. The second male nucleus fuses with the secondary nucleus to form endosperm nucleus
In flowering plants double fertilization occurs . In double fertilization there are two male nuclei traveling down the pollen tube and entering the ovule.
One of the male nuclei fuses with the egg cell nucleus to from the zygote. The zygote divides by mitosis and form an embryo, which consists of plumule and radicle. The second male nucleus fuses with the secondary nucleus to form the endosperm in which food is stored in some seeds.
The changes in a flower after fertilization
· The zygote divides by mitosis and form an embryo which consists of plumule and radical.
· The integuments of the ovule becomes the testa (seed coat) of the seed.
· The ovule turns into a seed.
· The food produced in the leaves is translocated to the wall of the ovary and form the fleshy pericarp of the fruit.
· The petals, anthers, stigma etc are withered away.
· The ovary turns into a fruit.
DISPERSAL OF FRUITS AND SEED
When the seeds are mature, the whole fruit or individual seeds fall from the parent plant to the ground and the seeds may then germinate. In many plants the seed or fruit is adapted in such a way that they are carried a long distance away from the parent plant. This scattering of seeds and fruits away from the parent plants is known as dispersal.
Advantages of Dispersal:
· Reduces the competition for light, water and mineral salts between the members of the same species.
· Avoids overcrowding of plants.
· It helps to colonies plants in a new area.
· It helps to obtain variation among the members by inter breeding with the existing members in an area.
TYPES OF DISPERSAL
Fruits and seeds may be dispersed by external agents such as animals, wind, water, humans or by self-dispersal explosion mechanisms.
The main adaptations of fruits and seeds to various dispersal methods are given in the below.
Main adaptation features of fruits and seeds:
· Small sized fruits or seeds.
· Parachute or tuft of hairs projecting from fruit or seed. Eg: Dandelion.
· Wing-like outgrowths from the ovary wall fruit. Eg: Sycamore, red maple etc.
· Open seed cases which are easily shaken by wind. Eg: small, light seeds of poppy.
Main adaptation features of fruits and seeds:
· Hooked and hairy fruits. Hooks get caught in the fur of passing animals and the seeds fall out as the mammals move about. Eg: Burdock,
Succulent, brightly coloured and scented fruits having small seeds with hard coats. When animals and birds eat these, the seeds pass out with the faeces without getting digested. Eg: papaya, mango, straw berry and tomato.
SELF DISPERSAL- EXPLOSIVE MECHANISM
· Pericarp dries in the sun and shrink. The tension generated splits the fruit longitudinally into two halves suddenly and ejects the seeds out. E.g.: pods of leguminous plants.