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Sexual Reproduction in Plants

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Sexual Reproduction in Plants - Lesson Summary

Reproduction
Reproduction is the process by which living organisms produce their young ones. Plants reproduce either sexually or asexually.

Sexual mode of reproduction
This type of reproduction involves two organisms of opposite sex, the male and the female. Reproductive organs in plants produce gametes – ovules (eggs) and pollen grains.
  • Egg is the female gamete produced by ovary.
  • Pollen grains are male gametes produced by stamens.
  • A zygote is the future individual formed by the fusion of an egg and a pollen grain. Zygote develops into a seed.
  • Seeds enclose embryo, the future individual until favourable conditions prevail for the development of embryo into a plantling.

Reproductive structures of a plant
Flowers are the reproductive structures which help the plant to undergo the process of sexual reproduction. These are the most attractive parts of the plant. A flower may have a stalk-like structure called the pedicel which helps in its attachment to the plant.

Structure of a flower
A complete flower is made up of four whorls on it. These are sepals, petals, stamens and the pistil. These four whorls are attached to flattened tip of the flower called as receptacle.
a) Sepals and petals are termed to be accessory whorls or vegetative whorls as they do not take part in the formation of zygote.
b) Stamens and pistil are termed to be reproductive whorls as they play vital role in the formation of zygote.
  • Sepals collectively form calyx. These protect the inner parts of the flower in its bud condition.
  • Petals collectively form corolla. These are the coloured structures which attract insects and birds for pollination.
  • Stamens are the male reproductive parts. Stamen is made up of two structures namely, a filament and an anther. Filament is a tubular structure. Anther is a knob-like structure containing pollen sacs filled with pollen grains. Pollen grains act as the male gametes or the male reproductive cells.
  • Pistil is the centrally located female reproductive part of a flower. It is made up of stigma, style and the ovary. The stigma is a flattened structure which receives the pollen. It is sticky in nature. The style is long tubular structure which conveys pollen to the ovary. The ovary contains numerous ovules. Each ovule contains a female gamete or egg cell.

Types of flowers
Flowers based on the reproductive whorls they carry, can be classified into unisexual flowers and bisexual flowers.
  • A unisexual flower is the flower which consists of either stamens or pistil. A flower with a whorl of stamens is called as male flower. A flower with just a pistil is called as a female flower.
  • A bisexual flower is the flower which consists both the whorls of stamens and pistil. i.e. male and female reproductive structures.

Pollination
Pollination is the transfer of pollen from the stamen to the stigma.
  • Different factors which help in the transfer of pollen are called as agents of pollination. Agents of pollination include air, water, birds, insects, animals etc.
  • Pollination can be of two types namely, self-pollination and cross-pollination. Self pollination is the transfer of pollen from the stamen to the stigma of the same flower or the flowers on the same plant. Cross-pollination is the transfer of pollen from the stamen of one flower to the stigma of another flower on a different plant of the same kind.

               SELF-POLLINATION

                     CROSS-POLLINATION

It occurs between the flowers of the same plant. 

It occurs between the flowers on the different plants of the same variety. 

It does not require any agents for pollination.

It requires agents like air, water etc for pollination.

It requires maturation of pollen and eggs at the same time.

 It is not necessary that pollen and eggs to mature at the same time.



Fertilisation
Stigma of the flower secretes sugary substances.
  • As the pollen grains from the stamen fall on the stigma, they recognise the sugary substance and germinate to produce a structure called as pollen tube.
  • Pollen tube grows in size through style to reach the egg in the ovary.
  • As it reaches the ovule, the tip of the tube opens and releases male gametes to fertilise the egg cell.
The fusion of a male gamete and a female gamete to produce a zygote - single cell. Zygote or the oospore develops into an embryo. Fertilised ovary becomes a fruit. The ovule develops into a seed.

Formation of seed
There occur some changes in the flower which underwent the process of fertilisation.
  • Sepals, petals, stigma and the style fall off leaving the fertlised ovary.
  • Zygote utilises the reserved food in the ovule to divide and transform into an embryo.
  • Walls of the ovule develop hard layers of the seed.
  • A seed contains a young plant and stored food in the form of cotyledons.

Formation of fruit
Ovary grows in size due to formation of seeds. This well-grown fertilised ovary becomes the fruit. A fruit can have any number of seeds in it. Fruits can be fleshy or dry in nature.

Seed dispersal
Seeds require sufficient amount of sunlight and water to germinate. Seeds falling under the parent plant compete for these factors. Hence, seeds get scattered for proper germination and growth. Seed dispersal is a method of scattering seeds by different agents like water, animals, birds, wind or insects to long distances.

Agents for seed dispersal
Nature appoints different agents for the dispersal of seeds. In some plants, fruits along with seeds are dispersed to long distances. But in many plants only the seeds get dispersed to long distances. Different agents of dispersal include wind, water, birds and animals.
Wind: Seeds which are light and have hairy structures are blown away by wind to long distances. e.g. Calotropis.
Water: Coconut seeds become light as they ripen and dry. As they fall into water, they float on the water surface. They are carried to long distances by flowing water. The pond water is adapted for seed dispersal through water.
Birds: Birds like parakeets feed on fruits along with seeds. As they fly to different places, seeds come out along with their droppings. e.g. Guava
Animals: Animals like monkeys feed on fruits. They throw the seeds at different places. This helps in seed scattering. Some seeds have hook like structures with which they cling on to animals moving to different places. e.g. Xanthium
Bursting: It is also a method of seed dispersal. Fruits in the form of pods burst open exposing the seeds to environment. The force with which they burst also scatters the seeds to long distances. e.g. Balsam. Drumstick seeds are also dispersed by the explosion of fruits.

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