Sexual reproduction in plants

  • Sexual reproduction fusion takes place between male and female haploid nucleus. This is called fertilization and leads to the formation of zygote. The zygote formed is genetically not identical to the gametes as they have diploid nuclei.
  • Sexual reproduction results in offspring geneticallydifferent from the parents.
  • All flowering plants (angiosperms) show sexual reproduction.
  • The flowers of the plants are the organs of sexual reproduction.
  • They are usually bisexual and sometimes unisexual.

Flower structure

Flower is a modified stem which functions as a reproductive organ and produces ova and/or pollen

flower-structure

 

  • Petal: Usually colorful modified leaves that make up the “flower”, collectively called the corolla. They may contain perfume and nectar glands.
  • Sepal: Protective leaf-like enclosures for the flower buds, usually green, collectively called
  • Stamen: Male flower organ.
    Anthers: Pollen-producing organs.
    Filament: Stalk supporting anthers.
  • Carpel:Central female organ of the flower. It is generally bowling-pin shaped and located in the center of the flower.
    Stigma: Receives pollen, typically flattened and sticky.
    Style: Connective tissues between stigma and ovary.
    Ovary: Contains ovules or embryo sacs.

Pollination

Pollination is the deposition of pollen from the anther of one flower onto the stigma of a different flower of the same species. When pollination occurs, the pollen grows a pollen tube down the stigma of the flower. The pollen tube carries the nucleus of the pollen into the ovary, where it fuses with an ovule.

Self pollination

Self pollination is the transfer of pollens from anther to stigma of the same flower or another flower of same plant.

self-pollination

Advantages of Self Pollination

  • It maintains the parental characters or purity of the race indefinitely.
  • The plant does not need to produce large number of pollen grains.
  • Only fertile parent is required.

Disadvantages of Self Pollination

  • New useful characters are seldom introduced.
  • Complete and continuous self pollination my lead to extinction.
  • Variability and hence adaptability to changed environment are reduced.

 

Cross pollination

Cross pollination is the transfer of pollens from anther of one flower to the stigma of another flower on a different plant of the same species.

cross-pollination

Advantages of cross pollination

  • Progeny shows enhanced vigor.
  • Offspring’s are more viable and resistant.
  • There is a possibility to get new desirable characters.

 

Disadvantages of cross pollination

  • Pollination may fail due to distance barrier.
  • Flowers have to totally depend on the external agencies for pollination.
  • It may introduce some undesirable characters

 

Wind pollinated flowers and insect pollinated flowers

The two most common agents for carrying pollen are wind and insects. Wind pollinated and insect pollinated flowers have structural difference adapted according to the method of pollination. Grasses have wind-pollinated flowers. Orchid is an insect pollinated flower.

 

Wind-Pollinated flower
Wind-Pollinated flower
Insect-pollinated flower
Insect-pollinated flower

Difference between wind and insect pollinated flowers

Wind pollinated Insect pollinated
Petals: Small inconspicuous, sometimes absent. If present, not brightly colored. Large, brightly colored, conspicuous and attractive to insects.
Pollen: Produced in large quantities, light, smooth pollen grains. Less produced pollen grains larger, sculptured walls to attachment to insects and to stigma.
Anthers: Move freely, so pollen is easily dispersed. Fixed to filaments and positioned to come into contact with visiting insects.
Stigma: Large often branched and feathery, hanging outside the flower to trap pollen. Small in closed within the flower, positioned to come into contact with visiting insects

Advantages and disadvantages of wind and insect pollinated flowers

Wind pollinated flowers Insect pollinated flowers
Advantages Seeds can travel a long on the wind, the plants migrate or move to other areas, and are not all bunched in one place.

 

Less wastage of pollen
Disadvantages High possibility that pollen grains won’t reach and fertilize or pollinate plant Some insects lay eggs in the ovary and larva feeds on developing embryo

 

Fertilization in flowering plants

  • Fertilization is the process of fusion of the female gamete, the ovum or egg and the male gamete produced in the pollen tube by the pollen grain.
  • Pollen tube will grow so that eventually the egg cell, hidden away in the embryo sac, can be fertilized.
  • The process of fertilization in plants occurs when gametes in haploid conditions meet to create a zygote which is diploid.
  • The male gametes of the flower are transferred on to the female reproductive organs through pollinators. The final product of this process is the formation of embryo in a seed.

fertilization-in-flowering-plants

The development of a seed and fruit

  • Once fertilization takes place, the ovule is called a seed. It remains attached to the plant and continuous to receive nutrients from it as it develops.
  • The fusion of male and female gametes produces a zygote. A zygote then develops into an embryo. An embryo consists of a tiny shoot, a root and two specialized leaves called cotyledons.
  • The cotyledons grow in size so they can enclose the embryo. The outer wall of an ovule called integument becomes harder and thicker to form a seed coat or testa.
  • The small gap in the integuments, the micropyle, remains as a tiny hole in the testa.
  • The petals and stamens shrivel and fall off.
  • The ovary becomes the fruit, and its wall becomes the pericarp of the fruit. The seeds are contained within the fruit, and the fruit is often adapted to disperse the seeds away from the parent plant.
development-of-seed
Development of Seed
development-of-fruit
Development of fruit

Fruit

  • Fruits are ripened ovaries containing seeds with sometimes additional flower or inflorescence tissues associated with them. Only angiosperms produce flowers and fruits.
  • Fruit consists of carpels where the ovules develop and the ovary wall, which may flesh or dry and hard.fruit

Types of fruits

  • A true fruit is one which develops from a single ovary of a single flower with no other part outside the ovary.
  • A fruit is false or spurious when other floral parts also take part in the formation of fruit.
  • Depending on flower structure and inflorescences type, fruits may be either simple, aggregate, multiple or accessory.
    1. Simple fruit- develops from a single ovary of a single flower; may be either dry or fleshy when mature.
    2. Aggregate fruit- develops from one flower with many ovaries
    3. Multiple fruit develops from ovaries of several flowers borne together on same stalk.
    4. Accessory fruit- fruit in which the edible part is primarily tissue other than ovary tissue.

types-of-fruits

Seed

  • Sugar and amino acids travel through the phloem of the parent and enter the seed.
  • Sugars are converted to starch and in some cases fat.
  • Amino acids are converted to protein. Starch and protein are then stored in the seed of the dicotyledonous plants in two large storage organs called cotyledons.

structure-of-a-seed

Dispersal of seeds

  • Spreading the seeds away from the parent plant is called dispersal.
  • Fruit and seed dispersed by animal, wind, self and water.

 

Advantage and disadvantage of seed dispersal

  • Help the plant to spread into new and different environment.
  • Prevent overcrowding.
  • The dispersal is dependent on external agents such as wind or animal.
  • The seeds may not end up in a place where they will be able to grow.

Animal dispersal

Animals disperse seeds in several ways. Some plants, like the burr at left, have that get tangled in animal fur, and are then carried to new sites.  Other plants produce their seeds inside fleshy fruits that then get eaten be an animal. The fruit is digested by the animal, but the seeds pass through the digestive tract, and are dropped in other locations.

burranimal-dispersal

Self-dispersal
Seeds are enclosed within a pod that explodes open when it becomes dry, e.g. pea.

self-dispersal

Wind dispersal
Seeds are generally very light and usually have some anatomical adaptation (hairs, wings) that enables them to be transported a long distance from parent plant, e.g. dandelion.

wind-dispersal

Water dispersal
Seeds are usually enclosed within an air-filled fruit that is capable of floating, e.g. water lilies.

water-dispersal

Germination of seed

  • Germination is the process of seeds developing into new plants.
  • When suitable environment conditions are available, the seed will germinate.

 

Process of germination

  • Water imbibation results in rupture of seed coat, uniform imbibation is important   and approximately optimum temperatures are required
  • The imbibition of the seed coat results in emergence of the radical and the plumule, the cotyledons get   unfolded. It is important that  the temperature and photo period  are required in optimum amounts
  • This marks the final step in the germination of the seed where the  cotyledons are expanded which are the true leaves.

germination-process

Conditions for germination
the environmental conditions necessary for germination include

  • Suitable temperature

Temperature influences the rate of enzyme controlled reactions going on inside the seed. Increasing temperature increases the rate of enzyme reactions up to the optimum temperature. Temperatures above this denature the enzymes.

  • Sufficient water

Seed is soaked into the water. It absorbs water. The water enters into seed through micropyle. The seed swells and breaks the testa. It gives way to the delicate embryo to grow out. The soaked seed also increases the permeability of testa to oxygen. Water provides medium for enzyme action

  • Oxygen
    Oxygen is required for aerobic respiration. Respiration provides the energy for metabolic reactions occurring inside the seed, which are necessary for growth.

 

Role of enzymes during germination

  • The seeds may contain starch, proteins or fats as storage material in their cotyledons.
  • When the seeds are soaked in water, the enzymes in the cotyledons become active.
  • They break large complex insoluble storage materials to the small simple soluble substances that can be transported to the growing region of the embryo where oxidation of the end products of digestion releases energy.germination-enzymes

Changes in the dry mass of seed during germination

Dry mass of the seed and seedling reduces till the development of the green foliage. After the green foliage develops, plants manufacture carbohydrates that are stored or converted to protoplasm, and then increase the dry mass of growing seedling.