Pollination is the manner in which the pollen grains transfer from the anther to the stigma of the same or different flower. (on same or different plant).The pollination occurs only in gymnosperms and angiosperms.
On the basis of the destination of pollination is of two type:-
- Cross pollination
Self-pollination is the transfer of the pollen grains from the anther of a flower to the stigma of either the same flower or genetically similar flower.
Cross pollination is the deposition of pollen grains from the anther of a flower to the stigma of a different flower of the other plant of same or different species.
Self-pollination is of two type:-
When the pollen grains are transferred from anther to the stigma of the same flower, it is called autogamy.
Condition for favoring autogamy
The essential conditions for autogamy are – Bisexual flower, Homogamy, Cleistogamy.
- Bisexual flower – The flower should have both male and female reproductive parts.
- Homogamy – In homogamy both male and female reproductive parts of a bisexual flower mature at the same time.
- Cleistogamy – The flower never opens, or which never expose their sex organs to the environment are called cleistogamous flower and this phenomenon is called cleistogamy. Example – Viola mirabilis, Oxalis acetocela.
The flower opens or exposes their sex organ to the pollinating agents are called chasmogamous flowers and the phenomenon is called Chasmogamy.
Commelina benghalensis have both cleistogamous and chasmogamous flowers. Its aerial flowers are chasmogamous (cross-pollinated by insects) and underground flowers on rhizome are cleistogamous (Self-pollinated).
When the pollen grains are transferred from anther to stigma of the different flower on the same plant is called geitonogamy. Genetically geitonogamy is equivalent to self-pollination.
Advantages of self-pollination
- It maintains the parental characters.
- It is used to maintain pure lines for hybridization experiments.
- The does not need to produce a large number of pollen grains.
- It eliminates some bad recessive characters.
Disadvantages of self-pollination
- Immunity against the diseases decreases.
- Vigour and Vitality of the race decrease with prolonged self-pollination.
- The Variability changes and hence adaptability to survive in changed environment are reduced.
Cross-pollination is the transfer of pollens from an anther to the stigma of the different flower on the different plant. It is also known as Allogamy or Xenogamy.
Contrivances for cross pollination
The main reasons or adaptations (contrivances) for cross-pollination in bisexual flowers are:-
- Dicliny or Unisexuality
- Self-sterility or Self-incompatibility
Dicliny or Unisexuality
There are two types of unisexual flowers are present in the plants:-
- Male flowers, having only stamens.
- Female flower, having only carpels.
When the plants have unisexual flowers then there are two conditions possible:-
- Monoecious plant – when both male and female unisexual flowers are present on the same plant then plant is known as the Monoecious plant. Example – Maize, Coconut, Mango, Grapes, etc.
- Dioecious plants – When male and female unisexual flowers are present on two different plants this condition is called Dioecious.
In the dioecious state, the plant bear male plant bears staminate flowers, and female plant bears pistillate flowers. Example – Papaya, Asparagus, Date palm, Spinach, etc.
Dichogamy is the condition of bisexual flowers in which male and female sex organs mature at different times. There are two conditions in dichogamy
- Protandry – It is the condition in which the anther mature before gynoecium is known as Protandry. Example – Sunflower, marigold, Cotton.
- Protogyny – It is the condition in which the gynoecium mature before anther is known as Protogyny. Example – Mustard, Banana, Four o’clock plant.
It is the presence of a natural and physical barrier between male and female reproductive parts which helps in avoiding self-pollination.Example – Calotropis stignui ( gynoecium is fused with anthers and form gynostegium disc).
In some plants, flowers have many forms of anthers and stigmas at different levels i.e the condition in which styles and filaments are of different heights, is called heterostyly. Example – Primula vulgaris.
Self-sterility or Self-incompatibility
The pollen grains of a flower has no fertilizing effect on the stigma of the same flower. Example – Radish, tea, Petunia axillaris.
Advantages of cross-pollination
- The Cross pollination involves genetic recombinations, and hence there is variation in the progeny.
- The offspring obtained from cross-pollination have better adaptability towards changes in the environment.
- Plants produced by cross pollination are more resistant to disease.
Disadvantages of cross-pollination
- In cross-pollination, plants have to produce a large number of pollen grains, so this is highly wasteful.
- In cross-pollination, it is possible that some unwanted characters creep in the race or some unique characters are spoiled.