Self-pollination and its type

February 24, 2019

Self-pollination is the transference of pollen grains from the anther of a flower to the stigma of either the same or genetically similar flower.
It is generally not dependent on any external agency for pollination.
It is of two types:

1.Autogamy

It is the transfer of pollen grains from anther of a bisexual flower to its stigma.
Both stamens and carpels mature at the same time.
It occurs in several cereals (wheat, rice), some peas and flower which do not open.

2.Geitonogamy

It is kind of self-pollination where flowers could be bisexual or unisexual but are borne by the same parent plant.
The pollens from one flower deposited on the stigma of another flower borne on the same plant.
It may require an external agency like insects or wind.
Self-pollination may occurs when the flowers are open (chasmogamy) or closed (cleistogamy).
Chasmogamous self-pollination occurs only when their anthers and stigma mature at the same time (homogamy).
The self-pollinated flowers are generally small, inconspicuous, colourless, odourless, and nectarless.
Self-pollination occurs by following methods:

a) Homogamy

In this case the anthers and stigmas of a bisexual flower mature simultaneously.
The pollen grains reach the mature stigma either by contact, wind, gravity, rain drop or even insects.
Self-pollination brought about by contact is called direct autogamy and remaining agencies perform indirect autogamy.

i) Direct autogamy

It is accomplished by the movement of floral parts on account of growth, bending or folding.
In Convolvulus, Ixoza, Catharanthus and Gardenia the anthers are borne at the mouth of the corolla tube.
With the growth of style the stigma comes in contact with the ripe anthers and pollination occurs.
Pollination takes place in Cotton when the stigma in being pushed out of the staminal tube due to the growth of the style.
In Mirabilis, the filaments of ripe anthers bend and bring in contact with stigma.
A similar curling of style takes place in potato.
The stigma of Sunflower has brushing hair which helps in pushing the pollen grains from the synantherous tube.
The stigma curls back and receives the pollen grains present on the brushing hair.
The closing of flowers at night performs self-pollination in Argemone mexicana.

ii) Indirect autogamy

It occurs without coming contact of anthers and stigmas.
The pollen grains reach the stigma by rain (e.g. Caltha) or gravity (e.g. Lilac).
In Lilac, the anthers lie exactly above the stigmas.
The pollen grains of the dehisced anthers fall over the low lying stigmas under the influence of gravity.
In Sanicula, male and female flowers grow side by side.
The long style of the female flower bends over the male flowers to get pollinated.

b) Cleistogamy

It occurs in bisexual flowers which are always closed.
Such closed flowers can perform only self- pollination.
It is generally accompanied by geocarpy when the fruits are formed underground, e.g. Groundnut.
The phenomenon of having both open and closed flowers is called chasmocleistogamy.

Advantages of Self-pollination

It does not depend on the external agency, so it is a sure method of pollination.
Some flowers e.g. Oxalis utilize self-pollination for seed production when cross-pollination fails. Pollen grains are not wasted.
Parental characters can be preserved by this pollination.
It causes no genetic variations and because of this property, it maintains the purity of the races from generation to generation.
It is utilized in production of homozygous parents for experimental hybridization and evolving useful varieties.

Disadvantages of Self-pollination

It produces weak progenies, generation after generation, which cannot adapt to the changing environment.
It does not help in evolution of new species, as it does not cause any variability.
It cannot give any new character or new ability to the plant.
Useful characters cannot be introduced by this method.
The undesirable and defect characters do not get eliminated from a plant.
The immunity of race towards infection decreases. Thus, the plants become susceptible to many diseases.

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