What are the Advantages and Disadvantages of Pollination ?

‘What are the Advantages and Disadvantages of Pollination ?‘, if you have the same question, stay tuned with this article because we are going to cover everything about ‘advantages and disadvantages of pollination‘ and ‘types of pollination‘.

The primary requirement for sexual reproduction is the complete fusion of male and female gametes by the process called fertilization.

But…

Before knowing more about the advantages and disadvantages of pollination, you should know ‘what is pollination ?‘ first.

So lets get start…

What is Pollination ?

The transfer and deposition of pollen grains from an anther to the stigma of a carpel in a flower is called pollination.

According to wikipedia, Pollination is the transfer of pollen from a male part of a plant to a female part of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by wind.

It was about the definition of pollination, lets discuss about the types of pollination.

Types of Pollination

Pollination is of two types i.e.

1. Self-pollination
2. Cross-pollination

Self-pollination or autogamy

When the pollen grains from the anthers of one flower are transferred to the stigma of the same flower or flowers borne on the same plant, it is called self- pollination or autogamy (auto-self, games-marriage).

Self-pollination mostly occurs in either bisexual flowers or monoecious plants (if flowers are unisexual). To achieve self-pollination, the flower must possess certain adaptations, such as:

1. Bisexuality (Hermaphroditism) – The flower should bear both the stamens and carpels so that, both the pollen grains and ovules are borne in the same flower.

2. Monoecious plants – If the flowers are unisexual, both the male and female flowers should be present on the same plant.

3. Homogamy – To ensure self-pollination, both the anthers and stigma of the bisexual flower or unisexual flowers present on the same plant, should mature simultaneously.

4. Cleistogamy – The pollen grains from the anthers get deposited over the stigma of the same bisexual flower, which never opens. For example some species of balsam, sundew, pansy, wood-sorrel, oxalis, commelina etc.

5. Bud pollination – In some plants e.g. pea, wheat and rice, the anthers and stigmas of bisexual flowers ripen before the opening of buds. Self-pollination occurs before the flower opens.

Advantages of self-pollination

The advantage of self-pollination are as follow :

• Chances of pollination to occur are greater if flowers show homogamy.
• Parental traits are preserved from generation to generation.
• Very large number of pollens, need not be produced. Small number of pollens will ensure pollination.
•  The flowers need not be large and showy.
• Scent and nectar need not be produced by the flowers for attracting pollinating agents.

Disadvantages of self-pollination

The disadvantage of self-pollination are as follow :

• Repeated self-pollination decreases the vigour and vitality of future generations.
• The weaker or defective characters are inherited as such and weaken the species.
• Genetic improvement cannot take place.

Cross-pollination

When the pollen grains from the anthers of one flower are transferred on the stigmas of flowers borne on different plants belonging to same species, it is called cross-pollination or allogamy or xenogamy.

Cross-pollination mostly occurs in dioecious plants having unisexual flowers. But, a large number of examples are available where in spite of bisexuality, plants prefer cross-pollination over self-pollination.

How nature favours cross-pollination ?

Nature always favours cross pollination because it helps in the formation of new and better new- combinations of characters. For the purpose of cross-pollination, following adaptations are found in the plants :

(i) Unisexuality – The plants bear either male or female flowers i.e. plants are dioecious e.g. palm, mulberry, papaya, etc.

(ii) Dichogamy – If the flowers are bisexual, the male and female organs mature at different times. The flower may be :

(a) Protandrous – When the anthers mature earlier than the stigma, the flower is said to be protandrous, and this phenomenon is termed as protandry e.g. Salvia, sweet pea, sunflower etc.

(b) Protogynous protogynous if the stigma matures earlier than the anthers. This phenomenon is called protogyny e.g. magnolia, custard apple, peepal, etc.

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(iii) Self-sterility – In this condition, the pollens from the anthers of a flower fail to germinate on the stigma of the flowers borne on the same plant. It happens because of genetic incompatibility e.g. Nicotiana, potato, orchids, etc.

(iv) Herkogamy (herkos-a fence barrier) – The flower possesses certain structural barrier. It does not allow self-pollination though, the flower is bisexual and both the anther and stigma mature simultaneously, e.g. in pansy and iris a hood covers the stigma.

(v) Heterostyly – In this condition the flower has style and anthers at different heights. Either the style is very long or anthers are at more height so that it discourages self- pollination e.g. primrose, Oxalis, Linum etc.

Agents of Cross Pollination

In cross-pollination, pollen grains from the anthers of one flower are transferred to the stigma of another flower.

The pollen grains need some abiotic or biotic agent in pollination, such as wind, water, insects, birds or animals to transport and bring about pollination. The most common agents are the wind and insects.

It is also done artificially by the plant breeders for producing useful and better varieties.

Anemophily

Flowers pollinated by wind have the following characteristics :

• Usually, they are neither large, nor showy or bright coloured. The corolla is usually small and inconspicuous. They are not scented.
• The stamens having long filaments sway easily with the wind.
• The pollens are produced in very large quantities.
• The pollen grains produced are very light in weight. They are usually provided with wings.
• The stigmas are usually large and feathery. The style is long enough to expose the stigma well above the flower.
• Often, the flowers appear and mature before new leaves arise. It avoids interference by the leaves. Examples – maize, bamboo, pine, etc.

Wind pollination in maize

In maize the cluster of male flowers are borne at the apex. The female flowers are borne near the base in the axil of leaf on the plants. The male flowers produce large quantities of pollen grains.

As the anthers burst, the light and dry pollen grains are blown by the wind. Many of them land on the silky and feathery stigma of neighbouring plant, hence, bringing about cross-pollination.

Hydrophily

Flowers are pollinated by water in case of aquatic plants, such as Hydrilla, Vallisnaria, etc. In such cases the following characteristics are found:

• Plants are dioecious.
• Flowers are small and inconspicuous. They are not scented as well.
• Pollen grains are produced in large quantities.
• The pollen grains have the same specific gravity as that of water, so that they can float in water.
• In some cases, the male flowers detach and float on water till they meet the female flowers.

In Vallisnaria, pollination is very interesting. The plants are dioecious. The female (pistillate) flowers have long stalks. Whereas, the male (staminate) flowers have short stalks and are submerged.

Before pollination…

The male flowers detach from the parent plant. They are carried by water currents up to the female flowers. In this way. pollination takes place when the anthers burst open setting the pollen grains free, after striking stigmas.

Zoophily

Cross-pollination caused by animals is called zoophily. Depending upon the agent used, cross- pollination is categorised as (A) Entomophily (by insects) ; (B) Ornithophily (by birds) ; (C) Chiropterophily (by bats) ; (D) Malacophily (by snails) ; (E) Artificial (by man), etc.

(A) Entomophily – Insect-pollinated flowers have the following characteristics :

• Day-blooming flowers have either very bright coloured attractive corolla (e.g. Crotolaria), small florets grouped into a head (e.g. sunflower) or some other conspicuous feature like coloured bracts (e.g. Bougainvillea) so as to attract the insects.
• Night-blooming flowers are often white coloured but strongly scented e.g. jasmine, coffee, etc.
• Flowers have nectaries which secrete nectar- a sugary fluid. The insects which come in search of nectar also carry pollen grains from one flower to another. Such flowers are called nectar flowers.
• The flowers produce a small number of pollens. The insects visit these flowers to eat the pollen grains which serve as food for them.
• The pollen grains have rough, sticky surface.
• The surface of stigma is also sticky as in the flowers of China rose, lotus, roses, etc.

Insect pollination in Salvia – Pollination in Salvia is brought about by bees for which it has specialized floral mechanism. The flower has a two-lipped corolla with two protandrous stamens attached to the corolla tube.

Only one lobe of each anther is fertile whereas, the other sterile lobe forms a plate. It lies at the mouth of the flower just above the lower lip of the corolla. When the insect (bee) enters the corolla tube, it pushes the lower sterile anther lobe upwards.

The fertile lobe swings down by lever mechanism and dusts the back of the bee with a cloud of pollens. When the bee visits another flower, it transfers the pollens to a mature stigma, thus causing cross pollination.

Insect pollination in Pisum sativum (Pea) – The sweet pea is pollinated by bumple bee. The flowers have five petals- one large outer or uppermost petal, two lateral petals and two innermost keel petals.

The bees are attracted by colour,scent and nectar. The two lateral wing petals act as landing platform for the bee. Whem the bee lands on the wing petals, the keel petals are also pushed down because of the weight of  the  bee.

As a result…

The stamens and stigma touch the undersurface of the bee that may be carrying pollens from another flower. It may transfer some of the pollens to stigma thus, causing cross-pollination.

B) Ornithophily – There are only a few flowers pollinated by birds as in canna, Bignonio. In such plants, the following characteristics are present.

• The flowers are usually present in groups.
• They are bright coloured or produce abundant nectar for the attraction of birds.
• The pollen grains are produced in large quantities so as to ensure the process of pollination.
• The floral parts are leathery.
• Corolla is funnel shaped.

(C) Chiropterophily –  It is cross-pollination performed by way of bats. The bats bring about pollination only in the tropical plants.

The bat- pollinated flowers are large and strongly scented as the bats are nocturnal e.g. Bauhinia megalandra. They produce abundant nectar and pollen.

(D) Malacophily – It is the pollination brought about by snails e.g. Arisaema.

(E) Flowers of Rafflesia found in Burma and Indonesia are pollinated by the elephants. The flowers of Rafflesia are found on the ground.

The pollen grains stick to the feet of elephants and are carried to the stigma of other flowers bringing about cross-pollination.

(F) Artificial pollination – Man plays an important role in bringing about pollination artificially for producing useful and better varieties. Pollen grains from the mature anthers of a flower from better yielding quality plant are carefully removed.

They are dusted on the mature stigma of other plant belonging to same or related species. These pollinated flowers are now covered with polythene bags till fruits and seeds are produced. This method is very common in garden plants.

Advantages of cross-pollination

Advantages of Cross Pollination are as follow  –

• It results in healthier offsprings.
• It helps to produce new variations and recombinations some of which are very useful.
• New and improved varieties of plants can be produced by this method.
• The plants produced from the seeds of cross- pollinated flowers are more resistant to diseases and other unfavourable conditions.
• The seeds produced by cross-pollination are much better, usually larger, healthy and more vigorous.
• The seeds produced by cross-pollination are much numerous.
• The yield by such seeds is very high and never falls below an average minimum.

Disadvantages of cross-pollination

Disadvantages of Cross Pollination are as follow :

• It is not a sure method. Plants have to depend upon certain external agencies for pollination which may or may not be available at the proper time.
• A large quantity of pollens needs to be produced to ensure cross-pollination, much of which goes waste.
• It is less economical. The plants have to develop many devices, such as scent, nectar, coloured petals etc. to attract various pollinating agencies.
• Many good characters of the plants may also be lost in recombinations.

SUMMARY