POLLINATION IN PLANTS

Pollination is defined as the transfer of mature pollen grains from the anthers of one flower to the mature stigma of the same flower or another flower of the same plant or closely related species. Pollination is the first step which leads to the eventual coming together of the male and female gametes for the sake of fertilisation.

 

TYPES OF POLLINATION

There are two types of pollination. These are self pollination and cross pollination.

1) Self pollination

Self pollination is the transfer of mature pollen grains from the anther of a flower to the stigma of the same flower or to that of another flower of the same plant, e.g. pea, cotton, tomato. For self pollination, only one parent plant is involved (fig. 30.1) (plant B).

 

2) Cross pollination

Cross pollination is the transfer of mature pollen grains from the anther of a flower to the stigma of a flower of another plant of the same or closely related species, e.g. morning glory, Hibiscus, pride of Barbados. For cross pollination to occur, two parent plants are involved (fig. 30.1) (Plants A and B).

 

AGENTS OF POLLINATION

Agents of pollination also called pollinators are organisms and other means by which pollen grains are transferred from the anthers to the stigma of flowers. Agents of pollination include insects, wind, water and other animals like snails, birds, bats and man. Common insects involved in pollination are butterfly, moth and bees.

 

FEATURES OF POLLINATED FLOWERS

Conditions or Devices which Aid Self Pollination

Some plants have conditions or devices which aid self pollination to take place. These conditions are homogamy and cleistogamy

 

A) Homogamy

Homogamy refers to the ripening of the anthers and stigmas of a bisexual flower at the same time. Under this condition, self pollination may occur in the following ways:

1. A gentle breeze may blow the mature pollen grains which may be shed on mature stigma that are situated below.
2. A visiting insect may transfer the mature pollen grains to the stigma of the same flower.
3. Self pollination may also occur when mature stigmas push their way out of the corolla tube during which they are brushed against the anthers and in the process pollen grains are collected.
4. In a situation where the filaments are longer than the stigma, the filaments may recoil to. touch the mature stigmas.
5. In like manner, self pollination may occur where the styles are longer than the filaments; the styles may also bend or recoil to make the stigmas touch the anther.

 

B) Cleistogamy

Cleistogamy is defined as a condition in which ripe pollen grains are deposited on the stigma which becomes ripened at the same time.

This situation usually occurs among closed flowers, that is, bisexual flowers which never open at all.

 

Advantages of self-pollination

1. Pollen grains are produced in small quantity and most of them are not wasted.
2. Once the pollen grains are mature, pollination is more certain to occur as it does not involve any distance.
3. The flower structures are less complicated and may not need any complex mechanism to achieve pollination.

 

Disadvantages of self-pollination

1. Seeds produced are less healthy than seeds produced from cross-pollination.
2. Plants produced from self-pollination are less likely to survive when there are changes in conditions of the environment. This is because they are usually more adapted to a particular condition.
3. There are little or no variations among species and the rate of evolution is slower.

 

Conditions or Devices which Aid Cross Pollination

Some plants may have conditions or devices which may aid cross pollination to take place. These are dichogamy, unisexuality and self-sterility.

 

A) Dichogamy

Dichogamy refers to the ripening of the anthers and stigmas of a bisexual flower at different times. Dichogamy occurs in two ways.

These are protandry and protogyny.

1) Protandry

Protandry refers to the condition in which the anthers of a flower mature earlier than the stigmas of that flower or other flowers of the same plant so that the mature pollen grains are only useful to flowers of other plants which have mature stigmas to receive them.

 

2) Protogyny

Protogyny refers to the condition in which the stigma of a flower matures earlier than its own pollen grains or those of other flowers of the same plant so that it can only receive pollen grains from flowers of other plants.

 

B) Unisexuality

Unisexuality is a situation in which some plants bear only male or female flowers and not both on the same plant, e.g. pawpaw. Such plants are said to be dioecious plants. Cross pollination may occur under this condition.

On the other hand, in a monoecious plant the male and female flowers are borne by the same plant, the female flowers are usually situated higher than the male flowers so that pollen grains may not reach the stigma of the female flowers. Hence, they will be received only by stigmas of female flowers of other plants.

 

C) Self-sterility

Self-sterility refers to situation in which some plants make themselves sterile. The presence of pollen on their stigmas is injurious to further development of the plant. For example, they may wither and die. However, when pollen grains come from other plants, fertilisation can take place in such plants. Examples are found in passion flowers and tea.

 

Advantages of cross pollination

1. It leads to the production of more seeds.
2. It produces healthier seeds.
3. The seeds develop into stronger plants, which will produce more fertile offspring.
4. Plants produced from cross-pollination are more likely to survive changes in environmental conditions since their genetical products are mixed up.
5. It leads to greater variations among species and a faster rate of evolution.

 

Disadvantages of cross-pollination

1. It is less certain to occur since it involves two separate plants which may be at a great distance.
2. Pollen grains are likely to be wasted during the process as the pollinating agents may be transferring them to wrong plants. To compensate for the waste, pollen grains must be produced in large quantity.
3. Cross-pollinated flowers are often more complicated in structure than self-pollinated flowers and often require some special mechanisms.

 

Differences Between Self pollination and Cross pollination

 

Self Pollination

1. Self pollination takes place only in bisexual plants.
2. Only one parent is involved.
3. Pollination may occur without an external agent.
4. It does not ensure new varieties.
5. Pollen grains are effectively utilised.

 

Cross Pollination

1. Cross pollination takes place in both unisexual and bisexual plants.
2. Two parents are involved.
3. This requires external agents, e.g. insects and wind.
4. It results in the formation of new varieties.
5. Much of the pollen grains are wasted.

 

Major Agents of Pollination

Two major agents of pollination are wind and insects.

 

Characteristics of Insect Pollinated Flowers

Insect pollinated flowers also called entomophilous flowers have the following characteristics:

1. They have large conspicuous petals/ sepals.
2. Flowers are usually brightly coloured.
3. They possess scent.
4. Nectar is also present.
5. Pollen grains are rough, sticky and relatively few.
6. The stigma is flat with sticky surface to enable it receive pollen grains.
7. Petals are shaped and arranged to enable visiting insects become dusted with pollen grains.

Examples of insect pollinated flowers are hibiscus, delonix, cowpea, crotolaria and pride of Barbados.

 

Process of Pollination by Insects (e.g. Pride of Barbados)

The insects that normally pollinate pride of Barbados are the swallow tail butterfly and bees. When the insect lands on the standard petal it uncoils its proboscis and inserts it through the furrow in the standard petal that leads to the nectar. During this process, the hairy body or wings of the butterfly or bee touches the pollen grains and these stick to the body or wings of the insect. When the insect visits another pride of Barbados flower, the pollen grains on its body or wings may touch the stigma of this flower thereby bringing about cross pollination.

 

Characteristics of Wind Pollinated Flowers

Wind pollinated flowers also called anemophilous flowers have the following characteristics:

1. They have small, inconspicuous petals / sepals.
2. Flowers are usually dull coloured.
3. There is absence of scent.
4. There is absence of nectar.
5. Large quantity of pollen grains are produced.
6. Pollen grains are small, smooth, light and not sticky.
7. Stigma is elongated and sticky with large surface area.
8. Anthers are attached to the flower in such a way that they readily swing in the air and release the pollen grains. Examples of wind pollinated flowers are maize, guinea grass, rice, millet and wheat.

 

Wind pollination

Structure and pollination of maize (Zea mays)

Structure: Maize has male and female flowers separately on the same plant and it is therefore monoecious.

 

Structure of a male flower

The male flowers are contained in a panicle of inflorescence in the uppermost part of the plant and they are made up of many spikelets. The spikelets are arranged in pairs on the main axis. A single spikelet consists of two green bracts or glumes which become brown at maturity. Inside the glumes are two sets of flowers each covered by two smaller bracts known as pales (outer and inner pales). One of the flowers matures before the other. At maturity, the pales open with three long stamens hanging freely in the air. At the base of the stamens are two small structures called lodicles or reduced perianths.

(There are no petals and sepals). The lodicles have no function except that they help to open the pales when they swell.

 

Structure of a female flower

The female flowers are borne in a cob which develops from the axis of a leaf far below the male inflorescence. The cob has numerous green bracts known as husk covering the spikelets of flowers inside. A spikelet also has a pair of glumes with a flower made up of an ovary with a long style ending in hairy stigma and two lodicles. The second flower in the glume is sterile.

 

Pollination

The male flowers mature before the female flowers, therefore, self pollination does not occur. When the stamens mature, the lodicles swell to open the pales. The elongated filaments of the stamen hang out the ripe anthers out of the glume. The slightest air current shakes the anthers which release a cloud of powdery pollen grains which are carried by wind. These pollen grains then land on the hairy sticky stigmas of cob thereby pollinating the flowers.

 

Pollination of grass

Many grasses have their flowers in compound raceme inflorescence or in spikes (figure 16.39 A and B). The flowers are borne in certain unit structures known as spikelets (figure 16.39 C).

A spikelet consists of three or more bracts enclosing one or two flowers. At maturity the bracts open to expose the flowers.

 

Insect pollination 

Pollination of flamboyant

Flamboyant is pollinated by large butterflies.

The butterfly is attracted by the brightly coloured petals of the flowers. The butterfly lands on one of the fan – shaped petals and inserts its long tongue into the tube of the folded standing petal which is believed to be directing the tongue to the nectaries at the base of the ovary. The posterior petal is thus known as “honey guide”. During this process the body of the insect becomes dusted with pollen grains. The flower is protandrous, that is, the stamens mature before the stigma. The butterfly carries the pollen grains to another flower whose stigma is mature. In an attempt to collect more nectar, the stigma makes contact with the body, brushing up some of the pollen grains from the previously visited flower. Cross-pollination has thus taken place.

 

Pollination of Allamanda

Allamanda is mostly pollinated by sunbirds. The sunbird has a long slender beak adapted for sucking flower juice. The bird lands on a stem close to the flower, and inserts its long beak into the corolla tube to collect some nectar. The anthers are normally protected inside the corolla tube and their pollen grains are retained after liberation from the anthers. The bird struggling to collect nectar pushes some of the pollen grains into the stigma which is closely below the anthers. This is self-pollination.

Self-pollination is also possible in Allamanda without the help of any pollinating agent. This is because the stigma can grow to meet the anthers above.