Pollinationis the science in which the male parts of a mature flower comes in contact with the female. The grains are shed from the male part- anther and they reach the stigma-female part of a pistil. Pollination of a flowering plant happens in a phased manner under a systematic environment. In the olden days, pollination was known to linked only with the honey bees however, that is not always true. The modern floriculture has deeply analysed to describe the complete mechanism of pollination. This article delineates the types, processes, and mechanism of pollination.
Types of pollination
There are a multitude of classifications of pollination but the most common classification is based on the source of pollen. Based on the source of where pollination occurs, pollination can be divided into autogamy, geitonogamy, and xenogamy.
It is otherwise known as self-pollination. It is a process involving the transfer of pollen grains from the anther of a flower to the stigma of the same flower. When the flower is matured, its external parts get exposed to bring in clear contact between anthers and the stigma however, this can occur only if the stigma and anther are compatible with each other. Furthermore, the time at which the pollen releases must coincide with the receptivity of the stigma. Depending upon whether the flowers get easily opened or not, there are 2 types of pollinating flowers namely chasmogamous flowers and the cleistogamous flowers. Plants like Viola, Oxalis, and Commelina belongs to chasmogamous as they easily open up to allow contact between anther and sigma. In the case of cleistogamous flowers, they don’t easily open to allow contact between anther and stigma. So, in order to pollinate the male and female parts must stay close to each other. However, Cleistogamous flowers produce assured seed-set even in the absence of pollinators. The advantage of self- pollination is that the breeds maintain purity and the flowers do not depend on external factors for their pollination. Self- pollination will help in eliminating the recessive characters of the breed.
It is a subtype of autogamy as there is a transfer of pollen grains from the anther of one flower to the stigma of another flower of the same plant (but not to the flower of some other plant). This type of pollination produces genetically the same offspring as that of autogamy process. Although it is a partial geitonogamy, genetically it is similar to autogamy since the pollen grains come from the same plant.
It is otherwise known as cross-pollination. Xenogamy is characterized by the transfer of pollen grains from the anther of one plant`s flower to the stigma of another plant`s anther. Xenogamy provides a great chance of variation in the newly formed generations, and they are genetically expressive.
Agents of Pollination
Pollination needs an external vehicle or medium. Different agents aid in transferring the anther to the stigma. Pollinating agents are classified as abiotic and biotic factors. Abiotic medium doesn't involve the biological organism (instead, they use wind and water) On the other hand, biotic agents use some animals as the agents to achieve pollination. Most plants use biotic agents as a medium of pollination.
In a normal case, pollen grains contacting the stigma of a flower are influenced by external factors like excess of wind and rain, so, it’s a chance factor in both wind and water pollination. To substitute this loss, plants are fortunate enough to have the ability to produce a maximum amount of pollen.
Abiotic pollination by wind
Abiotic pollination uses non-living methods such as wind and water to move pollen from one flower to another. This allows the plant to spend energy directly on pollen rather than on attracting pollinators with flowers and nectar. The wind is the most common medium of abiotic pollination. Wind helps only when the pollen grains are light, non-sticky, allowing them to pass easily on to the next flower. Such a transfer can happen only if the wind currents are within a healthy range.
Abiotic pollination by water
On the contrary, pollination by water is extremely rare and the resulting flowers are called Hygrophilous flowers. It is evident in only about 30 genera of monocotyledons. Some of the water pollinated plants are common waterweed, pondweed, eel-grass, hydrilla, and coontail. But surprisingly, not all aquatic plants use water for pollination. In a majority of aquatic plants such as water hyacinth and water lily, the flowers emerge above the surface level of water while they come in contact with the pollinating insects or wind. In plants like seagrasses, female flowers remain deeply submerged in water and the pollen grains will be released by the mature flower inside the water, in such cases, the grain is ribbon-like and very long and slender. These ribbon-like structures are transported passively inside the water and very few of them reach the stigma to achieve pollination. In order to protect the pollen grains underwater, they are being covered by a mucilaginous covering. Flowers pollinated by abiotic medium are usually not so colourful.
Biotic is the most prefered method of pollination in almost all the flowering plants. Some of the biological agents are insects, animals, and birds. Based on the biological pollination, flowers are classified as zoophilous flowers, entomophilic flowers, ornithophilous flowers.
Biotic pollination by honey bee- Image from wiki commons
Zoophilous flowers use animals like human beings, bats, birds for their pollination. These flowers have pollen that can easily stick on to the body of the animal to ensure it is easily carried from one flower to another. On the other hand, Entomophilic flowers are those that appear very attractive with bright petals, highly fragrant in order to attract the insects. They often have broader stigmas or anthers to allow the insect to perch on it. Many of the insect-pollinated flowers also secrete nectar which attracts bees. The pollen grains in these flowers are often spiny with extensions that help them to stick on to the body of the insects. Ornithophilous flowers are those flowers pollinated by birds and are rare.
Outbreeding devices help to deliberately block the self-pollination. This is because, a continued self-pollination for many generations (autogamy) can result in poor variation in the progeny. Few of the plants have spontaneously generated some techniques to block the self-pollination . 4 of the most commonly used techniques are :
Creating a situation in which there is poor synchronization between the time of pollen release and stigma receptivity ; this will ensure the failure of natural pollination.
In some species, wrongly positioned anther and stigma brings in difficulty for the contact between stigma and anther hence this makes pollination impossible.
In some cases, self-incompatibility is being instilled. It is a genetic mechanism to block self-pollination by suppressing the growth of the pollen tube in the pistil.
Another method is by producing unisexual flowers within a plant to a larger extent, this ensures there is no exchange of male and female gametes. It can be seen in plants like castor and maize (monoecious).
Events and Issues related to pollen-pistil interaction
Some of the sequential events that happen during pollen-pistil interaction are
The landing of a suitable pollen on a compatible pistil.
Germination of pollen and the formation of pollen tube where pollen grains release its contents.
Growth of pollen tube through the style of the pistil towards the ovary
The entry of male gametes into the ovule and then to synergid.
Knowledge about outbreeding devices and pollen-pistil interaction helped in economic and financial development. Generally, the pistil has the ability to recognize the pollen for its compatibility. Pistil decides whether to accept the pollen or to reject it based on a receptivity mechanism. Only if there is compatible pollination, the pollen grain undergoes proper matching and seeds gets germinated. This interaction where a pistil is capable of recognizing its pollen is the result of long term pollen-pistil interaction and chemicals released by pollen.