Types of Pollination & Pollen-Pistil Interaction & Outbreeding Devices - NEET PDF Download

The majority of flowering plants reproduce sexually i.e., through seed formation. We know sexual reproduction is incomplete without fertilization. The male and female gametes have to meet for fertilization and further developments.  Have you ever wondered how plants ensure their continuity on earth despite their immobile nature? Let us answer the same by having a brief discussion on a process called pollination.

Pollination in Plants

Reproduction is the life process which helps an organism to procreate its own offspring. There are a lot of events involved in this. In plants, pollination is one among them. Pollination can be defined as the pre-fertilization event or process where pollen grains from anther are transferred to the stigma of a flower.

Plants are immobile. Unlike animals, both male gamete and female gamete are immobile. They can’t copulate with each other by themselves. They need a vector for this. Pollination is the process that helps to unite the male and female gametes and thus helps in fertilization. It can be broadly classified into two, cross-pollination and self-pollination and this is achieved with the help of a variety of vectors/agents. For successful pollinations, it must occur between the same species.

Types of Self-pollination and Cross-pollination

Pollinations can occur either within a flower or between flowers of the same plant or flowers of different plants. Depending on this, pollinations are of three types, namely:

➤ Autogamy

It is a type of self-pollination where the transfer of pollen grains from the anther to the stigma takes place within the same flower. Opening and exposure of anther and stigma are necessary for autogamy. There are two conditions for autogamy to takes place:

• Anther-stigma synchronization; when the pollen is released, stigma should be ready to receive it.
• The position of or distance between anther and stigma. Both should be close enough for pollinations.

In chasmogamous flowers, anther and stigma are exposed. The exposed reproductive parts give a chance of cross-pollination in chasmogamous flowers. While in cleistogamous flowers anther and stigma are not exposed but lie close enough for transfer. Thus, the chances of cross-pollination in cleistogamous flowers are almost none. In addition, they barely require a pollinating agent.

➤ Geitonogamy

Geitonogamy is the type of self-pollinations where the transfer of pollen grains from the anther to the stigma takes place between different flowers in the same plant. Though it seems like cross-pollination and takes place with the help of pollinator, both the gametes have the same plant as their origin.

➤ Xenogamy

Xenogamy is the cross-pollination where the pollen grain transfer occurs across flowers of two different plants. In other words, the transfer of pollen from the anther of one plant to the stigma of another plant.

Each type has its own merits, like xenogamy, leads to a new variety whereas autogamy helps to preserve parental characters. Plants have various adaptations to accomplish this task. In addition, flowers depend on certain pollinating agents which can either be biotic or abiotic. These biotic and abiotic pollinating agents are collectively termed as pollinators.

Pollen-Pistil Interaction & Outbreeding Devices

Pollination is the process, which plants depend on to transfer pollen grains from anther to stigma or pistil. It can be either cross-pollination or self-pollination. Most of the plants are bisexual or hermaphrodite which promote self-pollination. Is this self-pollination desirable? What are the factors promoting cross-pollination?

What is pollen-pistil interaction?

The sequence of events, which is carried out from the time of pollen deposition over the stigma and till the entry of pollen tube inside the ovule is called pollen pistil interaction. The pollination, process of transferring pollen grains from the anther to the stigma of the same flower or from one flower to the different flower is the initial stage or the first step in pollen pistil interaction.

Outbreeding Devices

Self-pollination is common and more likely to happen in the case of hermaphrodite flowers but a successive series of self-pollination affects negatively and causes inbreeding depression. This also results in homozygous genes. Thus plants are adapted to promote cross-pollination. This is known as outbreeding. Factors which encourages cross-pollination are as follows:

• Unisexual flower: If a flower is unisexual i.e., contain only one sex either female or male, cross-pollination is the only choice.
• Non-Synchronization: Timing is important for successful pollination. Pollen release and receptivity of stigma should happen simultaneously. Sometimes, pollen matures and releases before the stigma is open which leads to loss of pollen vitality or vice-versa. This prevents self-pollination; even though the flower is hermaphrodite.
• Self-incompatibility: Incompatibility within a flower (or plant) includes self-sterility; structural barrier. Self-sterility means even though pollination takes place it can’t proceed to fertilization due to further pollen growth failure. Structural barriers include height difference between gynoecium and androecium and some structures which hinder the stigma from receiving pollen. These are the more or less genetic mechanism. 

Pollen–Pistil Interaction

All pollinations do not lead to successful fertilization because for successful fertilization, the pistil of a flower has to recognize the pollen of the same species. Therefore, the interaction between pollen grains and the stigma needs to be understood properly. Once compatible pollen is accepted by pistil, events for fertilization proceed, whereas incompatible pollens are rejected. 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.

Events of pollen-pistil interaction proceed as follows:

• The landing of true pollen on the compatible pistil.
• Germination of pollen and formation of pollen tube where pollen grains release its contents.
• Pollen tube growth through the style of the pistil towards the ovary.
• The entry of male gametes into the ovule and then to synergid.

Frequently Asked Questions

Q. What Is Pollination?
Ans: Pollination is a biological process in which the pollen grains are transferred from an anther (male part of a flower) to the stigma (female part of a flower). There are two types of pollination:

1. Self-Pollination
2. Cross-Pollination

Q. What are the Pollinating Agents?
Ans: The agents which are involved in transferring the pollen grains from one flower to another flower are called as the Pollinating Agents. Animals, birds, insects, wind and other biotic and abiotic agents are all examples of Pollinating Agents.

Q. How does Pollination occur in Plants?
Ans: There are two different types of pollinations in which the pollen grains are transferred from one flower to another. In both the process, pollen grains are transferred from a stamen to the stigma of the same plant or to a flower of different plants.

Q. Name the plants which undergo Self- pollinations?
Ans: Plants with smaller flowers use self-pollination. Peanuts, wheat, apricots, rice, tomatoes are some examples of self-pollinating plants.

Q. What Is Self-Pollination?

Ans: Self-pollination is referred to as the primary type of pollination, which occurs by transferring the pollen grains directly from anther into the stigma of the same flower.

Q. What are pollen grains?

Ans: Pollen grains are the granular microspores termed as the micro-gametophytes produced within the anther – male part of the flower.

Q. What is Cross-Pollination?

Ans: Cross Pollination is referred to as the complex type of pollination during which the pollen grains are transferred from the anther of one flower into the stigma of another flower.

Q. Name the plants which undergo Cross pollinations?

Ans: Most plants use cross-pollination. Cross-pollination is mainly seen in dark and bright colored flowering plants, where insects like butterflies and honey bees are attracted by their bright colored flowers. Apples, tulips, lavender, strawberries, beans, dandelions are some examples of plants with cross-pollination.