Class 12 CBSE

Chapter 2 Sexual Reproduction in flowering plants (PART-3)

POLLINATION & FERTILIZATION

Click here for Part- 1 Introduction and Male Reproductive organ

Click here for Part-2 Female reproductive whorl

Click here for Part-4 Fertilization

“Pollination is defined as the process of transfer of pollen grains from anther to the stigma. In flowering plants, both types of gametes (male gamete and female gamete) are non-motile and brought together for fertilization by pollination. Flowering plants have evolved an amazing array of adaptation to achieve pollination. They make use of external agents to achieve pollination. There are different types of pollination based on the source of pollen.

1. AUTOGAMY OR SELF POLLINATION: If the pollen grains are transferred from an anther to the stigma of the same flower, then it is called self-pollination or autogamy. In a normal flower that opens and exposes the anthers and stigma, complete autogamy is rather rare. Autogamy in such flowers requires synchrony in pollen release and stigma receptivity, and also the anther and the stigma should lie close to each other so that self-pollination can occur. Continued self-pollination results in inbreeding depression.
2. XENOGAMY: If the pollen grains are transferred from an anther of one flower to the stigma of a different flower on the same plant or on a different plant of the same species, it is called xenogamy.
3. GEITONOGAMY: If the pollen grains are transferred from an anther to the stigma of another flower on the same plant, it is called geitonogamy.”

When both male and female flowers are present on the same plant but are unisexual, the plant is called monoecious. Examples of monoecious plants include castor, cucurbits, coconut, and maize. Monoecious plants prevent autogamy but not geitonogamy.

On the other hand, when male and female flowers are present on different plants and are unisexual, the plant is called dioecious. Examples of dioecious plants include papaya and date palm. Dioecious plants prevent both autogamy and geitonogamy.

Contrivances or Adaptations for Self Pollination :

In the plant kingdom, pollination is the process of transferring pollen from the anther to the stigma, leading to fertilization and the production of seeds. While cross-pollination or pollination between different plants is more common, some plants have evolved mechanisms to promote self-pollination. Self-pollination is the transfer of pollen from the anther to the stigma of the same flower or different flowers on the same plant. In this article, we will explore the various adaptations or contrivances that plants have developed for self-pollination.

1. Cleistogamy: This is a mechanism where plants produce small, inconspicuous flowers that never open, but are self-pollinated inside the closed flower bud. Examples of plants that use this mechanism include Viola, Oxalis, and Commelina.
2. Homogamy: In this mechanism, the anthers and stigma of a flower mature at the same time, ensuring self-pollination before the flower opens. This occurs in many plants including tomato, pea, and bean.
3. Herkogamy: Herkogamy is a mechanism where the anthers and stigma of a flower are positioned in such a way that self-pollination is promoted. For example, the style of the flower may be curved or the anther may be positioned in a way that makes it more likely that the pollen will land on the stigma. This is seen in plants such as wheat and barley.
4. Self-sterility: This is a mechanism in which plants prevent self-pollination by rejecting their own pollen. Plants with this mechanism have a genetic system that allows the recognition and rejection of their own pollen. This can be seen in many fruit trees, such as apples and pears.
5. Dichogamy: This is a mechanism where the anthers and stigma of a flower mature at different times, preventing self-pollination. For example, some flowers have anthers that mature before the stigma, while others have the opposite pattern. This is seen in plants such as corn and holly.
6. Self-pollinating structures: Some plants have developed specialized structures to ensure self-pollination. For example, in the common garden pea, the anther and stigma are enclosed in a protective sheath called the keel, which allows self-pollination to occur.

Contrivances for Cross Pollination (Outbreeding devices)

Flowering plants have developed various mechanisms to ensure cross-pollination, which leads to genetic diversity and better adaptability. The process of cross-pollination or outbreeding occurs when pollen from one flower lands on the stigma of another flower, which is not genetically identical. The contrivances or adaptations for cross-pollination in flowering plants are known as outbreeding devices.

1. Dioecy: Dioecious plants have male and female flowers on separate plants, which forces them to rely on cross-pollination. For example, papaya and date palms are dioecious plants.
2. Dichogamy: This is a temporal separation of male and female organs, which reduces the chances of self-pollination. Protandry is the condition where the stamens mature before the stigma, while protogyny is the opposite. This arrangement prevents self-pollination and promotes cross-pollination. For example, in sunflowers, the male and female organs are temporally separated.
3. Heterostyly: It refers to the existence of two or three types of flowers within a species, which differ in the length of the style and stamen. Insects that visit long-styled flowers tend to pick up pollen from short-styled flowers, and vice versa. This arrangement increases the probability of cross-pollination. For example, in primroses, flowers of different styles grow in different positions on the plant.
4. Herkogamy: It refers to the spatial separation of male and female organs. This arrangement forces the pollinator to brush against the anthers and stigma, reducing the likelihood of self-pollination. For example, in spider flowers, the stamen and stigma are located at different positions in the flower.
5. Self-Incompatibility: This is a genetic mechanism where pollen and stigma of the same plant are prevented from fertilizing each other. Self-incompatibility promotes cross-pollination and genetic diversity. For example, in the brassica family, self-incompatibility is common.
6. Cleistogamy: It refers to the production of flowers that do not open and remain self-contained. These flowers are usually small and produce a small amount of pollen. They ensure pollination without relying on pollinators. For example, in violet flowers, the cleistogamous flowers produce viable seeds without cross-pollination.