11.2.2 Pollination

Definition of Pollination

• Pollination is the transfer of pollen from the anther to the stigma of the same flower or to the stigma of another plant of the same species. It is essential for the development of fruits and seeds.

• Pollination is classified into two types:

1. Self-pollination

2. Cross-pollination

Types of Pollination

1. Self-pollination:

• Occurs when pollen is transferred from the anther to the stigma of the same flower or between two flowers of the same plant.

• Example: Brassica, cucumber, and datura.

• Advantages:

• Minimal wastage of pollen.

• Not dependent on external agents (pollinators).

• Pollination is guaranteed.

• Disadvantages:

• No new characteristics are introduced.

• The plant’s ability to adapt is limited, which can lead to reduced vigor and eventual extinction if the environment changes.

2. Cross-pollination:

• Occurs when pollen is transferred between two different plants of the same species.

• Example: Cotton tree (Salmalia malabarica), papaya.

• Advantages:

• New genetic traits emerge.

• Increased seed germination rate and vigor.

• New varieties of plants are produced with better characteristics.

• Disadvantages:

• Dependent on external agents for pollination (such as wind or insects).

• Waste of pollen as not all pollen reaches a stigma.

• The purity of the species is potentially compromised.

Medium of Pollination

Pollination is often mediated by carriers (called pollen carriers) that transport pollen from one flower to another. These carriers can include:

• Air

• Water

• Insects (entomophily)

• Birds (ornithophily)

• Bats (chiropterophily)

• Humans and other animals.

Adaptations for Pollination:

• Insect-pollinated flowers:

• Large, colorful, and produce nectar.

• Pollen and stigma are sticky, and the flowers have fragrance.

• Example: Hibiscus, mustard, gourd.

• Air-pollinated flowers:

• Light and lack nectar-secreting glands.

• Stigmas are branched, sticky, or feathery to catch airborne pollen.

• Example: Rice (Oryza sativa).

• Water-pollinated plants:

• Small, lightweight, and able to float.

• No fragrance or nectar.

• Male flowers detach and float to female flowers for pollination.

• Example: Vallisneria.

• Animal-pollinated flowers:

• Typically large and have attractive colors and sometimes fragrance.

• Example: Kadam, cotton tree (shimul), and taro.

Microsporogenesis

• Microsporogenesis is the process where male gametophytes (pollen) develop from microspores.

• Pollen Mother Cell (2n) undergoes meiosis to produce four haploid pollen cells (n).

• The pollen cells mature in the pollen sac.

• Pollen grains consist of:

• Tube cell: Develops into the pollen tube.

• Generative cell: Divides further to form two male gametes.

• Development of Male Gametophyte:

• The pollen grain begins to germinate, and the tube cell forms the pollen tube which will carry the male gametes to the stigma for fertilization.

Megasporogenesis

• Megasporogenesis is the development of female gametophytes (embryo sac) from megaspores.

• A large cell in the nucellus of the ovule divides by meiosis to form four haploid megaspores.

• One of the megaspores survives and develops into the embryo sac.

• The surviving megaspore divides to form:

• Two haploid secondary nuclei.

• The two nuclei move to opposite poles, eventually fusing to form a diploid secondary nucleus.

• Egg Apparatus:

• The egg apparatus consists of:

• Egg cell (ovum): The female gamete.

• Synergids: Two smaller cells near the egg cell.

• The cells at the opposite pole are called antipodal cells.

This process results in the formation of the embryo sac, which houses the egg cell for fertilization.

Summary:

Pollination is the essential step in plant reproduction, enabling the fertilization of ovules and the production of seeds. It can be self-directed or cross-pollinated, with cross-pollination providing genetic diversity. Various agents such as air, water, insects, and animals help transfer pollen. Once pollen reaches the stigma, male and female gametophytes develop through microsporogenesis and megasporogenesis, leading to the formation of seeds.