Pollination refers to the transfer of pollen from anther of a stamen to stigma of a carpel.

WHAT IS A POLLEN ?

Pollen represents the male gametophyte of Angiosperms as well as Gymnosperms. A diploid Microspore Mother Cell divides by meiosis to form four haploid Microspores which further divides by mitosis to develop into Pollen. So, Pollen are formed in the Pollen sacs (Microsporangium) of the Anther by the process of Microsporogenesis. In Angiosperms, typically a pollen consist of two haploid cells i.e. a large Vegetative cell and a small Generative cell which floats in the cytoplasm of the Vegetative cell. This generative cell further divides by mitosis to form two male gametes. So, pollination is, in fact, a process of transfer of male gametes in Angiosperms.

TYPES OF POLLINATION

Based on the source of pollen, pollination can be divided into three main types:-

• Autogamy
• Geitonogamy
• Xenogamy

1) AUTOGAMY

• Autogamy refers to the transfer of pollen from anther to stigma of the same flower.
• Autogamy can only occur if the plant has bisexual flowers.
• Autogamy is true Self-pollination i.e. Genetically as well as functionally Self-pollination.
• Autogamy is not dependent on the presence of pollinators as pollination occurs within the same flower.

2) GEITONOGAMY

• Geitonogamy refers to the transfer of pollen from anther to stigma of different flower of the same plant.
• Geitonogamy can occur in two types of plants:-
  1. Monoecious plant having unisexual flowers i.e. Staminate (male) flowers and Pistillate (female) flowers on the same plant.
     Examples – Maize, Coconut, Castor, Cucurbits etc.
  2. Monoecious. plant having bisexual flowers.
• Geitonogamy is a complex case as genetically it is Self-pollination while functionally it is Cross-pollination.
• Geitonogamy is dependent on the presence of pollinators as pollens are transferred from one flower to another.

3) XENOGAMY

• Xenogamy refers to the transfer of pollen from anther to stigma of different plants of the same species.
• Xenogamy can occur in all types of flowering plants:-
  1. Dioecious plants i.e. male and female flowers are located on different plants and pollen are transferred from the male plant to the female plant.
     Examples – Date palm (Phoenix datylifera), Papaya etc.
  2. Monoecious plants having unisexual flowers.
  3. Monoecious plants having bisexual flowers.

• Xenogamy is true Cross-pollination i.e. Genetically as well as functionally Cross-pollination.
  Xenogamy is also dependent on the presence of pollinators as pollens are transferred from one plant to another.

AGENTS FOR POLLINATION

The pollinating agents are broadly divided into two groups:-
1. Abiotic agents
2. Biotic agents

ABIOTIC AGENTS

1. WIND POLLINATION / ANEMOPHILY
   • Wind is the most common among abiotic agents.
   • Pollens are small, light and non-sticky.
   • Carpel has feathery stigma and style to capture the pollen.
   • Flowers lack bright colors, fragrance and nectar
   • Large number of pollen are required as the process is non-directional.
   • Flowers are often found grouped into inflorescence.
2. WATER POLLINATION / HYDROPHILY
   • Rare type of pollination and is observed in less than 30 genera of angiosperms mostly monocots.
   • Pollens are surrounded by a mucilage layer to make it unwettable.
   • Flowers lack bright color, fragrance and nectar.
   • Large number of pollen are required as the process is non-directional.
   • Most aquatic angiosperms like Water hyacinth (Eicchornia), Water lettuce (Pistia) and Lotus (Nelumbo) perform wind or insect pollination.
   • Examples – Vallisneria, Hydrilla, Zostera.

BIOTIC AGENTS

1. INSECT POLLINATION / ENTOMOPHILY
   • A wide range of insects carry out pollination in Angiosperms including Bees, butterflies, moths, ants, flies, beetles etc.
   • Bees are the most common pollinating agent and contribute to about 75% of total pollination worldwide.
   • Pollen are surrounded by a sticky substance to help it attached to the insects body.
   • Flowers are brightly colored, fragrant and either large in size or grouped into inflorescence to attract insects.
   • Nectar, pollen and safe egg laying sites are the rewards offered by the plants to sustain the visits of insects.
   • Chances of pollination is high so less number of pollen are required.
   • Pollinators like flies and beetles are attracted towards foul smell.
2. BIRD POLLINATION / ENTOMOPHILY
   • Birds like Humming bird, sunbird have long bill and tongue which allows them to easily obtain nectar from the flower. During this process, pollen gets dusted on the bill of the bird and when this bird reaches another flower to obtain nectar pollination can occur.
3. HUMAN POLLINATION / ANTHROPOPHILY
   • Plants attract humans with edible fruits, aromatic flowers, or medicinal properties, encouraging cultivation and propagation.
   • Examples include crops like wheat (Triticum aestivum), which spread due to human agriculture, and ornamental plants like roses, selectively bred for desirable traits.
   • The selective breeding of plants for desirable traits by human is called Artificial hybridization.