Pollination Basics-Part 2: Mechanisms of Pollination

Plant Reproduction

Flowers are important for sexual reproduction by plants. They produce male sex cells and female sex cells. These must meet for reproduction to begin, a process called pollination.

All living organisms have one major goal in common, which is to pass along their genetic information to the next generation by creating offspring. Flowering plants create seeds, which carry the genetic information of the parents and develop into a new plant. In order for seeds to be created, pollination must occur.

Pollination Vocabulary

Antler The part of the stamen where pollen is produced
Cross-pollination A type of pollination in which the pollen from the anther of a flower is transferred to the stigma of a flower of another plant
Dioecious Species has male and female flowers on separate plants
Endosperm Nucleus The nucleus formed by the fusion of two polar nuclei in the embryo sac of a seed plant prior to fertilization.
Filament The stalk that holds the anther and attaches it to the flower.
Monoecious Species has flowers of both sexes on an individual plant
Nectary Nectaries are specialized nectar-producing structures of the flower
Ovary The enlarged basal portion of the pistil where ovules are produced.
Petal The parts of a flower that are often conspicuously colored
Pistil The ovule producing part of a flower. The ovary often supports a long style, topped by a stigma. The mature ovary is a fruit, and the mature ovule is a seed.
Self-pollination A type of pollination in which the pollen from the anther of the flower is transferred to the stigma of the same flower
Sepal The outer parts of the flower (often green and leaf-like) that enclose a developing bud.
Stamen The pollen producing part of a flower, usually with a slender filament supporting the anther.
Stigma The part of the pistil where pollen germinates.
Style This is the name for the stalk of the pistil. When pollen reaches the stigma, it begins to grow a tube through the style called a pollen tube, which will eventually reach the ovary. The style therefore acts as a buffer against pollen contamination, since only compatible pollen is able to grow a pollen tube.
Zygote A fertilized egg cell that results from the union of male (sperm) and female (ovum) gametes.
Graphic showing labeled parts of a flower

How Does Flower Pollination Work?

it all begins in the flower. Flowering plants have several different parts that are important in pollination. Flowers have male parts called stamens that produce a sticky powder called pollen. Flowers also have a female part called the pistil. The top of the pistil is called the stigma, and is often sticky. Seeds are made at the base of the pistil, in the ovule.

To be pollinated, pollen must be moved from a stamen to the stigma. When pollen from a plant's stamen is transferred to that same plant's stigma, it is called self-pollination. When pollen from a plant's stamen is transferred to a different plant's stigma, it is called cross-pollination.

Cross-pollination produces stronger plants. The plants must be of the same species. For example, only pollen from a daisy can pollinate another daisy.

Parts Of A Flower

It is important to consider that individual plants are bisexual as most plants have both "male" and "female" parts, situated apart from each other. Based on how a plant reproduced, this means that auto-pollination (i.e., self-reproduction) is not only likely but inevitable in some settings.

Plant Sex

The male sex cell in a plant, or more specifically the part that bears pollen, is called a stamen and consists of an anther and a filament.

The female part, which receives pollen grains, is called a pistil and includes an ovary, a stigma and a style.

Flowering plants contain gametes, which have half of the number of chromosomes in the plant. These must be combined with other gametes to create a seed with the full number of chromosomes. The male gametes of a flower are found in the pollen that grows on the stamen, whereas the female gametes are found in the ovule inside the pistil.

The stamen is made of the anther and the filament. The anther is where the pollen is located at the end of the stamen. The pistil is made of three parts: The ovary, the style and the stigma. The ovary contains the ovules, which need to be fertilized in order to produce a seed. The ovules contain an egg nucleus and polar nuclei.

Monoecious Versus Dioecious

Although many plants grow uni-sexual flowers that have both male and female parts, others grow flowers that have only a pistil or only a stamen. Some of these species have flowers of both sexes on an individual plant, known as a monoecious plant, while others only grow one or the other, known as a dioecious plant.

Dioecious plants must be located near each other in order to reproduce. Hibiscus and lily flowers are uni-sexual and have both parts. Squash are monoecious, which is why only half of the blossoms will ever grow into vegetables. Holly is an example of a dioecious plant as it only grows stamen on some plants, while others only grow pistils.

Fertilization

Graphic showing cross-pollination

When the pollen lands on the stigma, the fertilization process begins. The sperm nuclei then travel down the style through a pollen tube into the ovary, where it enters an ovule. At this point, one of the sperm nuclei will unite with the egg nucleus and create a zygote. The other will unite with two polar nuclei to create an endosperm nucleus. The egg and endosperm nucleus grow inside the fertilized ovule and develop into a seed eventually.

The ovary will then produce a fruit to protect the seed. This could be a fruit protecting a single seed, such as an avocado, while others have many seeds, such as a kiwi.

Seeds

Graphic showing parts of a seed

A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. The formation of the seed completes the process of reproduction in plants (started with the development of flowers and pollination), with the embryo developed from the zygote and the seed coat from the integuments (tough outer protective layer) of the ovule.

A seed develops from an ovule after fertilization. It consists of:

  The tesla is tough, hard, outer coat, the testa protects the seed from fungi, bacteria and insects. It has to be split open by the radicle before germination can proceed.
  The plumule is the embryonic shoot. In it two or more leaves are usually visible, with a growing point enclosed between them.
  The radicle is the embryonic root which grows and develops into the root system of the plant.
  The hypocotyl is the part of the stem of an embryo plant beneath the stalks of the seed leaves or cotyledons and directly above the root.
  The cotyledon is an embryonic leaf in seed-bearing plants, one or more of which are the first leaves to appear from a germinating seed.

Feeling confident? Test your knowledge of pollination, take the quiz: Pollination Quiz.

Further Reading:

 Pollination — Part 1, Types of Pollination
 Other Pollinaters — Not Just Bees and Butterflies
 Do Plants 'See'?
 Do Plants 'Breathe'?
 The Benefits of Pollen

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