What is Pollination? Self and Cross Pollination in Plants/flowers

What is pollination?

Pollination is simply defined as the transfer of a pollen grain (male gametophyte) to a flower’s stigma (receptive surface of the female reproductive organ), where it can germinate, grow through the style, and fertilize an ovule of a flower to produce seed. Every living organism, including plants, has the goal of producing offspring for the next generation and plants do so by producing seeds through the process of pollination.

Before we proceed further, let’s get an understanding of pollen and how this pollen pollinates plants. This means that pollen can be defined as fine dust-like fragments that form within the anther and accumulate on its surface. Another feature of pollen grains is that they are distinctive in their shape in such a way that they are different from the normal dust particles.

Furthermore, it is important to note that pollen must be transported to a stigma of the same or different plant species at the appropriate time for pollination to occur. This simply means that, in order for the whole process of pollination and eventual fertilization of the plant (angiosperm or gymnosperm) to happen, the pollinators need to carry the pollen that is necessary to pollinate at the right time. This whole process is important for the production of seeds.

Pollination process (mechanism)

pollination diagram
Pollination diagram showing a bee picking pollens from one flower to that of another.

The process of pollination also known as the pollination mechanism occurs when pollen grains from the male part of a flower known as the (anther) are moved to the female part (stigma) of another flower. After the pollination mechanism, the fertilized flowers produce seeds, which allow the associated plant to reproduce and/or form fruit.

The process begins when pollen grains land on a receptive stigma, they form a pollen tube that travels down the style to the ovary and the male genetic material travels through the pollen tube and fertilizes an ovule. Leading to the development of the ovules into seeds, and the ovary surrounding them developing into the fruit.

The aforementioned pollination mechanism can be achieved by abiotic pollinators such as wind and water, and animal pollinators as they try to obtain nutritional rewards such as pollen and nectar, or other resources. In order words, as they go out in search of food, they transfer pollen from one flowering plant to another flowering plant.

What are pollinators?

Pollinators are things either living or not that aids in the transfer of pollen from the stamen of the flower to the stigma of the same or a different flower. The transfer of pollen is requisite for plants to attain fertilization which eventually leads to the production of fruits, seeds, and the next generation of plants. Pollen carried by the wind or water can fertilize some plants, while pollen carried by the insects or bats can fertilize others. Pollen is intentionally collected by some pollinators, including many bee species whereas some other pollinators, such as most butterflies, birds, and bats, unintentionally help in the movement of pollen. This unintentional act happens when pollen gums to their bodies, while they drink or feed on nectar, found when there is a flower bloom. This pollen is then accidentally transported from flower to flower, resulting in pollination.
Pollinators are known to perform the following responsibilities;

  • Pollinators are important in the production of not only our food but also animal food and habitat. Because pollinators are responsible for one out of every three bites of food you eat, including many fruits, vegetables, and seeds.
  • Pollinators are influential in the health of diverse ecosystems. This is achieved as insects and animals pollinate at least 75% of all flowering plants on the planet. This equates to over 1,200 different types of food crops and 180,000 different types of plants—plants that help stabilize our soils, clean our air, provide oxygen, and support wildlife.
  • Pollination helps to boost the economy of society as it is known that honey bees contribute billions of dollars to the economy in crop production alone.

The association between plants and pollinators is generally believed to be mutualistic. This means that the plants are profiting from pollen transfer and pollinators on the order hand are receiving a nutritional or another reward. However, there are instances where the plants provide no gain for the pollinators and the pollinators may come in the form of seed predators.

On a whole, no matter the relationship between the plants and the pollinators, the important thing is that the pollination process is initiated.

For plants to be pollinated, they require pollinators. These pollinators are divided into either biotic or abiotic. The biotic pollinators include insects and animals and the abiotic include wind and water.

Insect Pollination

Plants have modified adaptations to help them foster symbiotic relationships with insects, which ensure pollination. Such insects involved in pollination include;

  1. Bees
  2. Flies
  3. Butterflies and moths
  4. Wasps
Pollination by bees

Bees are the primary pollinators of many garden plants and commercial fruit trees.  For example, bumblebees and honeybees are the most common bee species that participate in the pollination process of plants. In search of the energy needs required for their survival, bees collect energy-rich pollen or nectar as they are attracted to flowers that are open during the day. The characteristics of such flowers are;

  • They have brightly colored petals. The flowers mostly pollinated by bees are usually blue, yellow, or other colors but certainly not red flowers because bees cannot see red.
  • Possessing a strong aroma or scent,
  • Another characteristic of the flowers is that it has a tubular shape; with or without the presence of a nectar guide (this is a pattern on the flower that helps pollinators to located and sap nectar).

Bees are great pollinators because they spend the majority of their time collecting pollen, (which is a source of protein that they feed to their developing offspring) and during their buzzing in the vicinity of the flower with the above characteristics, the bee lands on a flower. Upon landing the hairs on its body uses electrostatic forces to attract pollen grains and the hairs found on their legs allow them to groom pollen into specialized brushes or pockets on their legs or bodies, which they then carry to another flower. This simply implies that pollen from one flower is more likely to be transferred to another flower of the same species by the same bee.

Another great feature about bees is that they are known to provide some essential services to the planet when they help in the pollination process and some of the important roles of bees in pollination are as follows;

  • Their role is evident in the food we eat. For instance, plenty of crops are grown for their fruits including vegetables like squash, cucumber, tomato, and eggplant, nuts, seeds, fiber, for example, cotton, and hay (alfalfa grown for livestock feed) require insect pollination.
  • Bees are known to play a critical part in maintaining natural plant communities and ensuring seed production in most flowering plants.

On a whole, bees are a critically relevant group of insect pollinators, pollinating 60–70 percent of the world’s total flowering plant species, along with nearly 900 food crops like apples, avocados, cucumbers, and squash. These crops account for 15–30 percent of global food production, and bees are responsible for making billions of dollars in annual crop productivity across the world stressing the economic importance of bees in pollination.

Pollination by flies

Numerous flies are drawn to flowers with a decaying odor or the odor of rotting flesh. These nectar-producing flowers are typically dull in colors, such as brown or purple. Flies can be found on corpse flowers (Amorphophallus), dragon arum (Dracunculus), and carrion flowers (Stapelia, and Rafflesia). The nectar gives energy, while the pollen gives protein.  When flies visit these flowers they unintentionally or intentionally pick pollen and transfer them to other flowers. By doing so, they help in the pollination process.

Pollination by butterflies and moths

Many garden flowers and wildflowers are pollinated by butterflies such as monarch butterflies. The flowers pollinated are characterized by brightly colored petals, a sweet fragrance, and the ability to bloom during the day. Pollen is collected and carried by the butterfly’s limbs when they perch on the flowers and they carry the pollen on their limbs to other flowers. This process of flying from one flower to another flower initiates pollination.

Moths, on the other hand, pollinate flowers late in the afternoon and late at night. Moth-pollinated flowers are pale or white in color and flat in shape, allowing the moths to land. The yucca plant, which is pollinated by the yucca moth, is a well-studied example of a moth-pollinated plant. The pollen gotten from one flower is deposited on the sticky stigma by the moth onto another flower as the moth prepares for fertilization (because the moth deposits its eggs on the flower after they have been fertilized). In this symbiotic relationship, both the insect and the flower benefit from each other. A similar relationship exists between the corn earworm moth and the Gaura plant.

Pollination by wasps

A handful of wasps feed pollen or nectar to their young. The targeted flowers by these insects are the brownish-purple-colored flowers with easily accessible nectar, such as those of figwort. Some Mediterranean and Australian orchids have flowers that so closely resemble the females of certain wasps (of the families Scoliidae and Ichneumonidae). This makes the males attempt copulation and receive pollen masses on their bodies as they come in contact with the flower. These pollen masses on their bodies are then transferred to other flowers. Also, tiny gall wasps (Blastophaga) use the diminutive flowers (within their fleshy receptacles) as incubators. That is the gall wasps lay their eggs on the fleshy receptacles of the diminutive flower species and by so doing, they transfer pollen and pollinate other flowers.

Pollination by animal pollinators

Plants have evolved specialized adaptations to take advantage of non-insect pollination methods. Such as;

  1. bats
  2. Birds
Pollination by bats

Bat pollination is a type of pollination in which bats pollinate flowers. Bats pollinate nocturnal flowers such as agave, guava, and morning glory in the tropics and deserts. The flowers are typically large in size and possess a color that is either white or pale, allowing them to be distinguished from their dark surroundings at night. The flowers have a strong, fruity, or musky scent and produce a lot of nectar. They are naturally large and wide-mouthed in order to accommodate the bat’s head. As the bats seek nectar, pollen gathers on their faces and heads, which is then transferred to the next flower.

Pollination by birds

Small bird species, for example, hummingbirds pollinate plants such as orchids and other wildflowers. The preferred type of flowers by these birds is generally fashioned in such a way that the birds can stay close to the flower without getting their wings entangled in the nearby flowers. The flower can be characterized by a curved and tubular shape, allowing access for the bird’s beak. Birds are known to pollinate flowers that are bright in color, odorless, and that open during the day. The pollen is deposited on the bird’s head and neck as it searches for nectar, before being transferred to the next flower it visits.

An example of pollination can be seen in the relationship between hummingbirds and the ornithophilous flower commonly found in the Americas. The hummingbirds fly around in search of nectar, and on discovering and feeding the nectar in the ornithophilous flower, they collect pollen and transfer this pollen to other flowers of the same species. Thereby kickstarting the pollination process.

Pollination by abiotic pollinators

  1. Wind
  2. water
Wind Pollination

Wind pollinates most conifer species and many angiosperms such as grasses, maples, and oaks. Pine cones that require wind pollination are brown and unscented, whereas wind-pollinated angiosperm flowers are usually green, small, with few or no petals, and produce a lot of pollen. Flowers pollinated by wind, unlike insect-pollinated flowers, do not produce nectar or scent. Wind-pollinated species have microscopic spores called microsporangia that hang out of the flower, and the lightweight pollen is carried with it as the wind blows. The flowers usually appear before the leaves in the spring, so that the leaves do not obstruct the movement of the wind. Plants of the same species that are pollinated by the wind often grow in dense populations. Because many species are dioecious or have separate male and female flowers on each plant (like hazel), the chance of self-pollination is reduced. Grass, rushes, sedges, cattail, sorrel, lamb’s-quarters, hemp, nettle, plantain, alder, hazel, birch, poplar, and oak are examples of flowering plants that rely on wind pollination. (Tropical oaks, on the other hand, maybe pollinated by insects.)

Water Pollination

Water pollinates several weeds, like the Australian seagrass and pond weeds. The pollen travels on the surface of the water. It is then deposited inside the flower when it comes into contact with it. Surface water pollination is exemplified by ribbon weed a dioecious plant, with male and female plants. The whole process starts when the female flowers that are tethered to the mother plant float on the surface of the water, creating a dimple. Whereas the male flowers are released from the male plant and float on the surface, relying on breezes or drift to reach a dimple. After their release, the female flower collides with the male flower as it slides down into the dimple, causing pollen to be catapulted to the stigma. Successful water pollination involves a lot of chance, which could explain why there are so few water pollinated plants.

Types of pollination

Pollination is of 2 types namely;

  1. Cross pollination
  2. Self pollination

Cross Pollination

Cross pollination, also referred to as heterogamy, is a type of pollination in which sperm-laden pollen grains are moved from one plant’s cones or flowers to another plant’s egg-bearing cones or flowers containing eggs ready to be fertilized to commence the pollination process. Cross-pollination occurs in both angiosperms (flowering plants) and gymnosperms (cone-bearing plants), and it aids in cross-fertilization and outbreeding.

Cross pollination evidently provides some evolutionary benefits such as;

  1. The outbreeding of seeds that may combine the hereditary traits of both parents, and the resulting offspring are generally more diverse.
  2. In a complex environmental setting, the genetic variability inside a cross-pollinated population may allow some individuals to adjust to a new situation, guaranteeing the species’ survival.
  3. Cross pollination produces more and higher quality seeds. A great illustration of this effect can be found in hybrid corn (maize), a superior product resulting from the cross-breeding of several specially bred lines. Natural cross pollination occurs in approximately half of the more important cultivated plants.

Plants have modified numerous strategies to avoid self pollination because cross-pollination allows for greater genetic diversity. The modified ways such as having separate locations of male and female gametes on the same plants are found in plants, such as cucumber, that have male and female flowers on opposite sides of the plant, making self pollination difficult. Furthermore, in some species, the male and female flowers are located on separate plants such plants are called dioecious plants.

Self Pollination

Self pollination or selfing is known as a type of pollination in which pollen from the anther is transferred to the stigma of a single flower of the same plant. A good example is the hermaphrodite flowers that self-pollinate because a hermaphrodite flower has both the stamen and the carpel. The stamen is the reproductive organ of the flower that produces pollen, whereas the carpel is the part of the flower that produces the ovule.

Self pollination is common in many plant species. It has the advantage of having a relatively stable genotype and not being dependent on pollinating agents. However, there are significant disadvantages to self-pollination over cross-pollination. One disadvantage is that it promotes inbreeding depression, which occurs when biological fitness in a given population is reduced due to inbreeding. Because the more diverse the genes or alleles found in a gene pool, the better protected the plants are from extinction.

In order for self-pollination or selfing to occur, the following scenarios must present themselves;

  • If cross pollination has not occurred successfully, self-pollination occurs at the end of a flower’s lifespan, in order to preserve and pass on its genetic materials to the next generation.
  • Self pollination can occur through the curving of stamens or styles, as seen in fireweed.
  • Selfing happens when biotic pollinators are temporarily scarce or when plants in a population are widely dispersed. This exact scenario can be an evolutionary advantage. Because selfing may be used to keep the species going until better conditions for outbreeding become available.

Selfing is common among annual plants, which must often produce an abundance of seed in order to colonize any bare ground that becomes available quickly and massively. If an annual plant produces no seed at all in a given year, the species’ survival may be jeopardized. Some plant species that have lost their natural pollinators appear to have successfully adopted a persistent habit of self-pollination. Many food-crop plants engage in continuous selfing as well. Some of these plants are termed cleistogamous (meaning that the flowers do not open), which is an extreme method of ensuring self-pollination.

Pollination in plants

Plants are unable to move resulting in the male and female gametes being immobile, unlike animals. This phenomenon leads to plant being unable to reproduce on their own. Thus, employing the help of a carrier (pollinators) to aid them in reproduction. A good example of pollination in plants can be seen in flowering plants as described below.

Pollination in flowers

Flowering plants use pollination as their reproduction system. It is the act of transferring pollen from a flower’s male “anther” to its female “stigma” in order to produce seeds and allow reproduction.

Once pollen settles on a receptive stigma, one of the pollen grain’s 2 internal cells germinates and forms a pollen tube, which is essentially a tunnel through the stigma and style that leads to an ovule within the flower’s ovary. The pollen grain’s second cell divides into two sperm cells, which travel down the pollen tube to the ovule. One sperm cell combines with an egg to form an embryo and, eventually, a seed. The other combines with an ovule cell to form endosperm, which provides nutrients for the embryo and, in some cases, the seed’s early growth. The whole process as described above is what is called double fertilization. Double fertilization in angiosperm (which is another name for flowering plants) produces seeds and the next generation.

Pollination and Fertilization

Pollination is simply defined as the transfer of a pollen grain (male gametophyte) to a flower’s stigma (receptive surface of the female reproductive organ), where it can germinate, grow through the style, and fertilize an ovule of a flower to produce seed. While fertilization occurs inside the ovary when the nucleus of a pollen grain fuses with the nucleus of an ovule to form a zygote.

The major difference between pollination and fertilization is that pollination is the deposition of pollen grains from the anther to a flower’s stigma, whereas fertilization is the fusion of haploid gametes to form a diploid zygote.

Today, we know of over 250,000 flowering plants that require pollination to reproduce. Approximately 80 percent of these require biotic pollination via animals while wind pollination accounts for 98 percent of the abiotic pollination of plants, and water pollination accounts for only 2 percent.

Flowers would not exist if pollination did not occur, laying more emphasis on why plants require pollination to reproduce, regardless of the type of pollination used that is either cross-pollination or self-pollination. Plants, like any other living things, have the desire to propagate their species and ensure that their genetics live on for many generations resulting in the use of pollination to help them in the process.


What are the agents of pollination?

The following are the agents of pollination

  • Insects
  • Birds
  • Bats
  • Wind
  • Water

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