Introduction
Pollination is of two types: cross-pollination and self-pollination. Regardless of the type, pollination is essential for seed production. In cross-pollination, the agents of pollination carry the pollen from the stamen/anther of a male flower to a female flower’s pistil/stigma. The pollen arrives at the ovule causing fertilization, which results in seed production.
In self-pollination, the pollen from the same plant is transferred to the stigma or the ovule without the aid of pollinating agents like wind, insects, water, animals, etc. The process is known as autogamy if the pollen arrives at the stigma of the same flower and geitonogamy if the pollen from a flower’s anther fertilizes the stigma of another flower on the same plant.
Cross-Pollination Vs. Self-Pollination
The main distinction between cross and self-pollination is the number of plants involved in the process. Self-pollination refers to pollen transfer happening in the same flower or a different flower of the same plant. On the other hand, cross-pollination refers to pollen transfer from a plant’s flower to another plant’s flower.
Difference Between Cross Pollination And Self-Pollination In Tabular Form
| Parameters of Comparison | Cross-Pollination | Self-Pollination |
| Meaning | Cross-pollination occurs when the stigma of another flower in a different plant belonging to the same species is pollinated. | Self-pollination occurs when the pollens are transferred from the anther to the stigma of the same flower. |
| Transfer of pollen | Transfer of pollen occurs with the help of pollination agents. | Pollination occurs without the aid of pollination agents. The flower’s anther directly sheds the pollen into the stigma. |
| Variety of progeny | A wide variety of progeny is created, as the genetic makeup varies between the plants even though they belong to the same species. | Self-pollination results in a limited variety of progeny. |
| Occurs in | It occurs in imperfect or incomplete flowers. That is, in flowers that have either male (anther) or female (stigma) parts. | It occurs in complete or perfect flowers. That is, in flowers with anther and stigma. |
| Energy requirements | Cross-pollination requires the flowers to exert more energy in attracting pollinating agents. | Self-pollinating plants do not require much energy exertion, as they do not rely on pollinators for pollination to occur. |
What Is Cross-Pollination?
Cross-pollination is a type of reproduction that results in genetic diversity. Since failure to evolve will cause a species to become extinct, plants have come up with several ways to ensure survival through cross-pollination. In some plants, the flowers’ anthers and stigma mature at different times making self-pollination impossible. Some species have male and female flowers bloom in different plants. Such barriers ensure that cross-pollination occurs with the help of pollinators lured by the plants’ sweet nectar.
Apples, raspberries, daffodils, lavender, heather, and most other flowering plants rely on biotic agents (of pollination) like bees, flies, butterflies, birds, etc. for pollination. Maple, grass, catkin, dandelion, and so on rely on abiotic agents like wind and water for pollination. However, dandelions are capable of self and cross-pollination.
Cross-pollination results in hybrid creations. That is, the resultant product has the characteristics of different parent plants belonging to the same species. On extremely rare occasions, cross-pollination takes place between plants belonging to different species. Cross-pollination may occur natural or human-induced. However, developing a hybrid with only the desirable characteristics and ensuring it is marketable requires a lot of effort. Some plant experts spend years trying to find a way to create the best possible hybrid through trial and error. The Better Boys Tomatoes is an excellent example of a hybrid born out of human-induced cross-pollination.
Popular Agents Of Pollination
Bees are the most popular agents of pollination, as they do not gather nectar from more than one species while searching for it. Therefore, the pollen that sticks to them eventually falls into the stigma of the flower, which is compatible with cross-pollination. Butterflies act as pollinators for bright-colored flowers, while moths are attracted to flowers with strong fragrances.
Flies and birds, too, are popular natural pollinating agents. However, a unique agent is a bat (no, bats are not the blood-sucking vampires that roam in the night, as fiction sets them out to be.). The long-nosed bat feeds on flowers that bloom at night and help pollinate them. Candle trees and durian are examples of flowers that bats help to pollinate.
Ways To Improve Cross-Pollination
Most people with backdoor gardens (whether or not they grow hybrid varieties) love to experiment with cross-pollination. The following are some ways in which chances of cross-pollination may be improved:
Planting Different Varieties
Planting at least two varieties of the same species capable of cross-pollination close together increases the chances of cross-pollination. Usually, the relative closeness is enough to ensure cross-pollination; however, some plant types require to be grafted together to achieve better results.
People must be careful to avoid creating any obstacles between the plants like fencing them individually, using meshes, or planting hedges between them, as that will prevent air-borne pollen from reaching the flowers.
Attracting Cross-Pollinators
Butterflies and other insects in gardens are excellent pollinating agents. However, bees (especially bumblebees and honeybees) are the best pollinator one can ask for. People may try setting up a beehive or renting them from beekeepers for improving the likelihood of cross-pollination. However, in some regions, using drones is a more cost-effective alternative to renting bees due to the scarcity of bees.
Human Intervention
Apparently, the wind and the insects’ work that results in cross-pollination can be mimicked using soft paintbrushes. Pumpkins and butternut squashes are ideal for cross-pollination by hand. People who unfortunately planted (without prior knowledge) a species that requires specific pollinators that are scarce in their area will benefit by switching to manually intervening in the cross-pollination process.
Use Of Drones
Drones are ideal for large orchards/areas. They spray pollen on the flowering plants or trees. Some pollen is successfully transferred to the required part (stigma) at the time of spraying, while the wind picks up the fallen pollens, and gravity ensures that the pollens sticking to the plants reach their destination. That is, natural and man-made mechanisms work together to make cross-pollination successful.
Ways To Reduce Cross-pollination
Excessive cross-pollination will exhaust the mother plant and cause it to perish sooner than it ought to. It is impossible to keep microscopic pollen from being transferred; however, people can take certain measures to reduce the chances of cross-pollination. They are as follows:
Increasing The Distance Between Plants
Planting plant varieties of the same species far apart or on opposite ends helps reduce cross-pollination. Planting a different species between the plant varieties of the same species will also hinder cross-pollination. An even more effective tactic is to plant varieties of the same species at different times so that at a given point in time, no more than one type of plant belongs to a particular species.
Hinder Pollinating Agents
Preventing pollinators from performing their work dramatically reduces the chances of cross-pollination. Building fences around the plants, using meshes, or planting them in a greenhouse that (pollinating) insects cannot breach are other ways to stop cross-pollination.
Growing Varieties That produce Only Minimal Pollen
Some plant varieties produce only minimal pollen, while some do not have pollen at all. For example, a few apple varieties like Winesap, Baldwin, and Cat’s Head do not have pollen to pollinate. People do not have to worry about pollination at all if they plant them (sounds like less work and more fun!).
Cutting off Certain Flowers
This tactic is for hybrid developers and is highly effective for creating specific types of hybrids. Removing the male flowers from the mother plant will lead to pollens from the desired father plant reaching the female flowers of the former. It is a cumbersome process but yields good results if people understand the process well.
What Is Self-Pollination?
Self-pollination is a type of reproduction that occurs without the help of pollen vectors. Some plants have inbuilt mechanisms to avoid self-pollination, while others adapt themselves to self-pollinate due to the chances of cross-pollination being negligible. Some plants, to ensure autogamy and avoid cross-pollination, have flowers that do not open. After all, cross-pollination cannot occur when the flower is not open.
A disadvantage of self-pollination is the creation of uniform progeny, which may result in inbreeding depression or the species’ reduced health. However, self-pollinating plants have the advantage of being able to spread to areas that pollinators do not frequent. Moreover, they do not have to spend energy on producing nectar to lure in pollinating agents, and less pollen is wasted during pollination. Besides, the progeny created have uniform characteristics which means the species’ pure features are maintained.
Most hermaphrodites (contain both sexes in the same flower) and monoecious species self-pollinate unless some mechanism prevents them from doing so. Hermaphrodites constitute 80% of the flowering plants, while 5% are monoecious. The other 15% are dioecious.
Onions, pepper, corns, carrots, and beans are some examples of self-pollinating vegetables. Several types of orchids, Red Shepard’s purse, and mouse-ear cress are self-pollinating plants. Other examples of self-pollinating plants are wheat, barley, apricots, oats, etc.
Mixed Mating
Only 10 – 15% of flowering plants are self-pollinating. A quarter of the total flowering plants, which is around 40%, exhibit a mixed mating system. In dimorphic cleistogamy, a type of mixed mating system, a plant produces flowers by closed self-pollination or open cross or self-pollination process.
Self-pollination In Different Species
The following are some examples of how several species self-pollinate:
Orchids
In the orchid Paphiopedilum parishii, the anther (pollen sac) changes from a solid to a liquid state and falls on the stigma’s surface. Therefore, pollinating agents are not necessary for this process. In Holcoglossum amesianum, the bisexual flowers turn their anthers against gravity (a self-pollination mechanism) to drop the pollens into their own stigma cavities. People theorize that these orchids adapted to their environment (windless conditions and insect scarcity) and started self-pollinating.
Caulokaempferia Coenobialis
Caulokaempferia coenobialis, a Chinese herb, has a unique way of self-pollinating. A film of pollen slides from its anther, slides along the style, and drops into the stigma. Now, the process is the same (pollen transfers from anther to stigma); however, the flow of pollen is due to the flower’s oily emulsion that helps spread the film of pollen and not gravity as in other plants.
Red Shepard’s Purse
Capsella rubella or the Red Shepard’s purse became self-pollinating species about 50,000 – 100,000 years ago. This indicates that plants can evolve into self-pollinating ones, and the adaptation can persist for generations.
Arabidopsis Thaliana
The chances of an Arabidopsis thaliana, a predominantly self-pollinating species, out-crossing (cross-pollinating), even in the wild, is around 0.3%. This species started self-pollinating about one million years or more ago.
Main Difference Between Cross-Pollination And Self-Pollination (In Points)
- Cross-pollinating plants have large quantities of pollen, whereas self-pollinating plants have smaller quantities.
- Self-pollination can occur when the flower is closed; however, cross-pollination can occur only when the flower is open.
- Holly shrubs, pawpaw, sweet corn, and various fruit plants rely on cross-pollination. Legumes like peanuts, soybeans, and peas, and flowers like orchids, sunflowers, and so on self-pollinate.
- Self-pollination causes inbreeding, whereas cross-pollination causes outbreeding.
- Though self-pollinating plants rely on internal factors to pollinate, some are receptive to pollination that occurs with the help of wind or insects. That is, cross-pollination is possible in self-pollinating plants. On the other hand, cross-pollinating plants will always rely only on pollinating agents for pollen transfer.
- Pollen wastage is minimal or negligible during self-pollination as opposed to cross-pollination. The higher pollen wastage in cross-pollination is a result of the plant’s dependence on pollinators. Some pollens are lost in the wind and water or drop off the insects before they reach a flower in another plant.
- Cross-pollinating plants are colorful and have an attractive scent, which helps them attract pollinators. On the other hand, self-pollinating plants are comparatively less colorful and do not require attractive scents as they have no need for pollinating agents.
Conclusion
People need to know what kind of plants they are planting in their gardens. Agreed, not everyone is a green thumb; however, one can do everything in their power to grow at least half-decent varieties, right? Anyway, knowledge about cross and self-pollinating plants and how to induce the desired pollination will have a huge impact on one’s gardening skills.
References
- https://courses.lumenlearning.com/wm-biology2/chapter/self-pollination-and-cross-pollination/
- ttps://www.thespruce.com/cross-pollination-7111528
- https://www.nature-and-garden.com/gardening/increase-cross-pollination.html
- https://www.nature-and-garden.com/gardening/avoid-cross-pollination.html
- https://owlcation.com/stem/6-Agents-of-Pollination
- https://en.wikipedia.org/wiki/Self-pollination
