How to cross pollinate?

Unfounded fears are at the heart of much cruelty toward wild animals, from the wolf to the wolf spider and everyone in between. But a lack of respect for natural behaviors causes harm too. A little empathy goes a long way toward compassionate coexistence.

Though we may not realize it, coyotes and other wild animals are living in close proximity to us, no matter how urban our environs. (Photo by John Harrison)

Wherever you are in the continental U.S., a coyote may be less than a mile away. If you live in the city, you’re more likely than your rural cousins to encounter raccoons. And regardless of geography, you probably share your home with dozens of species of insects and spiders.

These facts aren’t meant to scare you, though they’re often exploited by pest control companies for exactly that purpose. Proximity to wildlife should be cause for celebration—reminders that, no matter how many walls and fences we construct, we’re an integral part of the natural world.

On private property, we treat wildlife as intruders, alerting police when coyotes wander the neighborhood. On public lands, we ourselves are the intruders, taking selfies with seals and cuddling up to buffalos.

Yet instead of awe and wonder, fear is our default reaction, often leading to trapping, poisoning, or, in the case of small creatures, the angry stomp of a shoe. In a society where federal scientists declared a century ago that predators “no longer have a place in our advancing civilization” and modern consumers can purchase weapons of mass habitat destruction at local hardware stores, our conflicted relationships with wild animals large and small aren’t surprising. Many of us grew up in denuded landscapes without large carnivores or herbivores, from wolves to deer, and without context for how to coexist. We take our cues from pop culture, learning about animals in faraway places while perpetuating myths about those in our backyards.

Scenes from Selfie Land: On the beaches of Poipu, Hawaii (above), lifeguards routinely warn tourists to keep their distance from green sea turtles. In La Jolla, California (below), a woman with a selfie stick gets too close to a sea lion, drawing surrounding children into her orbit. Minutes later, a man who tries to touch another sea lion jumps away after being barked at. (Photos by Nancy Lawson)

Caught in the crossfire of our contradictory attitudes are living beings with few real homes to call their own. On private property, we treat wildlife as intruders, alerting police when coyotes wander the neighborhood. On public lands, we ourselves are the intruders, taking selfies with sea lions and cuddling up to buffalos. On the surface, the two types of behaviors appear dissimilar; one results from misguided fears and the other from seemingly no fears at all. But they derive from the same view of wild animals as abstractions, a collection of anonymous creatures who are alternately “pests,” “nuisances” or perfect vacation pictures.

“More people could probably tell you about an African lion or an elephant than they could about a gray fox or an opossum or something right here,” says Christine Barton, director of operations for the Fund for Animals Wildlife Center in Ramona, California, a care center operated by HSUS affiliate the Fund for Animals.

Nothing to be afraid of: Wasps in the Tachytes genus are distinguishable by their large green eyes. They’re among the common solitary wasps you might see in the garden, as they go about their business feeding on flower nectar and gathering katydids, grasshoppers and crickets to provision their nests. (Photo by Nancy Lawson)

Ironically, that lack of knowledge engenders fears of some of our closest and most harmless wild neighbors. Many spiders lack the ability to bite us, skunks would rather walk away than spray, and opossums don’t attack. Most bee and wasp species are solitary and never sting, and many snakes present no threat. Yet viral videos about single incidents can spark outsized fears of entire classes of wildlife.

Healthy fears can be lifesaving—a lesson learned too late by tourists who treat wildlife as objects of entertainment. But fear doesn’t have to turn into loathing. “It’s OK to be afraid of spiders,” says Kerry Wixted of the Maryland Department of Natural Resources, “but it’s not OK to kill the spider because you’re afraid of it.”

Instead of annihilating perceived dangers, we have an ethical responsibility to question assumptions, says HSUS urban wildlife director John Griffin. “We all have a right to be here,” adds Barton, and we should should keep in mind a few key principles when learning about wildlife among us:

Wild animals are individuals.

It’s a myth that foxes, raccoons and other animals only venture out at night. (Photo by John Harrison)

“Everybody’s always asking, ‘What’s his story?’ ” says chief animal control officer Jennifer Toussaint of the dogs and cats at the Animal Welfare League of Arlington, Virginia. “Well, wildlife have stories, too.”

One local favorite is “Ma,” a fox who suns herself on a busy sidewalk while her kits play. She’s been doing so for years, but Toussaint still finds herself assuring panicked residents that because fox parents hunt for food almost around the clock, Ma is “trying to catch a moment and just catch a few rays.” The information delights callers: “Every mother understands that moment.”

Even the smallest creatures lead fascinating lives. Wolf spider mothers carry egg sacs around and allow hatched babies to travel with them. Male spiders often work hard to attract females. “Some spider males have to bring gifts,” says Wixted. “Some of them sing and dance. Some of them have to build elaborate displays.”

Their motivations are survival-based.

Just like people, animals go to great lengths to protect their young. Mammal mothers keep alternate den sites in case their first choices become dangerous or unsuitable. (Photo: HSUS)

When animals get too close for comfort, it often means they’re hungry, thirsty, or protecting their young.

Bobcats and mountain lions venture near homes looking for water, says Barton, especially during wildfires that force them farther into human territories. Snakes inhabit porch mailboxes to eat insects attracted to lights; going dark may encourage them to move along. Raccoon mothers who rip through plywood or other repairs to a hole in a wall are “not plotting to come down one day and kill you and your children,” says Toussaint. But if these DIY coverings have inadvertently trapped kits inside, they’ll “brave fire—they will brave darn near anything to get back to their children.” Toussaint advises callers to remove makeshift barriers and wait at least 24 hours; wild moms who feel unsafe will relocate kits to alternate den sites. (It’s best to work with a humane service to gently exclude wildlife before you get to this point; see “Choosing a Wildlife Control Company” for more information.)

They all have essential jobs to do.

A yellow jacket feeds on nectar from a garden camellia blossom. Yellow jackets are important predators, gathering insects to provision their nests. (Photo by Vicki DeLoach/Creative Commons)

Wixted has been stung enough in the past to be wary of yellow jackets. But she appreciates their dedication to their young and their role as wild garden helpers who gather insects for their nests.

When a nest expanded so quickly she worried neighborhood boys might come too close, another wild garden helper stepped in to take care of it. “That weekend, a raccoon dug it all up and ate everybody,” Wixted says. It’s a lesson she often teaches: Let stink bugs stay; spiders will eat them. Let spiders stay; hummingbirds will snatch them up. And so on, until everyone has had their fill in an elaborate web of life that, with just a little knowledge, we can all learn to appreciate.

Related content: For tips on helping others address their fears of the natural world, see this extended Q&A with Kerry Wixted.

A version of this article appeared in the Nov-Dec 2018 issue of All Animals magazine. Learn more about the natural behaviors of larger wildlife at, and read about the fascinating lifestyles of arthropods at Bug Eric, the blog of Eric Eaton, the principal author of the Kaufman Field Guide to Insects of North America.

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Cross-pollinating plants


This family is represented by plants whose flowers are generally unisexual, i.e. with male and female flowers on the same plant. The male flowers, at the tip of a long peduncle, fertilize the female flowers, in which the ovule is already in the shape of a miniature fruit. This means that different plants must be cross-pollinated, either naturally (by insects or the wind) or manually.

Cucurbitaceae easily form hybrids between species, producing different cultivars. Be sure to plant any cultivar whose seeds you intend to keep well away from any other sources of Cucurbitaceae pollen.

Harvesting seeds from various cross-pollinating vegetables

Squash and pumpkins


Harvest your squash and pumpkins once they are fully ripe. The skin should be thick and not yield to pressure from a fingernail.

Leave the seeds to mature fully for at least three weeks after you harvest the plants. Then cut the fruit in two and remove the seeds, separating them from the flesh. Blot them dry and spread them out on a cookie sheet or waxed cardboard for two to three weeks.


Allow your cucumbers to become over-ripe before harvesting them – they will turn soft and dark yellow, orange or white, depending on the cultivar.

Cut them in two and use a spoon to remove the seeds and pulp. Transfer it all to a glass jar and add as much water as there is pulp, then place the jar in a dark spot for 48 hours. This will allow it to ferment, which will clean the seeds. Any imperfect seeds and the pulp will float to the surface, so you can skim them off. Rinse the remaining seeds in a sieve and dry them as you would squash or pumpkin seeds.


Harvest your melons not long after they ripen (but before they rot or get eaten by rodents!). Remove and rinse off the seeds and dry them as you would squash seeds.

N.B.: You can grow muskmelons and watermelons together and harvest their seeds without worrying about them cross-pollinating, since they belong to two different genera and species.

On the other hand, you must make sure that your neighbours are not growing them within 300 metres of your garden.


Once the pods holding the seeds have turned brown, you can cut down the plants and lay them on a sheet of fabric to catch the seeds when the siliques burst.

Keep only the main flower head and prevent any secondary shoots from developing.

Do not choose any plants that bolt, or flower prematurely.

N.B.: Broccoli must be grown one kilometre away from other members of the cabbage family, in particular different varieties of Brassica oleracea. On the other hand, you can grow other broccoli plants and pick them before they flower.

Summer radishes

Harvest the seed pods before they open and place them in a paper bag. Choose the latest-flowering radishes.

N.B.: Radishes cross-pollinate with each other and with turnips, rutabagas, Chinese cabbage, colza (rape) and mustards. Do not allow more than one cultivar to flower, or keep them one kilometre apart.

Based on an article by Nathalie Leuenberger in Quatre-Temps magazine, Vol. 23, No.4

The process of transfers of pollen grains from anthers to the stigma of a flower, but when this process is between the flowers of the same plant than the term is called as self-pollination, while when the transfers are between the flowers of the different plant of same species is called as cross-pollination. Secondly, pure line progeny is obtained in the self-pollination.

The target of every living organisms is to create their young ones and transfer their characters to them. Pollination is also the same process occurring in plants, where the reproduction and fertilization are processed in the flowers, which further produce seeds. When pollens gets transferred between the flowers, seeds are produced which contains the genetic information of a plant and is capable of producing offspring.

In general sense, we say that in pollination the pollen grains are then transferred from the anther of a flower to the stigma. If we observe the flower of any plant, like China rose, we will see different parts like sepals, petals, stigma, style, anther, filaments, stamens, pistil, pedicel, thalamus, ovary and ovules. Anther and filaments are said as the male reproductive parts of a flower, while pistil which has the stigma, style and ovary are mentioned as the female reproductive parts of a flower.

The term pollination gains its attraction when Gregor Mendel successfully cross-pollinated the garden-peas. There are the different method of pollination which depends on the type of plants like in angiosperms it takes place between the flowers of the same plants (self-pollination), while in gymnosperms it occurs between two different plants with same or different species (cross-pollination).

In this content, we will be studying the two primary type of pollination and the ground points on which they can be distinguished.

Content: Self-Pollination Vs Cross-Pollination

  1. Comparison Chart
  2. Definition
  3. Key Differences
  4. Conclusion

Comparison Chart

Basis for Comparison Self-Pollination Cross-Pollination
Meaning The inbreeding process in plants where pollens are transferred from the anthers to the stigma of the same flowers or another flower but of the same plant. The outbreeding process between the two plants of the same species and different flowers.
It involves Single plant. Two different plants of the same species.
Occurs in Self-pollination occurs in genetically same plants. Cross-pollination occurs in genetically different plants of the same species.
External pollinating agents There are no requirements of the pollinating agents. External pollinating agents are required like water, animals, wind and insects.
Perfect flowers Self-pollination occurs in perfect flowers only. It occurs in both imperfect and perfect flowers.
Flowers of the plants These plants have small flowers. These types have the scent, nectar and bright-coloured petals.
Pollen grains Less number of pollen grains are produced. A large number of pollen grains are produced.
Reproduction Geitonogamy and autogamy are the two types of the process of reproduction. Allogamy kind of reproduction occurs here.
Genetic variability With the self-pollination, pure line progeny is obtained and so no role in genetic variations. With the cross-pollination, genetic variations and genetic recombinations are observed.
Characters Desirable characters can be obtained, but undesirable character cannot be eliminated. Desirable characters can be obtained and undesirable character can be removed.
Offspring It results in homozygous offspring. It results in heterozygous offspring.
Examples Wheat, rice, pea, orchids, barley, tomatoes, peaches, apricot. Mulberry, maize, pumpkins, strawberries, blackberries, plums, grapes, daffodils, maple, catkins, grasses.

Definition of Self-pollination

When a flower of a plant has the capability to produce a seed by pollinating itself and do not need another flower or when the pollination occurs within the flowers of the same plant is called as self-pollination. Monoecious species and the hermaphrodites are mainly known for self-pollinating.

The flowers of plants which perform the self-pollination have the male and female reproductive part in the same flower. It is the simple method and does not need any investment, here the pollens get assemble in the anther (male reproductive part), and transferred to the stigma (female reproductive part) of a flower and thus the process of fertilization is accomplished.

Autogamy is the process when the flower reproduces within itself, in this, the pollens are transferred from male to the female part of the flower, while Geitonogamy is little different and occurs in different flowers of the same plant. Cleistogamy is the process where flower does get opens and gets pollinated.

The advantage of self-pollination is that the plant does not need any agent like wind, insects to get fertilized, this method even does not require any extra investment. Though genetic variation is absent and thus the same product is obtained all time.

Definition of Cross-pollination

When flower of a plant gets pollinates with another flower of another plant, it is called as cross-pollination. This process is performed artificially also, to get the new and good quality of flowers, fruits, and vegetables. In this, the two old varieties produce a new variety. For example, a cross between two varieties of mangoes just to get a new good yield. So in cross-pollination genetic variation and genetic recombination is seen.

Cross-pollination is not possible for between entirely different species, like for example tomato cannot be pollinated with the potato or onion, but one variety of tomato can be crossed with another species of tomato.

Key Differences Between Self Pollination and Cross-Pollination

Given the below points will highlight the distinction between the two types of pollination:

  1. Self-pollination is the inbreeding process occurring in between two flowers of the same plants, in this pollens are transferred from the anthers to the stigma. Cross-pollination is the outbreeding process between the two plants of the same species and of different flowers, in this also the pollens are transfers from the anthers to the stigma.
  2. In self-pollination, there is the involvement of the single plant, while in cross-pollination two different plants of same species, though genetically distinct are involved.
  3. External pollinating agents are not required in the self-pollination, but on the contrary, external pollinating agents like water, animals, wind, and insects are required.
  4. Self-pollination occurs in perfect flowers only, and the plants have small flowers, while cross-pollination occurs in both imperfect and perfect flowers, and the flowers of the plants have the scent, nectar and bright-colored petals.
  5. Pollen grains produced are less in number in self-pollination, whereas large numbers of pollens are produced in the cross-pollination.
  6. Geitonogamy and autogamy are the two types of the process of reproduction occur in self-pollination, while allogamy kind of reproduction occurs in cross-pollination.
  7. Genetic variability is not observed in the, and pure line progeny is obtained, while in cross-pollination, genetic variations and genetic recombinations are observed.
  8. Self-pollination results in homozygous offspring, whereas cross-pollination results in heterozygous offspring.
  9. Desirable characters can be obtained, but the undesirable character cannot be eliminated in case of self-pollination, while in cross-pollination desirable characters can be obtained and undesirable character can be removed.
  10. Few examples of the plants which follow the process of self-pollination are wheat, rice, pea, orchids, barley, tomatoes, peaches, apricot, while mulberry, maize, pumpkins, strawberries, blackberries, plums, grapes, daffodils, maple, catkins, grasses are examples of the cross-pollination.


We can summarise the article by saying that we came to know about the types of reproduction and fertilization that occur in planta and are of two types. Self-pollination is equally important as cross-pollination, and are used to increase the yield and varieties of crops. As sometimes we need the same range of plants every time, and sometimes modifications are also required and also it is the need for the plants to get cross-pollinated.


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Cross-pollination is defined as the transfer of pollens from one flower to another of a different plant. Some examples of plants that exhibit this phenomenon are cucurbits, blueberries, cherry trees and apple trees.

Amongst sexually reproducing plants or flowering plants, pollination is the key process for fertilization. It is defined as the transfer of pollens from the anther to stigma, either in the same plant or to a different plant. When pollens are delivered to the same flower or different flower of the similar plant, it is known as self pollination or autogamy. Contrary to this, a flower is pollinated by pollens of a different plant’s flower in cross-pollination. As ovum and spermatozoa come from different plants, it is also referred to as allogamy.

What is Cross-Pollination?

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Most of us are well acquainted with cross-pollination definition, which states it as, the transfer of pollens from the male reproductive organ (stamen bearing anther) to the female reproductive part (pistil bearing stigma), of a different plant. In the phenomenon, the pollens containing male gametes or sperms are deposited in the receptive part of a flower, from which they are channeled to the female gamete or ovule. In gymnosperms, pollens are directly deposited in the micropyle of the ovule; whereas, pollens are collected by the stigma in angiosperms.

In cross-pollination, the pollinating agents play a crucial role in carrying out fertilization. Hence, for it to take place, there should be prospective agents, which may be biotic pollinators (e.g. insects, birds) or abiotic factors (e.g. wind, water). The biotic pollination contributes to more than 80 percent of fertilization cases. An adaptation in cross-pollinated plants is bearing of stamens that are taller than the ovule bearing structures called pistils. Usually, the flowers have colorful petals and strong fragrance to attract cross pollinating agent.


The major benefit of cross-pollination is giving rise to offspring that are genetically different from the parent plants. Thus, plant hybrids with desirable characters are produced as a result of it. In nature, it increases the survival rate of plants. In plant breeding experiments, flowers of various plant species are cross-pollinated manually. All you need is a collection of the pollens at a specific time and place them into stigma of another plant of the same species.

Cucurbits: At times, cross-pollination takes place between two related species, though they are not identical to each other. An example is pollination between zucchini and acorn squash, both of which belong to the Cucurbitaceae family. In short, plants classified under the same botanical species can be cross-pollinated successfully.

Blueberries: Cross-pollination of fruit trees is studied in detail, as it indirectly affects the fruit production. In case of blueberries, bumble bees and other bee types, pollinate the flowers. For production of larger berries and also to ensure good yield, grow different cultivars of blueberries. Nevertheless, while selecting the varieties, make sure that they are compatible to each other.

Cherry Trees: Sweet cherry cultivars are cross-pollinated species, while sour varieties undergo self pollination. This is the main reason why, fruits cultivators often grow many cherry trees (that bloom at the same time) in proximity to each other. For planting in your orchard, choose compatible cherry species by consulting your local horticulturist.

Apple Trees: Another example of cross-pollination in fruits trees is apple. For a flower to produce apple fruit, it has to be fertilized by pollen of another flower from a different apple plant. In this process, honey bees land on the apple flower, collect nectar and pollens, and then visit another flower. Thus, the pollens are carried to another flower of a different tree, resulting in cross-pollination.

The end result is fertilization, which is followed by formation of seeds in the ovule. This is the main difference concerning pollination vs. fertilization. A major concern in agriculture and horticulture industries is loss of biotic pollinators in the last few decades. This pollinator decline leads to reduced rate of cross-pollination of flowers. Consequently, fertilization is affected negatively, which in turn causes decrease in fruit yield and crop production. The probable reasons for loss of pollinators are application of pesticides, insecticides, and habitat destruction.


Assorted References

  • major reference
    • In pollination: Types: self-pollination and cross-pollination

      An egg cell in an ovule of a flower may be fertilized by a sperm cell derived from a pollen grain produced by that same flower or by another flower on the same plant, in either of which two cases fertilization is said to…

  • method in plant breeding
    • In plant breeding: Mating systems

      …of the same plant and cross-pollinated (an “outcrosser” or “outbreeder”) if the pollen comes from a flower on a different plant. About half of the more important cultivated plants are naturally cross-pollinated, and their reproductive systems include various devices that encourage cross-pollination—e.g., protandry (pollen shed before the ovules are mature,…

  • method of cross-fertilization
    • In cross-fertilization

      …plants, cross-fertilization is achieved via cross-pollination, when pollen grains (which give rise to sperm) are transferred from the cones or flowers of one plant to egg-bearing cones or flowers of another. Cross-pollination may occur by wind, as in conifers, or via symbiotic relationships with various animals (e.g., bees and certain…

  • role of bees
    • In bee

      …for it often results in cross-pollination of plants. The practical value of bees as pollinators is enormously greater than the value of their honey and wax production.

What happens when you cross a checkerboard with a midnight snack? You get edible checkers, sold with the motto “beat ’em and eat ’em.” What if you cross high-heeled shoes with a tricycle? You get pumps with training wheels. Or, what do you get when you cross a dessert plate with an ice-cube tray? An ice cream bowl that melts after use so you don’t have to wash it.

These are just a few of the wonderfully fanciful ideas in John Cassidy and Brendan Boyle’s The Klutz Book of Inventions. The goal of their book is to help readers become comfortable creating ridiculous ideas, since many brilliant ideas seem really crazy when they are initially conceived. The playful inventions they describe result from connecting and combining objects and concepts that on the surface seem unrelated. By exploring ways to fuse them together, we see many surprising and interesting ideas surface.

This is similar to the philosophy behind the Japanese art of chindōgu, which involves coming up with “unuseless” inventions. Essentially, chindōgu involves combining products that are completely unrelated to create inventions that are wonderfully unusual. For example, an outfit worn by a baby with a mop on its belly that allows the baby to clean the floor while crawling around; a shirt with a matrix on the back, so that you can tell someone exactly where to scratch; an upside-down umbrella that allows you to collect water when you are walking in the rain; or eyeglasses with arms that can be removed to be used as chopsticks. These inventions might not be immediately practical, but each one opens a door to new ideas that just might be.

1. Combine Unlike Ideas

Being able to connect and combine nonobvious ideas and objects is essential for innovation and a key part of the creative-thinking process. Along with your ability to reframe problems, it engages your imagination and thereby unlocks your innovation engine. Essentially, you need to be able to reorganize and rearrange the things you know and the resources you have in order to come up with brand-new ideas.

Alan Murray, head of the School of Design at the Edinburgh College of Art, gave his former graduate students at the Technical University of Eindhoven a surprising assignment to help them hone these skills. He challenged them to invent a “sextron.” He told them they needed to combine two different household devices, such as a coffee machine and a blow dryer or a telephone and an electric toothbrush, to create something new, and it had to function as a sex toy. They then had to design a formal user’s manual for the new device. This was certainly an edgy project! His goal was to inspire these students in ways they had never imagined. Not only did they have a wild time taking on this provocative assignment, but they also learned that by connecting devices that had never been connected before, they could come up with surprisingly innovative products that stimulate both the mind and the body, from ears to toes, in unusual ways.



This study describes a new and perhaps a most striking example of a structural adaptation that promotes cross-pollination in angiosperms. The results show that the sheaths surrounding the basal axis of the inflorescence of C. rigida, a self-incompatible orchid, constitute a perch for attracting and positioning foraging sunbirds to conduct efficient and orderly cross-pollination, which is responsible for essentially all the seed production of C. rigida, to ensure its reproductive success. Simultaneously, C. rigida offsets insect-mediated self-pollination, which causes infertility and incurs mating cost, through gamete discounting. Consequently, C. rigida gains not only mating and fertility advantages and genetic variability from crossing, but more importantly, it also endures reproductive success through subtle structural adaptation by merely adding a perch to the basal axis instead of altering the inflorescences of multiple flowers. The development of the structure for bird cross-pollination is likely an evolutionary response to self-pollination by insects, which is rendered infertile by self-incompatibility and incurs a high mating cost, as well as to the lack of cross-pollination from insects.

Except for its discernible scent and prominent sheaths, C. rigida does not conform to the general description of bird-pollinated plants because its flowers are not brightly colored. The reward offered to pollinators is nectar, with sugar content that is lower in this species than in other insect-pollinated species because birds cannot sip nectar that is excessively viscous .

All the sunbirds we observed alighted on the coriaceous sheaths, which cover the basal axis of inflorescence, before probing the flowers for nectar. Given that the inflorescence of C. rigida is pendent with flowers horizontally opening along the soft axis, sunbirds must bend their heads to reach the nectar in the flowers. The sheathed perch is located at the base of the inflorescence directly above the flowers and has a conical shape. The sheath serves as an alighting perch and a “grasping pole” that is sufficient for the sunbirds to land and to grasp securely. Upon landing, sunbirds grasp the perch by using their feet. The birds then lean down to access nectar by inserting their curved beaks into the disc from one side of the flower. In this way, the floral stigma and the pollinarium of C. rigida successively come in contact with the bird’s beak, enabling the pollinaria to be accepted by the bird and to be extracted from the flowers. Considering the confined standing position, sunbirds bend their bodies and elongate their tongues to visit different flowers with the same inflorescence. Different parts of their beaks, rather than their tongues, touch different floral columns and receive pollinaria. Thus, the pollinaria can only be delivered by the sunbird separately to the flowers of another inflorescence with corresponding distances between bird perch and flowers. Considering that the distances between a sunbird and different flowers are dissimilar and that the bird probes the flowers for nectar in a regular sequence, the pollinaria will not overlap with one another on the bird’s beak or will not mix in the flowers, thereby avoiding self-pollination. Sunbirds do not revisit flowers within the same inflorescence or the same clone, thereby also avoiding selfing among those flowers and ensuring the successful transfer of pollen among different plant clones.

Experiments with perch cover removal (peeling off of the sheaths around axis) immediately before blooming demonstrate that the sheaths are not required for floral or seed development but are important factors in attracting and keeping the visiting sunbirds in position for cross-pollination. Sheath removal did not preclude visitations by sunbirds because the remaining bare axis could still serve as a damaged perch, but it markedly reduced the frequency and duration of bird visitations and, consequently, the fruit setting rate of the orchid. The sunbirds strongly prefer landing and feeding on inflorescences with intact perches than on those with damaged perches. The sheaths function as a signal for sunbirds to visit the flowers. Intact perches are constructed in such a way that helps sunbirds complete flower visitation, thus increasing pollination efficiency. Our observations show that an axis tightly covered by coriaceous sheaths at the inflorescence base is 3 cm to 4 cm long, with its lower part enlarged, vertically forming a narrow, conical perch that signals sunbirds to land and provides them with visiting perches. The sheathed perches help sunbirds grasp tightly and avoid slipping, which allows them access to flowers in the lower part of the inflorescence. If a sunbird heavier than 18 g lands directly on the flowers, as some bird species do , , the soft rachis and fragile flowers of C. rigida will not be capable of supporting its weight. Therefore, the stable sheaths guarantee sunbirds sufficient time to alight on a firm foothold until they have finished probing all open flowers on the inflorescence. Particularly, when probing flowers on the far end, the birds spend longer times hanging on the perch. The sheathed perch is an important factor in attracting birds and controlling their visiting positions to enable them to pick up and deliver pollen using different parts of their beaks. Thus, once the sheaths are removed, bird visitation decreases, which limits cross-pollination and fruit setting.

Although C. rigida visitations by male and female sunbirds were equally affected by perch cover removal, notable differences were observed in the visiting behavior of male and female birds. The male birds, which are distinguishable from the female birds by their size and color, seem more diligent in foraging and are more alert. On average, the males visited the orchid populations more frequently but stayed for a shorter time per visit. For couples of visiting birds, the males usually reached flowering populations earlier than females. Males may need more food because they are larger and have a greater responsibility to hunt for food when females are nesting. For males, staying on the perches to probe flowers could be less convenient because of their longer tail feathers and greater weight. Furthermore, their colorful feathers could make them more vulnerable to predation if they stay at one site for too long.

We also examined the function of the floral lip, the other landing plate for visiting insect pollinators. Similar to sunbirds, wasps and honeybees are attracted by the nectar of the C. rigida flower. Both insects land on the epichile of the lip and then crawl into the disc to probe for nectar. While backing out, the forehead of wasps and honeybees come into contact with the rostellum, which facilitates pollen transfer. Considering that insects tend to visit flowers on the same inflorescence and in the same plant clone continuously and repeatedly, their pollen transfer is virtually all self-pollination, which results in abortive fertilization because of self-incompatibility, as well as in the wasting of pollen and ovules (gamete discounting) that could be used for fruitful cross-pollination.

C. rigida often forms a large number of plant clones, with 100 to 300 inflorescences opening simultaneously. We compared birds and insects that alight on different landing plates and found that sunbirds visit only one inflorescence in a clone before flying to another clone (a different plant), whereas wasps and honeybees visit different flowers of the same inflorescence or plant repeatedly for 3 min to 8 min or even longer to visit a clone, which results in self-pollination within an inflorescence or a clone. Pollen used for infertile selfing rather than fruitful outcrossing reduces usable pollen and male fitness as well as usable ovules and female fitness because of the lack of seeds from ovules. This behavior is unfavorable to C. rigida breeding. The natural fruit setting of inflorescences with all their pollinaria removed is significantly higher than that of unmanipulated inflorescences, which indicates that fruitless self-pollination occurs in C. rigida and is unavoidable because of visitation by insects. The insects repeatedly and continuously visit an inflorescence and then transfer their pollen to flowers of the same plant clone. This behavior diminishes the pollen available for pollinating other clones and the stigmas available for accepting foreign pollen, thereby contributing to pollen and ovule discounting. Removing the pollen of inflorescences offsets ovule discounting by enabling stigmas to accept foreign pollen, thus resulting in an increase in fruit set.

Under natural conditions, the contribution made by insects to the cross-pollination of C. rigida is difficult to quantify experimentally. Given that the insects rarely visit flowers in another plant clone, cross-pollination by the insects must be very low. Furthermore, C. fimbriata, an orchid related to C. rigida, has mostly the same characteristics, including growth in the same habitat with numerous clones, self-incompatibility, and the same insect pollinators except for birds because of the absence of the sheathed perch, i.e., it is exclusively self-pollinated by insects. Thus, C. fimbriata can be used as a suitable reference and control for C. rigida in studies on sheaths and pollination by insects and birds. We determined that although the fruit setting rate from the artificial cross-pollination of C. fimbriata (72.75%) was equivalent to that of C. rigida, the natural fruit setting rate of C. fimbriata, which exclusively results from cross-pollination by insects because selfing is infertile, was evidently low at 0.5%, as previously described . Thus, in natural C. rigida populations, cross-pollination by insects more likely results in extremely low levels of fruit setting, similar to that in C. fimbriata. The remaining bulk of the total natural fruit set in C. rigida (26.33%) is likely contributed by cross-pollination by sunbirds, which indicates that bird perch-enabled cross-pollination is responsible for essentially all instances of sexual reproductive success of C. rigida.

In other words, if C. rigida did not develop the bird perch or if its bird perches were all eliminated, its cross-pollination and natural fruit setting would be extremely low, as found in C. fimbriata. Conversely, if C. fimbriata added such a perch for bird cross-pollination, its natural cross-pollination and natural fruit setting would have been significantly higher as in the case of C. rigida. Self-incompatibility is generally accompanied by an adaptive change in floral components to prevent self-pollination. Neither of these two species evolved such an adaptive mechanism to avoid self-pollination directly. However, C. rigida has developed a bird perch to promote cross-pollination directly and to offset self-pollination by insects, a feat unachieved by C. fimbriata.

The evolution of such a seemingly simple structural adaptation in C. rigida, which added sheaths around the flower-bearing basal axis instead of altering the inflorescence of many flowers, is remarkable. The development of such an optimal perch for attracting sunbirds and precisely positioning them for efficient and orderly pollen dispersal for cross-pollination to ensure reproductive success and to reduce mating costs is more striking and delicate than the previously reported case of Babiana ringens. For B. ringens, a self-compatible iris with a stand-alone perch enhances cross-pollination by sunbirds while the plant also reproduces via self-pollination by the birds . However, the underlying mechanism is unclear, and cross-pollination is not required for the reproduction of the species.

With self-incompatibility, C. rigida requires a cross-pollination mechanism for successful sexual reproduction. The sheathed perch-enabled cross-pollination by sunbirds in C. rigida is ingenious and advantageous. Compared with changing the design of multiple individual flowers on each inflorescence, making one perch in each inflorescence (i.e., treating an inflorescence as an organized unit) is significantly more economical and enables more orderly control of pollination. Sunbirds are highly alert and active in a wide area, move rapidly, and spend a long time foraging to meet their large food requirements . The perches on the C. rigida inflorescences confine sunbird movements to ensure the attachment of pollinaria to different parts of their beaks for cross-pollination. Pollinators such as sunbirds enable efficient and orderly pollen transfer between different plants to achieve cross-pollination, which also counteracts infertile self-pollination by insects to reduce genetic costs. Consequently, C. rigida does not need to modify its floral component to prevent self-pollination or to promote cross-pollination.

A basic characteristic of orchids is a specialized lip that is suitable for insect visitation . Although C. rigida occupies a more recently evolved position in the evolution of orchids, its lip is inherited from its ancestors and may have been derived from adaptation to pollination by bees. The lip of C. rigida (Epidendroideae) may share the same history with that of other species in the subfamily Orchidoideae. These clues are helpful in understanding the evolutionary implications of the bird-pollination mechanism to the plant breeding system.

C. rigida provides a uniquely interesting example in which the self-pollination mode complements the cross-pollination mode but is rendered infertile by a self-incompatible genetic mechanism. Self-pollination by insects is aided by the floral lip, whereas cross-pollination by birds is facilitated by the sheathed perch. The species likely reproduce through insect-mediated self-pollination, but the resultant inbreeding depression facilitated the evolution of self-incompatibility (to avoid inbreeding). Self-incompatibility necessitates and favors the development of an outbreeding mechanism, an example of which is the perch-facilitated cross-pollination by birds, to ensure reproductive success while reducing the gamete discounting (mating cost or waste) of self-pollination. The results and analysis suggest that in C. rigida, the mechanism of self-incompatibility may have evolved from that of self-compatibility, its outcrossing may have originated from selfing, and its bird-pollination mechanism may have evolved more recently. This finding would be consistent with the hypothesis that selfing is part of a larger process that promotes outcrossing , or, at least, that the two pollination modes can develop into each other.

Recently, conflicting selection of floral traits by different pollinators has been thought to be important in the evolution of specialized species –. In C. rigida, bi-modal pollination systems coexist, wherein two types of visitors (birds and insects) can serve as pollinators, with birds strongly promoting cross-pollination and insects promoting geitonogamy. The selection forces acting on floral and inflorescence traits by pollinators must be closely related to the variation of the traits selected and to the plant reproductive success rate. C. rigida would probably develop an efficient variation of floral traits to prevent self-pollination caused by insect visits because auto-pollination is useless for a self-incompatible plant. However, ensuring the successful reproduction of C. rigida is a “task” of top priority, which has been fulfilled by its sheathed perch and special pollinator, the sunbird. C. rigida may require more time to change its floral traits to respond to new conditions.

Through the addition of sheaths around the axis of inflorescence to make a specialized perch that attracts and positions foraging sunbirds for orderly cross-pollination, C. rigida gains mating and fertility advantages and genetic variability. More importantly, the structure ensures reproductive success. This situation provides a new and striking example of a structural (non-floral) adaptation that promotes cross-pollination in angiosperms. This structural adaptation may shed light on the evolution of multi-flowered inflorescences in a large number of plants. Furthermore, the adaptation of inflorescence structure for bird pollination may represent an evolutionary trend in Coelogyne. Similar sheaths occur in other Coelogyne species, especially in the sections Elatae and Proliferae. The sheaths are found in all members of these two sections at the base of a pendent inflorescence or the apex of an erect one, i.e., the potential bird perches are invariably situated above all flowers of the inflorescence probably to facilitate pollen dispersal by birds effectively, as in C. rigida. Thus, our findings on C. rigida as a model may have broad implications for the evolution of flowering plants, particularly those with multi-flowered inflorescences, and their mating systems and strategies.

In angiosperms, pollination is defined as the placement or transfer of pollen from the anther to the stigma of the same flower or another flower. In gymnosperms, pollination involves pollen transfer from the male cone to the female cone. Upon transfer, the pollen germinates to form the pollen tube and the sperm for fertilizing the egg. Pollination has been well studied since the time of Gregor Mendel. Mendel successfully carried out self- as well as cross-pollination in garden peas while studying how characteristics were passed on from one generation to the next. Today’s crops are a result of plant breeding, which employs artificial selection to produce the present-day cultivars. A case in point is today’s corn, which is a result of years of breeding that started with its ancestor, teosinte. The teosinte that the ancient Mayans originally began cultivating had tiny seeds—vastly different from today’s relatively giant ears of corn. Interestingly, though these two plants appear to be entirely different, the genetic difference between them is miniscule.

Pollination takes two forms: self-pollination and cross-pollination. Self-pollination occurs when the pollen from the anther is deposited on the stigma of the same flower, or another flower on the same plant. Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of the same species. Self-pollination occurs in flowers where the stamen and carpel mature at the same time, and are positioned so that the pollen can land on the flower’s stigma. This method of pollination does not require an investment from the plant to provide nectar and pollen as food for pollinators.

Explore this interactive website to review self-pollination and cross-pollination.

Living species are designed to ensure survival of their progeny; those that fail become extinct. Genetic diversity is therefore required so that in changing environmental or stress conditions, some of the progeny can survive. Self-pollination leads to the production of plants with less genetic diversity, since genetic material from the same plant is used to form gametes, and eventually, the zygote. In contrast, cross-pollination—or out-crossing—leads to greater genetic diversity because the microgametophyte and megagametophyte are derived from different plants.

Because cross-pollination allows for more genetic diversity, plants have developed many ways to avoid self-pollination. In some species, the pollen and the ovary mature at different times. These flowers make self-pollination nearly impossible. By the time pollen matures and has been shed, the stigma of this flower is mature and can only be pollinated by pollen from another flower. Some flowers have developed physical features that prevent self-pollination. The primrose is one such flower. Primroses have evolved two flower types with differences in anther and stigma length: the pin-eyed flower has anthers positioned at the pollen tube’s halfway point, and the thrum-eyed flower’s stigma is likewise located at the halfway point. Insects easily cross-pollinate while seeking the nectar at the bottom of the pollen tube. This phenomenon is also known as heterostyly. Many plants, such as cucumber, have male and female flowers located on different parts of the plant, thus making self-pollination difficult. In yet other species, the male and female flowers are borne on different plants (dioecious). All of these are barriers to self-pollination; therefore, the plants depend on pollinators to transfer pollen. The majority of pollinators are biotic agents such as insects (like bees, flies, and butterflies), bats, birds, and other animals. Other plant species are pollinated by abiotic agents, such as wind and water.

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