Year of Pollination: Botanical Terms for Pollination, part one

When I began this series of posts, I didn’t have a clear vision of what it would be. I had a budding interest in pollination biology and was anxious to learn all that I could. I figured that calling 2015 the “Year of Pollination” and writing a bunch of pollination-themed posts would help me do that. And it has. However, now that the year is coming to a close, I realize that I neglected to start at the beginning. Typical me.

What is pollination? Why does it matter? The answers to these questions seemed pretty obvious; so obvious, in fact, that I didn’t even think to ask them. That being said, for these last two “Year of Pollination” posts (and the final posts of the year), I am going back to the basics by defining pollination and exploring some of the terms associated with it. One thing is certain, there is still much to be discovered in the field of pollination biology. Making those discoveries starts with a solid understanding of the basics.

Pollination simply defined is the transfer of pollen from an anther to a stigma or – in gymnosperms – from a male cone to a female cone. Essentially, it is one aspect of plant sex, albeit a very important one. Sexual reproduction is one way that plants multiply. Many plants can also reproduce asexually. Asexual reproduction typically requires less energy and resources – no need for flowers, pollen, nectar, seeds, fruit, etc. – and can be accomplished by a single individual without any outside help; however, there is no gene mixing (asexually reproduced offspring are clones) and dispersal is limited (consider the “runners” on a strawberry plant producing plantlets adjacent to the mother plant).

To simplify things, we will consider only pollination that occurs among angiosperms (flowering plants); pollination/plant sex in gymnosperms will be discussed at another time. Despite angiosperms being the youngest group of plants evolutionarily speaking, it is the largest group and thus the type we encounter most.

A flower is a modified shoot and the reproductive structure of a flowering plant. Flowers are made up of a number of parts, the two most important being the reproductive organs. The androecium is a collective term for the stamens (what we consider the male sex organs). A stamen is composed of a filament (or stalk) topped with an anther – where pollen (plant sperm) is produced. The gynoecium is the collective term for the pistil (what we consider the female sex organ). This organ is also referred to as a carpel or carpels; this quick guide helps sort that out. A pistil consists of the ovary (which contains the ovules), and a style (or stalk) topped with a stigma – where pollen is deposited. In some cases, flowers have both male and female reproductive organs. In other cases, they have one or the other.

photo credit: wikimedia commons

photo credit: wikimedia commons

When pollen is moved from an anther of one plant to a stigma of another plant, cross-pollination has occurred. When pollen is moved from an anther of one plant to a stigma of the same plant, self-pollination has occurred. Cross-pollination allows for gene transfer, and thus novel genotypes. Self-pollination is akin to asexual production in that offspring are practically identical to the parent. However, where pollinators are limited or where plant populations are small and there is little chance for cross-pollination, self-pollination enables reproduction.

Many species of plants are unable to self-pollinate. In fact, plants have evolved strategies to ensure cross-pollination. In some cases, the stamens and pistils mature at different times so that when pollen is released the stigmas are not ready to receive it or, conversely, the stigmas are receptive before the pollen has been released. In other cases, stigmas are able to recognize their own pollen and will reject it or inhibit it from germinating. Other strategies include producing flowers with stamens and pistils that differ dramatically in size so as to discourage pollen transfer, producing separate male and female flowers on the same plant (monoecy), and producing separate male and female flowers on different plants (dioecy).

As stated earlier, the essence of pollination is getting the pollen from the anthers to the stigmas. Reproduction is an expensive process, so ensuring that this sex act takes place is vital. This is the reason why flowers are often showy, colorful, and fragrant. However, many plants rely on the wind to aid them in pollination (anemophily), and so their flowers are small, inconspicuous, and lack certain parts. They produce massive amounts of tiny, light-weight pollen grains, many of which never reach their intended destination. Grasses, rushes, sedges, and reeds are pollinated this way, as well as many trees (elms, oaks, birches, etc.) Some aquatic plants transport their pollen from anther to stigma via water (hydrophily), and their flowers are also simple, diminutive, and produce loads of pollen.

Inforescence of big bluestem (Andropogon gerardii), a wind pollinated plant - pohto credit: wikimedia commons

Inflorescence of big bluestem (Andropogon gerardii), a wind pollinated plant – photo credit: wikimedia commons

Plants that employ animals as pollinators tend to have flowers that we find the most attractive and interesting. They come in all shapes, sizes, and colors and are anywhere from odorless to highly fragrant. Odors vary from sweet to bitter to foul. Many flowers offer nectar as a reward for a pollinator’s service. The nectar is produced in special glands called nectaries deep within the flowers, inviting pollinators to enter the flower where they can be dusted with pollen. The reward is often advertised using nectar guides – patterns of darker colors inside the corolla that direct pollinators towards the nectar. Some of these nectar guides are composed of pigments that reflect the sun’s ultraviolet light – they are invisible to humans but are a sight to behold for many insects.

In part two, we will learn what happens once the pollen has reached the stigma – post-pollination, in other words. But first, a little more about pollen. The term pollen actually refers to a collection of pollen grains. Here is how Michael Allaby defines “pollen grain” in his book The Dictionary of Science for Gardeners: “In seed plants, a structure produced in a microsporangium that contains one tube nucleus and two sperm nuclei, all of them haploid, enclosed by an inner wall rich in cellulose and a very tough outer wall made mainly from sporopollenin. A pollen grain is a gametophyte.”

A pollen grain’s tough outer wall is called exine, and this is what Allaby has to say about that: “It resists decay, and the overall shape of the grain and its surface markings are characteristic for a plant family, sometimes for a genus or even a species. Study of pollen grains preserved in sedimentary deposits, called palynology or pollen analysis, makes it possible to reconstruct past plant communities and, therefore, environments.”

Scanning electron microscope image of pollen grains from narrowleaf evening primrose (Oenothera fruticosa) - photo credit: wikimedia commons

Scanning electron microscope image of pollen grains from narrowleaf evening primrose (Oenothera fruticosa) – photo credit: wikimedia commons

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Year of Pollination: Mosquitoes as Pollinators

It is difficult to have positive feelings about mosquitoes, especially during summer months when they are out in droves and our exposed skin – soft, supple, and largely hair-free – is irresistible to them. We are viewed as walking blood meals by female mosquitoes who are simply trying to produce young – to perpetuate their species just like any other species endeavors to do. Unfortunately, we are left with small, annoying bumps in our skin – red, itchy, and painful – risking the possibility that the mosquitoes that just drew our blood may have passed along any number of mosquito-borne diseases, some (such as malaria) that potentially kill millions of people every year. For this, it is okay to hate mosquitoes and to long for the day of their complete eradication from the planet. However, their ecological roles (and yes, they do have some) are also worth considering.

There are more than 3,500 species of mosquito. Luckily, only 200 or so consume human blood. Mosquitoes go back at least 100 million years and have co-evolved with species of plants and animals found in diverse habitats around the world. Adult mosquitoes and their larvae (which live in standing water) provide food for a wide variety of creatures including birds, bats, insects, spiders, fish, frogs, lizards, and salamanders. Mosquito larvae also help break down organic matter in the bodies of water they inhabit. They even play an important role in the food webs found inside the pitchers of northern pitcher plants (Sarracenia spp.). Interestingly enough, Arctic mosquitoes influence the migration patterns of caribou. They emerge in swarms so big and so voracious that they have been said to kill caribou through either blood loss or asphyxiation.

However, blood is not the main food source of mosquitoes; flower nectar is. Males don’t consume blood at all, and females only consume it when they are producing eggs. Any insect that visits flowers for nectar has the potential to unwittingly collect pollen and transfer it to a nearby flower, thereby aiding in pollination. Mosquitoes are no exception. They have been observed acting as pollinators for a handful of species, and could be acting as pollinators for many more.

Bluntleaved orchid (Platanthera obtusata) is pollinated by mosquitoes. phot credit: wikimedia commons

Bluntleaved orchid (Platanthera obtusata) is pollinated by mosquitoes. photo credit: wikimedia commons

The scientific literature describes the pollination by mosquitoes of at least two plant species: Platanthera obtusata (syn. Habenaria obtusata) and Silene otites. P. obtusata – bluntleaved orchid – is found in cold, wet regions in North America and northern Eurasia. It is pollinated by mosquitoes from multiple genera including several species in the genus Aedes. Mosquitoes visit the flowers to feed on the nectar and, subsequently, pollinia (clusters of pollen) become attached to their eyes and are moved from flower to flower. This scenario likely plays out in other species of Arctic orchids as well*.

S. otites – Spanish catchfly – is a European species that is pollinated by mosquitoes and moths. Researches have been studying the floral odors of S. otites that attract mosquitoes, suggesting that determining the compounds involved in these odors “might lead to the development of new means of pest control and mosquito attractants and repellents.”

Northern House Mosquito (Culex pipiens) - one of the species of mosquitoes that has been observed pollinating Silene otitis. photo credit: www.eol.org

Northern House Mosquito (Culex pipiens) – one of the species of mosquitoes that has been observed pollinating Silene otites. photo credit: www.eol.org

Despite the list of functions that mosquitoes serve in their varied habitats, an article that appeared in Nature back in 2010 argues for wiping mosquitoes off the Earth, stating that “the ecological scar left by a missing mosquito would heal quickly as the niche was filled by other organisms.” And even though “thousands of plant species would lose a group of pollinators,” mosquitoes are not important pollinators of the “crops on which humans depend,” nor do they appear to be the sole pollinator of any single plant species [the species mentioned above are pollinated by other insects as well]. Eliminating mosquitoes, however, is more of a pipe dream than a realistic possibility as our “best efforts can’t seriously threaten an insect with few redeeming features.”

*Speaking of orchids and pollination, endless posts could be written about this incredibly fascinating and diverse group of plants and their equally fascinating and complex mechanisms surrounding pollination. There will be more to come on such topics. Meanwhile, it should be noted that orchids are also a notoriously threatened group of plants. To learn more about orchids and orchid conservation in North America, visit North American Orchid Conservation Center.

Read more about mosquito pollination here.

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