A Few Fun Facts About Pollen

Sexual reproduction in vascular plants requires producing and transporting pollen grains – the male gametophytes or sperm cells of a plant. These reproductive cells must make their way to the egg cells in or order to form seeds – plants in embryo. The movement of pollen is something we can all observe. It’s happening all around us on a regular basis. Any time a seed-bearing plant (also known as a spermatophyte) develops mature cones or flowers, pollen is on the move. Pollen is a ubiquitous and enduring substance and a fascinating subject of study. In case you don’t believe me, here are a few fun facts.

Bee covered in pollen – photo credit: wikimedia commons

Pollen is as diverse as the species that produce it. Pollen grains are measured in micrometers and are so tiny that the only reason we can see them with the naked eye is because they are often found en masse. Yet they are incredibly diverse in size, shape, and texture, and each plant species produces its own unique looking pollen. With the help of a good microscope, plants can even be identified simply by looking at their pollen. See images of the pollen grains of dozens of plant species here and here.

Pollen helps us answer questions about the past. Because pollen grains are so characteristic and because their outer coating (known as exine) is so durable and long-lasting, studying pollen found in sediments and sedimentary rocks helps us discover all sorts of things about deep time. The study of pollen and other particulates is called palynology. Numerous disciplines look to palynology to help them answer questions and solve mysteries. Its even used in forensics to help solve crimes. Criminals should be aware that brushing up against a plant in bloom may provide damning evidence.

Pollen oddities. While all pollen is different, some plants produce particularly unique pollen. The pollen grains of plants in the orchid and milkweed families, for example, are formed into united masses called pollinia. Each pollinium is picked up by pollinators and transferred to the stigmas of flowers as a single unit. A number of other species produce other types of compound pollen grains. The pollen grains of pines and other conifers are winged, and the pollen grains of seagrass species, like Zostera spp., are filamentous and said to hold the record for longest pollen grains.

The pollinia of milkweed (Asclepias spp.) look like the helicopter-esque fruits of maple trees. photo credit: wikimedia commons

Pollen tube oddities. In flowering plants, when pollen grains reach the stigma of a compatible flower, a vegetative cell within the grain forms a tube in order to transport the regenerative cells into the ovule. This tube varies in length depending on the length of the flower’s style. Because corn flowers produce such long styles (also know as corn silk), corn pollen grains hold the record for longest pollen tube, which can measure 12 inches or more. Species found in the mallow, gourd, and bellflower families produce multiple pollen tubes per pollen grain. Hence, their pollen is said to be polysiphonous.

Pollen is transported in myriad ways. Plants have diverse ways of getting their pollen grains where they need to be. Anemophilous plants rely on wind and gravity. They produce large quantities of light-weight pollen grains that are easily dislodged. Most of this pollen won’t make it, but enough of it will to make this strategy worth it. Hydrophilous plants use water and, like wind pollinated plants, may produce lots of pollen due to the unpredictably of this method. Some hydrophilous plants transport their pollen on the surface of the water, while others are completely submerged during pollination.

Employing animals to move pollen is a familiar strategy. Entomophily (insect pollination) is the most common, but there is also ornithophily (bird pollination) and chiropterophily (bat pollination), among others. Plants that rely on animals for pollination generally produce pollen grains that are sticky and nutritious. They attract animals using showy flowers, fragrance, and nectar. The bodies of pollinating insects have modifications that allow them to collect and transport pollen. Certain bees, like honey bees and bumblebees, have pollen baskets on their hind legs, while other bees have modified hairs called scopae on certain parts of their bodies.

Pollen is edible. Some animals – both pollinating and non-pollinating – use pollen as a food source. Animals that eat pollen are palynivores. Bees, of course, eat pollen, but lots of other insects do, too. Even some spiders, which are generally thought of as carnivores, have been observed eating pollen that gets trapped in their webs.

Pollen is thought to be highly nutritious for humans as well, and so, along with being taken as a supplement, it is used in all sorts of food products. To collect pollen, beekeepers install pollen traps on their beehives that strip incoming worker bees of their booty. Pollen from various wind pollinated plants, like cattails and pine trees, are also collected for human consumption. For example, a Korean dessert called dasik is made using pine pollen.

pine pollen – photo credit: wikimedia commons

Pollen makes many people sick. Hay fever is a pretty common condition and is caused by an allergy to wind-borne pollen. This condition is also known as pollinosis or allergic rhinitis. Not all flowering plants are to blame though, so here is a list of some of the main culprits. Because so many people suffer from hay fever, pollen counts are often included in weather reports. Learn more about what those counts mean here.

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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

Poisonous Plants: Castor Bean

A series of posts about poisonous plants should not get too far along without discussing what may be the most poisonous plant in the world – one involved in high and low profile murders and attempted murders, used in suicides and attempted suicides, a cause of numerous accidental deaths and near deaths, developed for use in biological warfare by a number of countries (including the United States), and used in bioterrorism attacks (both historically and presently). Certainly, a plant with a reputation like that is under tight control, right? Not so. Rather, it is widely cultivated and distributed far beyond its native range – grown intentionally and used in the production of a plethora of products. In fact, products derived from this plant may be sitting on a shelf in your house right now.

Ricinus communis, known commonly as castor bean or castor oil plant, is a perennial shrub or small tree in the spurge family (Euphorbiaceae) and the only species in its genus. It is native to eastern Africa and parts of western Asia but has since been spread throughout the world. It has naturalized in tropical and subtropical areas such as Hawaii, southern California, Texas, Florida, and the Atlantic Coast. It is not cold hardy, but is commonly grown as an ornamental annual in cold climates. It is also grown agriculturally in many countries, with India, China, and Mozambique among the top producers.

Silver maple leaf nestled in the center of a castor bean leaf.

Silver maple leaf nestled in the center of a castor bean leaf.

Castor bean has large palmately lobed leaves with margins that are sharply toothed. Leaves are deep green (sometimes tinged with reds or purples) with a red or purple petiole and can reach up to 80 centimeters (more than 30 inches) across. Castor bean can reach a height of 4 meters (more than 12 feet) in a year; in areas where it is a perennial, it can get much taller. Flowers appear in clusters on a large, terminal spike, with male flowers at the bottom and female flowers at the top. All flowers are without petals. Male flowers are yellow-green with cream-colored or yellow stamens. Female flowers have dark red styles and stigmas. The flowers are primarily wind pollinated and occasionally insect pollinated. The fruits are round, spiky capsules that start out green often with a red-purple tinge and mature to a brown color, at which point they dehisce and eject three seeds each. The seeds are large, glossy, bean-like, and black, brown, white, or often a mottled mixture. They have the appearance of an engorged tick. There is a small bump called a caruncle at one end of the seed that attracts ants, recruiting them to aid in seed dispersal.

Female flowers and fruits forming on castor bean.

Female flowers and fruits forming on castor bean.

All parts of the plant are toxic, but the highest concentration of toxic compounds is found in the seeds. The main toxin is ricin, a carbohydrate-binding protein that inhibits protein synthesis. The seeds need to be chewed or crushed in order to release the toxin, so swallowing a seed whole is not likely to result in poisoning. However, if seeds are chewed and consumed, 1-3 of them can kill a child and 2-6 of them can kill an adult. It takes several hours (perhaps several days) before symptoms begin to occur. Symptoms include nausea, vomiting, severe stomach pain, diarrhea, headaches, dizziness, thirst, impaired vision, lethargy, and convulsions, among other things. Symptoms can go on for several days, with death due to kidney failure (or multisystem organ failure) occurring as few as 3 and as many as 12 days later. Death isn’t imminent though, and many people recover after a few days. Taking activated charcoal can help if the ingestion is recent. In any case, consult a doctor or the Poison Control Center for information about treatments.

The seeds of castor bean are occasionally used to make jewelry. This is not recommended. In The North American Guide to Common Poisonous Plants and Mushrooms, the authors warn that “drilling holes in the seeds makes them much more deadly because it exposes the toxin.” Wearing such jewelry can result in skin irritation and worse. The authors go on to say that “more than one parent has allowed their baby to suck on a necklace of castor beans.” I doubt such parents were pleased with the outcome.

castor bean seeds

Castor beans are grown agriculturally for the oil that can be extracted from their seeds. Due to the way its processed, castor oil does not contain ricin. The leftover meal can be fed to animals after it has been detoxified. Castor oil has been used for thousands of years, dating as far back as 5000 BC when Egyptians were using it as a fuel for lamps and a body ointment, among other things. Over the centuries it has had many uses – medicinal, industrial, and otherwise. It makes an excellent lubricant, is used in cosmetics and in the production of biofuel, and has even been used to make ink for typewriters. One of its more popular and conventional uses is as a laxative, and in her book, Wicked Plants, Amy Stewart describes how this trait has been used as a form of torture: “In the 1920’s, Mussolini’s thugs used to round up dissidents and pour castor oil down their throats, inflicting a nasty case of diarrhea on them.”

A couple of years ago, I grew a small stand of castor beans outside my front door. I was impressed by their rapid growth and gigantic leaves. I also enjoyed watching the fruits form. By the end of the summer, they were easily taller than me (> 6 feet). I collected all of the seeds and still have them today. I knew they were poisonous at the time, but after doing the research for this post, I’m a little wary. With a great collection of castor bean seeds comes great responsibility.

The castor beans that once grew outside my front door.

The castor beans that once grew outside my front door.

There is quite a bit of information out there about castor beans and ricin. If you are interested in exploring this topic further, I recommend this free PubMed article, this Wikipedia page about incidents involving ricin, this article in Nature, and this entry in the Global Invasive Species Database. Also check out Chapter 11 (“Death by Umbrella”) in Thor Hanson’s book, The Triumph of Seeds.

Ethnobotany: White Man’s Foot, part one

“Plantains – Plantago major – seem to have arrived with the very first white settlers and were such a reliable sign of their presence that the Native Americans referred to them as ‘white men’s footsteps.'” – Elizabeth Kolbert (The Sixth Extinction)

“Our people have a name for this round-leafed plant: White Man’s Footstep. Just a low circle of leaves, pressed close to the ground with no stem to speak of, it arrived with the first settlers and followed them everywhere they went. It trotted along paths through the woods, along wagon roads and railroads, like a faithful dog so as to be near them.” – Robin Wall Kimmerer (Braiding Sweetgrass)

photo credits: wikimedia commons

photo credit: wikimedia commons

Plantago major is in the family Plantaginaceae – the plantain family – a family that consists of at least 90 genera, several of which include common species of ornamental plants such as Veronica (speedwells), Digitalis (foxgloves), and Antirrhinum (snapdragons). The genus Plantago consists of around 200 species commonly known as plantains. They are distributed throughout the world in diverse habitats. Most of them are herbaceous perennials with similar growth habits, and many have ethnobotanical uses comparable to P. major.

Originating in Eurasia, P. major now has a cosmopolitan distribution. It has joined humans as they have traveled and migrated from continent to continent and is now considered naturalized throughout most temperate and some tropical regions. In North America, P. major and P. lanceolata are the two most common introduced species in the Plantago genus. P. major has a plethora of common names – common plantain being the one that the USDA prefers. Other names include broadleaf plantain, greater plantain, thickleaf plantain, ribgrass, ribwort, ripplegrass, and waybread. Depending on the source, there are various versions of the name white man’s foot, and along the same line, a common name for P. major in South Africa is cart-track plant.

P. major is a perennial – albeit sometimes annual or biennial – herbaceous plant. Its leaves form a rosette that is usually oriented flat against the ground and reaches up to 30 cm wide. Each leaf is egg-shaped with parallel veins and leaf margins that are sometimes faintly toothed. The inflorescence is a leafless spike up to 20 cm tall (sometimes taller) with several tiny flowers that are a dull yellow-green-brown color. The flowers are wind pollinated, and the plants are highly prone to self-pollination. The fruits are capsules that can contain as many as 30 seeds – an entire plant can produce as many as 14,000 – 15,000 seeds at once. The seeds are small, brown, sticky, and easily transported by wind or by adhering to shoes, clothing, animals, and machinery. They require light to germinate and can remain viable for up to 60 years.

An illustration of three Plantago species found in Selected Weeds of the United States - Agriculture Handbook No. 366 circa 1970

An illustration of three Plantago species found in Selected Weeds of the United States – Agriculture Handbook No. 366 circa 1970

P. major prefers sunny sites but can also thrive in part shade. It adapts to a variety of soil types but performs best in moist, clay-loam soils. It is often found in compacted soils and is very tolerant of trampling. This trait, along with its low-growing leaves that easily evade mower blades, explains why it is so commonly seen in turf grass. It is highly adaptable to a variety of habitats and is particularly common on recently disturbed sites (natural or human caused) and is an abundant urban and agricultural weed.

Even though it is wind pollinated, its flowers are visited by syrphid flies and various bee species which feed on its pollen. Several other insects feed on its foliage, along with a number of mammalian herbivores. Cardinals and other bird species feed on its seeds.

Humans also eat plantain leaves, which contain vitamins A, C, and K. Young, tender leaves can be eaten raw, while older leaves need to be cooked as they become tough and stringy with age. The medicinal properties of  P. major have been known and appreciated at least as far back as the Anglo-Saxons, who likely used a poultice made from the leaves externally to treat wounds, burns, sores, bites, stings, and other irritations. Native Americans, after seeing the plant arrive with European settlers, quickly learned to use the plant as food and medicine. It could be used to stop cuts from bleeding and to treat rattlesnake bites. Apart from external uses, the plant was used internally as a pain killer and to treat ulcers, diarrhea, and other gastrointestinal issues.

P. major has been shown to have antibacterial, anti-inflammatory, antioxidant, and other biological properties; several chemical compounds have been isolated from the plant and deemed responsible for these properties. For this reason, P. major and other species of Plantago have been used to treat a number of ailments. The claims are so numerous and diverse that it is worth exploring if you are interested. You can start by visiting the following sites:

"White man's footstep, generous and healing, grows with its leaves so close to the ground that each step is a greeting to Mother Earth." - Robin Wall Kimmerer, Braiding Sweetgrass (photo credit: www.eol.org)

“White man’s footstep, generous and healing, grows with its leaves so close to the ground that each step is a greeting to Mother Earth.” – Robin Wall Kimmerer, Braiding Sweetgrass (photo credit: www.eol.org)

Other Ethnobotany Posts on Awkward Botany: