Seed Oddities: Apomixis and Polyembryony

Plants have uncanny ways of reproducing themselves that are unparalleled by most other living things. Offshoots of themselves can be made by sending out modified stems above or beneath the ground which develop roots and shoots (new plants) at various points along the way. Various other underground stem and root structures can also give rise to new plants. Small sections of root, stem, or leaf can, under the right conditions, push out new plantlets in a fashion that seems otherworldly. (Picture chopping off a bit of your finger and growing a whole new you from it.)

These are some of the ways in which plants reproduce asexually, and it’s kind of freaky if you think about it. Plants can clone themselves. But one major disadvantage of reproducing this way is that clonal offspring are genetically identical to the parent plant, which truncates any advantage that might be gained by genetic mixing between two separate plants. For one, it means that a plant population composed of all clones is at risk of being wiped out if something in the environment comes along (such as a disease or change in climate) and none of the plants in the population have adapted any sort of resistance to it.

New plants forming along the lateral stems of Ranunculus flammula – via wikimedia commons

That’s where seeds come in. Seeds are produced sexually, when the gametes of one plant fuse with the gametes of another. Genetic recombination occurs, and a genetically unique individual is born, housed within a seed. Unless, of course, that seed is produced asexually. Then the seed is a clone, and we’re back to where we started.

Apomixis is the process by which seeds are produced asexually. In flowering plants, this means that viable seeds are formed even when flowers haven’t been pollinated. In some cases, pollination stimulates apomixis or is required to produce endosperm; but either way, the result is the same: an embryo containing an exact copy of the genes of its single parent plant.

To understand this process, it’s important to familiarize yourself with the basic anatomy of an ovule, the part of a plant where embryos are formed and which ultimately becomes a seed. In gymnosperms, ovules sit inside cones; in angiosperms, they are surrounded by an ovary. The wall of the ovule is called an integument. A small opening at the top of the ovule, known as a micropyle, is where the pollen tube enters. Diploid cells of the nucellus compose the interior of the ovule, and within the nucellus resides the megasporocyte, which is where meiosis occurs and egg cells are produced. In sexual reproduction, a germ cell introduced through the pollen tube fuses with the egg cell to form a zygote and eventually an embryo. In the case of apomixis, the fusion of germ cells isn’t necessary for an embryo to form.

ovule anatomy via wikimedia commons

There are three main types of apomixis: diplospory, apospory, and adventitious embryony. In diplospory, the megasporocyte skips meiosis and produces diploid cells instead of haploid cells (germ cells). These unreduced cells go on to form an embryo inside of the embryo sac, just like an egg cell would if it had been fertilized with a pollen cell. Additional unreduced cells go on to form endosperm, and the ovule then matures into a seed. This type of apomixis is common in dandelions (Taraxacum officinale). As much as bees love visiting dandelion flowers, their pollination services are not required for the production of viable seeds. Yet another reason you are stuck with dandelions in your yard whether you like it or not.

In apospory, an embryo develops inside of an embryo sac that has been formed from cells in the nucellus. Embryo development within the megasporocyte is bypassed; however, pollination is usually necessary for endosperm to form. Plant species in the grass family commonly produce seeds using this type of apomixis.

Adventitous embryony is also known as sporophytic apomixis because an embryo is formed outside of an embryo sac. Cells from either the integument or the nucellus produce an embryo vegetatively. In this case, a sexually produced embryo can form along with several vegetatively produced embryos. Extra embryos die off and a single, surviving embryo is left inside the mature seed. But not always. Two or more embryos occasionally survive, including the sexually produced one. The mature seed then consists of multiple embryos. This phenomenon is called polyembryony and is common in citrus and mangoes. When it comes to plant breeding, polyembryony is incredibly useful because the asexually derived seedlings are exact copies of their parent, which means the desirable traits of a specific cultivar are retained.

Depiction of seed with three viable embryos after germination.

Polyembryony can occur in a number of ways, and not always as a result of apomixis. In some species, additional embryos “bud off” from the sexually produced embryo. This is called cleavage polyembryony and is known to happen frequently in the pine family (Pinaceae), as well as other plant families. Another common form of polyembryony in gymnosperms is simple polyembryony, in which several egg cells in a single ovule are fertilized resulting in the development of multiple embryos. This doesn’t always mean there will be multiple seedlings sprouting from a single seed. Most embryos usually fail to mature, and only one prevails. However, sometimes more than one survives, and if you’re lucky, you’ll find a seed with multiple plant babies pushing out from the seed coat.

Up Next: Vivipary!

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Inside of a Seed: Gymnosperms

“Every tree has to stay where it put down roots as a seedling. However, it can reproduce, and in that brief moment when tree embryos are still packed into seeds, they are free. The moment they fall from the tree, the journey can begin.” — The Hidden Life of Trees by Peter Wohlleben

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Seed plants – also known as spermatophytes – make up the largest group of plants on earth. Seed plants consist of five divisions, and among them the angiosperm division (a.k.a. flowering plants) dominates in its number of species. The four remaining divisions are referred to collectively as gymnosperms. This incudes the cycads (Cycadophyta), Ginkgo biloba (the only living species in the division Ginkgophyta), gnetophytes (Gnetophyta), and the conifers (Coniferophyta). Conifers are by far the largest and most widespread gymnosperm division.

Angiosperms and gymnosperms have different evolutionary histories, resulting in their distinct genetic and morphological differences. That being said, an overly-simplistic way of differentiating the two groups is to say that, while both groups produce seeds, angiosperms produce flowers and fruits while gymnosperms produce pollen cones and seed cones. There are always exceptions (Ginkgo biloba, for example, doesn’t produce cones), but for the most part, this is the case.

Pollen cones (top) and seed cones (bottom) of mugo pine (Pinus mugo) via wikimedia commons

Sexual reproduction in gymnosperms follows a familiar pattern. Pollen, which contains the male sex cells, is produced in pollen cones, which are essentially miniature branches with modified leaves called scales that house the male reproductive organs. Mature pollen is shed and carried away by the wind. Lucky pollen grains make their way to the female cones, which are also modified branchlets, but are a bit more complex. Scales sit atop bracts, and on top of the scales are ovules – the female reproductive structures. During fertilization, the bracts open to collect pollen and then close as the seed develops.

When pollen lands on an ovule it forms pollen tubes that help direct the male sex cells to the egg cells inside. The process is similar to pollen tubes extending down the style of a flower. In flowering plants, additional pollen cells combine with cells in the ovule to produce endosperm, a storage tissue that feeds the growing embryo. This doesn’t happen in gymnosperms. Instead, haploid cells within the ovule develop into storage tissue and go on to serve the same role.

The ovule eventually matures into a seed, and the cone opens to release it. The seed sits atop the scale rather than enclosed within a fruit, as it would be in an angiosperm. For this reason gymnosperms are said to have naked seeds. The development of seeds can also be much slower in gymnosperms compared to angiosperms. In some species, seeds don’t reach maturity for as long as two years.

Seed cones and winged seeds of mugo pine (Pinus mugo) via wikimedia commons

Seeds in the genus Pinus are excellent representations of typical gymnosperm seeds. Their basic components are essentially identical to the seeds of angiosperms. The seed coat is also referred to as an integument. It was once the outer covering of the ovule and has developed into the seed covering. A micropyle is sometimes visible on the seed and is the location where the pollen cells entered the ovule. The storage tissue, as mentioned above, is composed of female haploid cells that matured into storage tissue in the ovule. Like angiosperms, the embryo is composed of the radicle (embryonic root), the hypocotyl (embryonic shoot), and cotyledons (embryonic leaves).

Angiosperms can be divided into monocotyledons and dicotyledons according to the number of cotyledons their embryos have (monocots have one, dicots have two). Gymnosperms are considered multi-cotyledonous because, depending on the species, they can have a few to many cotyledons.

Seedling of Swiss pine (Pinus cembra) showing multiple cotyledons via wikimedia commons

For the sake of this introduction to gymnosperm seeds, I have offered a simple overview of the production of seeds in the conifer division. Sexual reproduction and seed formation in the other three gymnosperm divisions is a similar story but varies according to species. Even within the conifers there are differences. For example, the “seed cones” of several gymnosperm species can actually be quite fruit-like, which serves to attract animals to aid in seed dispersal. Also, the pollen of gymnosperms is often thought of as being wind dispersed (and occasionally water dispersed in the case of Ginkgo biloba and some cycads); however, researchers are continuing to discover the pivotal role that insects play in the transfer of pollen for many cycad species, just as they do for so many species of angiosperms.

All of this to say that Botany 101 is simply a window into what is undoubtedly an incredibly diverse and endlessly fascinating group of organisms, and that, as with all branches of science, there is still so much to discover.

The Dragon of Yankee Fork: Spalding Viaduct

This is a guest post by Martha Dalke Hindman. It is an excerpt from her upcoming book, The Dragon of Yankee Fork. This is the final of three posts. See also: Devil’s Washbasins and Grave Markers.

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Spalding Viaduct, from the Old Mission site over the Railroad tracks
Built in 1924, steel rods, concrete construction, supporting arches and pillars.
570 feet long, 20 feet wide, a Link in U.S. Highway 95 connecting Idaho, North to South.
Today, a Chain Link Fence and Steel Gates surround the structure.

Black Locust trees, cascading white or lavender flowers
Compound leaves, dark brown seed pods.
Shade and shelter for family picnics and softball games
Beside the Spalding Viaduct, the Nez Perce Tribal Cemetery

black locust (Robinia pseudoacacia)

Travel with my Dad was always an adventure. However, the Spalding Viaduct on U.S. Highway 95, was the scariest stretch of concrete highway, we ever encountered.

Dad slowed to a stop as we approached the Spalding Viaduct. A semi tractor-trailer rig was traveling north and in the center of the viaduct, leaving no room for any other vehicles. I watched as the driver carefully drove his rig down the narrow span. The driver slowed, saluted his “thanks” to us, and continued his journey. Normal traffic, backed up at either end of the viaduct, continued to their destinations. Dad and I were on our way to Boise – Idaho’s Capitol City.

Dad continued the story about how the Spalding Viaduct on U.S. Highway 95 connects the State of Idaho, North to South and South to North.

“The Spalding Viaduct is the vital link for travelers to stay within the geographical boundaries of the State of Idaho. Otherwise, to reach Boise from Lewiston, travel west and south on U. S. Highway 12 to Walla Walla, Washington, cross the Columbia River south to Umatilla, Oregon Highway 82. Outside Umatilla, connect with U.S. Highway 84 east through Southeast Oregon, across the Snake River at Ontario, Oregon and into Boise. The only other way to reach Boise from our home in Moscow, is to travel south on U.S. Highway 95 to Lewiston, turn east on U.S. Highway 12 into Hamilton, Montana. From Hamilton, south on U.S. Highway 93 through the magnificent Bitterroot Valley, to the Montana, Idaho border. U.S. Highway 15 takes you to Idaho Falls and U.S. Highway 86 into Boise. Coming from our home in Moscow, travel time to Boise would be two days. That is why the Spalding Viaduct is so vital to North-South and South-North traffic.”

“Dad, do we change time zones before we reach Boise??”

“Yes, Martha Lee. Time zones change from Pacific Standard Time, to Mountain Standard Time when we cross the Salmon River Bridge at Riggins. Because we are traveling south, we lose one hour. Watch for a roadside park with a picnic table. Mother packed a lunch full of sandwiches, goodies and a thermos full of ice cold water. We should arrive in Boise about 7 pm. Mountain Standard Time. In the meantime, enjoy the scenery.”

“Dad, there is JUST the place for our picnic, under the shade of the black locust trees.”

We parked our car at the picnic area next to the Spalding Viaduct. The black locust trees in bloom, a gentle breeze brought “fishy” smells from the Clearwater River, the water still high and rapid from the spring rains. The log drive was finished, only a few pieces of debris floated by.

Several families were finishing their noon meal, as we sat down at the long wooden table and unpacked our lunch. Mother had prepared peanut butter and honey sandwiches, potato salad, liverwurst slices, cheddar cheese, carrots and strawberries from our garden, chocolate chip cookies, and a large Coleman thermos of cold water. What a feast!

The afternoon sun was warm, the breeze calm, just the recipe for a game of softball. Dad batted first. A gentle swing to left field. Home Run!!

My turn to bat! I swung, missing the first ball. The second pitch, I hit, but it flew straight into the Clearwater River. The strong current carried my softball downstream. I could not catch it. End of Game!

I cried. Dad put his strong arms around my sagging shoulders. “It’s OK, Martha Lee. I’ll see to it that you have another softball. We will never know how far your old ball will travel. It may get stuck in debris along the river banks, or it may end up in the Pacific Ocean, riding the ocean currents to the shores of the Hawaiian Islands, or maybe even into Shanghai Harbor, China!”

Several days later, Dad came home with a package under his arm. With a twinkle in his eye, Dad gave me the box. I opened the shiny, square box. Inside was a new softball ready to be played with and loved just as much as the old one.

Thanks, Dad.

Additional Information:

The Spalding Viaduct is falling apart, literally. Large pieces of the concrete columns become loose and fall to the ground. This once vital link on U.S. Highway 95 is closed with steel gates and a chain link fence. Moss grows where semi trucks carrying goods and produce South to North and North to South, passenger cars and buses, traveling South to Boise and North to Canada, watched out for each other on the narrow roadway.

Modern day travelers and truck traffic continues to travel North-South and South-North on U.S. Highway 95, over a new bridge (built in 1962) across the Clearwater River to the East of the Spalding Viaduct. A new stretch of highway was constructed from the river bridge, over the railroad tracks, up the hill, and onto the Camas Prairie. U.S. Highway 95 continues south to Grangeville, Riggins, and ends at Middleton, Idaho. U.S. Highway 44 connects with U.S. Highway 84 into Boise.

Updates were made to the current bridge in 2014. Click here for more information.

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Poetry, personal stories, images, journal entries, recipes for Springerle, Cinnamon Rolls, Fried Cakes, “a little bit of science thrown in for good measure,” print and online resources, all define “The Dragon of Yankee Fork,” an Idaho Alphabet from A to Z. It all began on a long piece of cream colored shelf paper! (Visit the Go Fund Me page to learn more about the project and contribute to its creation.)

Martha Dalke Hindman’s outdoor classroom was the travel adventures she shared with her father around the State of Idaho. From dusty roads, fishing expeditions, and a keen sense of observation, learning about Idaho’s heritage gave Ms. Hindman her voice in poetry and personal short stories. She may be reached at martha20022 [at] gmail [dot] com.

Introducing Herbology Hunt

This is a guest post by Jane Wilson.

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Many people are “plant blind”. They walk through areas of fantastic wildlife or just down their street without noticing what grows there. Even plants growing in the gutter have an interesting backstory.

The term “Plant Blindness” was first put forth by Wandersee and Schlusser in 1998. Without an appreciation of plants in the ecosystem, people will be less likely to support plant research and conservation.

Herbology Hunt was born out of a love of plants and wild places and a determination to get kids outdoors and really looking at their environment. One of the founders started Wildflower Hour on Twitter – a place for people to share photos of wildflowers found in Britain and Ireland – and from this was stemmed a children’s version, which became Herbology Hunt. The Herbology Hunt team put together spotter sheets for each month of the year. Each sheet includes five plants that can be found throughout the month. They were made available as a free download, so schools and individuals can print them for use on a plant hunt.

By the end of 2018, we had created a year’s worth of spotter sheets. We are now looking to promote their use throughout Great Britain. Eventually we want to reward children who find 50 of the plants with a free T-shirt, and we will be looking for sponsors to support this. We have been supported by the Botanical Society of Britain and Ireland and the Wild Flower Society who have made the monthly spotter sheets available. They can be downloaded here or here.

Herbology Hunt Spotter Sheet for January

The Wild Flower Society has a great offer for Juniors interested in plants – it costs £3 to join and you get a diary to record your finds.

Going outdoors and noticing wildlife has been shown in some scientific studies to improve cardio-vascular health and mental health. A herbology hunt must surely be a good thing to do with children to help them get into a better lifestyle that will benefit their future health. We hope that many families and schools will use our spotter sheets as a way to help children become more passionate about the environment and enjoy the benefits of being outdoors.

Check out the Wildflower Hour website for more information about Herbology Hunt, along with instructions on how to get involved in #wildflowerhour, plus links to social media accounts and the Wild Flower (Half) Hour podcast.

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Also: Check out Jane Wilson’s website – Practical Science Teaching – for more botany-themed educational activities.

Inside of a Seed: Two Monocots

“Seeds are travelers in space and time – small packages of DNA, protein, and starch that can move over long distances and remain viable for hundreds of years. These packages have everything they need not only to survive, but also to grow into a plant when they encounter the right conditions.”      The Book of Seeds by Paul Smith

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As illustrated in last week’s post, the mature seeds of dicots – depending on the species – can be either with or without endosperm (a starchy food packet that feeds a growing seedling upon germination). Seeds without endosperm store these essential sugars in their cotyledons. Monocotyledons (or monocots, for short) are a group of flowering plants (i.e. angiosperms) whose seedlings are composed of a single cotyledon. With the exception of orchids, the seeds of monocots always contain endosperm.

The first of two examples of monocot seeds is the common onion (Allium cepa). The embryo in this seed sits curled up, surrounded by endosperm inside of a durable seed coat.

If you have ever sown onion seeds, you have watched as the single, grass-like cotyledon emerges from the soil. The seed coat often remains attached to the tip of the cotyledon like a little helmet as it stretches out towards the sky. Soon the first true leaf appears, pushing out from the base of the cotyledon. The source of this first leaf is the plumule hidden within the cotyledon.

The fruit of plants in the grass family – including cereal grains like wheat, oats, barley, rice, and corn – is called a caryopsis. In this type of fruit, the fruit wall (or pericarp) is fused to the seed coat, making the fruit indistinguishable from the seed. The embryos in these seeds are highly developed, with a few more discernible parts. A simplified diagram of a corn seed (Zea mays) is shown below. Each kernel of corn on a cob is a caryopsis. These relatively large seeds are great for demonstrating the anatomy of seeds in the grass family.

In these seeds there is an additional layer of endosperm called aleurone, which is rich in protein and composed of living cells. The cells of the adjacent endosperm are not alive and are composed of starch. The embryo consists of several parts, including the cotyledon (which, in the grass family, is also called a scuttelum), coleoptile, plumule, radicle, and coleorhiza. The coleoptile is a sheath that protects the emerging shoot as it pushes up through the soil. The plumule is the growing point for the first shoots and leaves, and the radicle is the beginning of the root system. The emerging root is protected by a root cap called a calyptra and a sheath called a coleorhiza.

Germination begins with the coleorhiza pushing through the pericarp. It is quickly followed by the radicle growing through the coleorhiza. As the embryo emerges, a signal is sent to the endosperm to start feeding the growing baby corn plant, giving it a head start until it can make its own food via photosynthesis.

corn seeds (Zea mays)

Up Next: We’ll take an inside look at the seeds of gymnosperms.

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Inside of a Seed: Two Dicots

“A seed is a living thing that embodies roots, stems, leaves, and fruit in an embryonic state and retains the ability to convert the sun’s energy into a source of food.” — Seedtime by Scott Chaskey

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Few things are more miraculous than seeds. Within them is a living plant in embryonic form. Under the right conditions, these tiny objects expand, pushing out the beginnings of the most minuscule weed to the most humongous tree. Looking at these otherwise unassuming specks, you would hardly guess that they held such potential.

Housed in a seed is the genetic material necessary for growth and reproduction, along with some stored sugars to get the plant started. All of this is enclosed in a protective case. It is a rare moment in a plant’s life – a time when it isn’t rooted in place and can, for a brief period, move around. With the help of agents like wind, water, and animals it can travel anywhere in the world, venturing as far as inches or miles from its parent plant. As long as it finds a suitable place to grow, its voyage is not in vein.

Seeds are the result of sexual reproduction in plants (with rare exceptions, which we will cover in a future post). After pollination, a pollen grain sends three haploid cells into the ovule of a flower. These cells unite with the haploid cells found within. One germ cell from the pollen grain goes to the formation of an embryo, while the other two cells help form endosperm, the food source for the developing embryo. The wall of the ovule becomes the outer layer of the seed, known as the seed coat or testa. The seed matures as the fruit it is nested in ripens. Eventually, the fetal plant within the seed is ready to find a new home.

Seed heads of rubber rabbitbrush (Ericameria nauseosa) – the fuzzy pappus attached to the fruits allows seeds to float in the breeze and travel away from their parent plant.

As with so many things in biology, there is no single type of seed. When it comes to seed anatomy, most seeds consist of the same basic components, but each species of plant has its own unique seed. In fact, a well-trained taxonomist can identify plants simply by observing their seeds. With such a wide variety of seeds, it is difficult to organize them into discrete categories, but we still try. What follows is an introduction to two types of seeds – endospermic and non-endospermic – using two basic examples.

The first thing you should know about these two examples is that both species are dicotyledons (or dicots, for short). This means that when the baby plant emerges, it has two cotyledons, which are also called embryonic leaves because they look like little leaves. All flowering plants have been divided into two groups based on the number of cotyledons they have, the second group being the monocotolydons (or monocots) which have only one cotyledon. This is an old-fashioned way to classify plants, but it is still useful in some instances.

Endospermic Seeds

The seeds of the castor bean plant (Ricinus communis) are endospermic seeds. This means that they retain the endosperm that was formed when two pollen grain cells joined up with the haploid cell in the ovule. The endosperm will help feed the growing embryo as it germinates. The two cotyledons are visible within the seed, but they are thin and broad, leaving plenty of space in the seed for the endosperm. The cotyledons are part of the embryo and are attached to the radicle, which is the embryonic root. The radicle is the first thing to emerge from the seed upon germination. The area between the radicle and the cotyledon is known as the hypocotyl. It becomes the stem of the germinating seedling.

An elaisome is attached to the outside of the seed coat of castor bean seeds. This fleshy, nutrient-rich appendage is particularly attractive to ants. They carry the seeds back to their colony and feed the elaisome to their young. The seeds, however, remain unconsumed. In this way, the ants aid in the seeds’ dispersal.

seeds of castor beans (Ricinus communis)

Non-endospermic Seeds

The seeds of plants in the bean family (Fabaceae) are non-endospermic seeds. This means that as the embryo develops, it uses up the majority of the endosperm within the seed. The food necessary for the seedling to get its start is all stored in its cotyledons. The common pea (Pisum sativum) is a good example of this. The embryo – which consists of the cotyledons, plumula (or plumule), hypocotyl, and radicle – takes up all available space inside of the seed coat. After germination, as the seedling develops, the plumule appears above the cotyledons and is the growing point for the first true leaves and stems.

seeds of the common pea (Pisum sativum)

In future posts, we will look at a few other types of seeds, as well as discuss various other seed-related topics. If you have a story to share about seeds, please do so in the comment section below.

2018: Year in Review

Another year has passed, which means it’s time again for a Year in Review. As I have done in the past, I am including links to a selection of posts that came out in 2018. Most of these posts are part of ongoing series. You can find the rest of the posts from 2018 in the Archives widget on the right side of the screen (or at the bottom of the page if you are viewing this on a mobile device).

Among many memorable happenings this year, the one I feel most compelled to highlight here is a short radio show that a friend and I started doing on Radio Boise, our community radio station. The show is called Boise Biophila, and each week Casey O’Leary and I spend about a minute each talking about something biology or ecology related with the goal of encouraging people to get outside and take a closer look at the natural world around them. After the shows air, I put them up on our Soundcloud page for all to hear for years to come. You can follow us there, as well as on our Facebook page.

Ficus carica via PhyloPic

In the spring of 2018 we set up a Donorbox account, which is a simple way for people who enjoy Awkward Botany and want to see it continue to give us a little financial encouragement. We received several donations at that time, and we are very grateful to those that contributed. But just like public radio or any other organization that would like to receive your support in the form of money, we continue our plea. If Awkward Botany means something to you and you feel compelled to share some of your hard-earned dollars with us, we are happy to receive them and promise to put them to good use.

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Money aside, a major contribution you can make to the success of Awkward Botany is to share it with your friends. Spread the word in conversation, through the postal system, over the phone, or through one or more of the myriad social media platforms. However you choose to share is none of our business. We are just happy that you do.

You are also encouraged to follow our various social media pages: Twitter, Tumblr, and Facebook. Above all, keep reading. We have lots more posts in the works for 2019, and we wouldn’t want you to miss out on the fun. Our appreciation for plants and the natural world is a constant, and we hope you will continue to share in our botany nerd revelry throughout the coming year.

Fragaria vesca via PhyloPic

Book Reviews:

Botany in Popular Culture:

Tiny Plants:

Field Trip:

Eating Weeds:

Two-parters:

Guest Posts: