Dispersal by Open Sesame!

In certain instances, “open sesame” might be something you exclaim to magically open the door to a cave full of treasure, but for the sesame plant, open sesame is a way of life. In sesame’s case, seeds are the treasure, which are kept inside a four-chambered capsule. In order for the next generation of plants to have a chance at life, the seeds must be set free. Sesame’s story is similar to the stories of numerous other plant species whose seeds are born in dehiscent fruits. But in this instance, the process of opening those fruits is fairly unique.

Sesamum indicum is a domesticated plant with a 5000 plus year history of cultivation. It shares a genus with about 20 other species – most of which occur in sub-Saharan Africa – and belongs to the family Pedaliaceae – the sesame family. Sesame was first domesticated in India and is now grown in many other parts of the world. It is an annual plant that is drought and heat-tolerant and can be grown in poor soils and locations where many other crops might struggle. However, the best yields are achieved on farms with fertile soils and adequate moisture.

image credit: wikimedia commons

Depending on the variety and growing conditions, sesame can reach up to 5 feet tall and can be unbranched or highly branched. Its broad lance-shaped leaves are generally arranged directly across from each other on the stem. The flowers are tubular, similar in appearance to foxglove, and are typically self-pollinated and short-lived. They come in shades of white, pink, blue, and purple and continue to open throughout the growing season as the plant grows taller, even as fruits formed earlier mature. The fruits are deeply-grooved capsules with at least four separate chambers called locules. Rows of tiny, flat, teardrop shaped seeds are produced in each chamber. The seeds are prized for their high oil content and are also used in numerous other ways, both processed and fresh. One of my favorite uses for sesame seeds is tahini, which is one of the main ingredients in hummus.

The fruits of sesame are dehiscent, which means they naturally split open upon reaching maturity. Compare this to indehiscent fruits like acorns, which must either rot or be chewed open by an animal in order to free the seeds. Dehiscence is also called shattering, and in many domesticated crop plants, shattering is something that humans have selected against. If fruits dehisce before they can be harvested, seeds fall to the ground and are lost. Selecting varieties that hold on to their seed long enough to be harvested was imperative for crops like beans, peas, and grains. In domesticated sesame, the shattering trait persists and yield losses are often high.

Most of the world’s sesame crop is harvested by hand. The plants are cut, tied into bundles, and left to dry. Once dry, they are held upside down and beaten in order to collect the seeds from their dehisced capsules. When harvested this way, naturally shattering capsules may be preferred. But in places like the United States and Australia, where mechanical harvesting is desired, it has been necessary to develop new, indehiscent varieties that can be harvested using a combine without losing all the seed in the process. Developing varieties with shatter-resistant seed pods, has been challenging. In early trials, seed pods were too tough and passed through threshers without opening. Additional threshing damaged the seeds and caused the harvest to go rancid. Mechanically harvested varieties of sesame exist today, and improvements in these non-shattering varieties continue to be made.

In order to develop these new varieties, breeders have had to gain an understanding of the mechanisms behind dehiscence and the genes involved in this process. This research has helped us appreciate the unique way that the capsules of the sesame plant dehisce. As in the seed bearing parts of many other plant species, the capsules of sesame exhibit hygroscopic movements. That is, their movements are driven by changes in humidity. The simplest form of hygroscopic movement is bending, which can be seen in the opening and closing of pine cone scales. A more complex movement can be seen in the seed pods of many species in the pea family, which both bend and twist as they split open. In both of these examples, water is evaporating from the plant part in question. As it dries it bends and/or twists, thereby releasing its contents.

dehisced capsules of sesame (Sesamum indicum); photo credit: wikimedia commons (Dinesh Valke)

The cylindrical nature and cellular composition of sesame fruits leads to an even more complex form of hygroscopic movement. Initially, the capsule splits at the top, creating an opening to each of the four locules. The walls of each locule bend outward, then split and twist as the seed falls from the capsule. In a study published in Frontiers in Plant Science (2016), researchers found that differences in the capsule’s inner endocarp layer and outer mesocarp layer are what help lead to this interesting movement. The endocarp layer is composed of both transvere (i.e. circumferential) and longitudinal fiber cells, while the mesocarp is made up of soft parenchyma cells. The thicknesses of these two layers gradually changes along the length of the capsule. As the mesocarp dries, the capsule initially splits open and starts bending outwards, but as it does, resistance from the fiber cells in the endocarp layer causes further bending and twisting (see Figure 1 in the report for an illustration). As the researchers write, “the non-uniform relative thickness of the layers promotes a graded bi-axial bending, leading to the complex capsule opening movement.”

All this considered, a rock rolling away from the entrance of a cave after giving the command, “Open sesame!” almost seems simpler than the “open sesame” experienced by the fruit of the sesame plant.

See Also: Seed Shattering Lost – The Story of Foxtail Millet

Burr Tongue, or The Weed That Choked the Dog

It is said that the inspiration for Velcro came when Swiss inventor, George de Mestral, was removing the burrs of burdock from his dog’s coat, an experience we had with Kōura just days after adopting her. I knew that common burdock was found on our property, and I had made a point to remove all the plants that I could easily get to. However, during Kōura’s thorough exploration of our yard, she managed to find the one plant I had yet to pull due to its awkward location behind the chicken coop.

I knew when I saw the clump of burrs attached to her hind end that we were going to spend the evening combing them out of her fur. However, not long after that we discovered that Kōura had already started the process and in doing so had either swallowed or inhaled some. What tipped us off was her violent hacking and gagging as she moved frantically around the living room. She was clearly distraught, and so were we. Recognizing that she had probably swallowed a burr, we made a quick decision to take her to an emergency vet. This was our unfortunately timed (this happened on Christmas Eve) introduction to burr tongue and all the frightening things that can happen when a dog swallows burdock burrs.

The roots, shoots, and leaves of both greater burdock (Arctium lappa) and common burdock (Arctium minus) are edible, which I have already discussed in an Eating Weeds post. The burrs, on the other hand, are clearly not. While sticking to the fur of animals and the clothing of people is an excellent way for a plant to get their seeds dispersed, the sharp, hooked barbs that facilitate this are not something you want down your throat. When this occurs, the natural response is to try to hack them up, which Kōura was doing. Salivating heavily and vomiting can also help. In many cases, this will be enough to eliminate the barbs. However, if they manage to work their way into the soft tissues of the mouth, tongue, tonsils, or throat and remain there, serious infection can occur.

burr of common burdock (Arctium minus)

A paper published in The Canadian Veterinary Journal in 1973 describes the treatment for what is commonly known as burr tongue and technically referred to as granular stomatitis. The paper gives an account of what can happen when “long-haired breeds of dogs … run free in areas where [burdock] grows” and the hooked scales of the burrs consequently “penetrate the mucous membrane of the mouth and tongue.” Dogs with burrs imbedded in their mouths may start eating less or more slowly, drinking more water, and drooling excessively. As infection progresses, their breath can start to stink. A look inside the mouth and at the tongue will reveal lesions where the burrs have embedded themselves. Treatment involves putting the dog under anesthesia, scraping away the infected tissue, and administering antibiotics. Depending on the severity of the lesions, scar tissue can form where the barbs were attached.

To prevent infection from happening in the first place, a veterinarian can put the dog under anesthesia and use a camera inside the dog’s mouth and throat to search for pieces of the burr that may have gotten lodged. There is no guarantee that they will find them all or be able to remove them, and so the dog should be monitored over the next several days for signs and symptoms. At our veterinary visit, the vet also warned us that if any burrs were inhaled into the lungs, they could cause a lung infection, which is another thing to monitor for since it would be practically impossible for an x-ray or a camera to initially find them.

Luckily, now more than three weeks later, Kōura appears to be doing fine, and the offending burdock has been taken care of. One thing is for sure, as someone who is generally forgiving of weeds, burdock is one weed that will not be permitted to grow at Awkward Botany Headquarters.


For more adventures involving Kōura, be sure to follow her on Instagram @plantdoctordog.

The Serotinous Cones of Lodgepole Pine

Behind the scales of a pine cone lie the seeds that promise future generations of pine trees. Even though the seeds are not housed within fruits as they are in angiosperms (i.e. flowering plants), the tough scales of pine cones help protect the developing seeds and keep them secure until the time comes for dispersal. In some species, scales open on their own as the cone matures, at which point winged seeds fall from the tree, taking flight towards their new homes. In other species, the scales must be pried open by an animal in order to free the seed. A third group of species have what are called serotinous cones, the scales of which are sealed shut with resin. High temperatures are required to soften the resin and expose the seeds.

Serotinous cones are a common trait of pine species located in regions where wildfire naturally and regularly occurs. One such species is lodgepole pine (Pinus contorta), which is found in abundance in forests across much of western North America. Lodgepole pine is a thin-barked tree species that burns easily and is often one of the first plants to recolonize after a stand-replacing wildfire. There are 3 or 4 subspecies of lodgepole pine. The one with the largest distribution and the one that most commonly exhibits serotinous cones is P. contorta subsp. latifolia, which occurs throughout the Rocky Mountains, north into the Yukon, and just west of the Cascade Range.

needles of lodgpole pine (Pinus contorta)

Lodgepole pine grows tall and straight, generally maxing out at around 80 feet tall. Its needles are about two and a half inches long, are borne in bundles of two, and tend to twist away from each other, which is one explanation for the specific epithet, contorta. Its cones are egg-shaped with asymmetrical bases, measuring less than two inches long with prickly tips at the ends of each scale. The seeds of lodgepole pine are tiny with little, papery wings that aid in dispersal. The cones can remain attached to the tree for 15-20 years (sometimes much longer), and the seeds remain viable for decades. In non-serotinous cones, the scales start opening on their own in early autumn. Serotinous cones require temperatures of 45-50°C (113-122°F), to release the resin bond between the scales. Some cones that happen to fall from the tree can open when exposed to particularly warm temperatures on the ground. Otherwise, it takes fire to free the seeds.

Serotinous cones aren’t a guarantee, and the percentage of trees with serotinous cones compared to those with non-serotinous cones varies widely across the range of lodgepole pine, both in space and in time. One reason for this is that trees with serotinous cones don’t develop them until they reach a certain age, generally around 20-30 years old, or perhaps as old as 50 or 60. The cones of young trees are all non-serotinous. But some trees never develop serotinous cones at all. Serotiny is a genetic trait, and there are various factors that either select for or against it. A number of factors are at play simultaneously over the life of a tree and across a population of trees, so it is difficult to determine exactly why the percentage of serotinous cones is so variable across the range of the species. What follows are a few potential explanations for this phenomenon.

closed cone of lodgepole pine (Pinus contorta)

As a fire-adapted, pioneer species, lodgepole pine has evolved to live in environments where fire is predictably common. Serotinous cones help ensure that a population won’t be wiped out when a massive wildfire comes through. After the fire has passed and the seeds are released, lodgepole pine can quickly repopulate the barren ground. As long as fire occurs within the lifespan of a population of similarly aged trees, it is advantageous for the majority of individuals to maintain their serotinous trait. If the population is located in an area that historically does not see much fire, serotinous cones may be a disadvantage and can have adverse effects on the longevity of that population.

A study published in Ecology in 2003 looked at the influence that the frequency of fire has on lodgepole pine stands found at low and high elevations in Yellowstone National Park. At lower elevations, where summer temperatures are warmer and precipitation is relatively minimal, fires occur more frequently compared to higher elevations, which tend to be cooler and wetter. The researchers found that at lower elevations when fires occurred at short intervals (less than 100 years between each fire), lodgepole pine was slower to repopulate compared to longer intervals. This suggests that the percentage of serotiny found in stands that experienced short fire intervals was low, and that stands with long fire intervals exhibit a higher percentage of serotiny. After all, as mentioned above, lodgepole pines don’t start developing serotinous cones until later in life.

At higher elevations, where fire occurs less frequently, lodgepole pines were found to have a low percentage of serotinous cones regardless of the age of the stand. Because the trees at high elevations are more likely to die of old age rather than fire, maintaining serotinous cones would be a disadvantage. Open cones are preferred. Thus, at least in this study, a greater percentage of serotinous cones was found in lodgepole pines at lower elevations compared to those at higher elevations. Latitude, elevation, mountain pine beetle attacks, and other environmental factors have all been used to explain differences in serotiny. However, the factor that seems to have the greatest influence is the frequency of fire. As James Lotan writes in a 1976 report: “A high degree of cone serotiny would be expected where repeated, high-intensity fires occur. Where forest canopies are disrupted by factors other than fire, open cones annually supply [seed] for restocking disturbances such as windfalls.”

That being said, one other factor does appear to play a critical role in whether or not lodgepole pines produce serotinous cones, and that is seed predation by squirrels. In a paper published in Ecology in 2004, researchers wondered why the percentage of serotinous cones wasn’t even higher in populations where fire reliably occurred during the lifetime of the stand. To help answer this question they looked at the activities of pine squirrels, which are the main seed predator of lodgepole pine seeds. Pine squirrels visit the canopy of lodgepole pines and consume the seeds found in serotinous cones. Because non-serotinous cones quickly shed their seeds, serotinous cones are a more reliable and accessible food source, and because pine squirrels are so effective at harvesting the seeds of serotinous cones, the researchers concluded that, “in the presence of pine squirrels, the frequency of serotiny is lower and more variable, presumably reflecting,” among a variety of other factors, “the strength of selection exerted by pine squirrels.”

A study published in PNAS in 2014 added evidence to this conclusion. While acknowledging that fire plays a major role in the frequency of serotinous cones, the researchers asserted that “squirrels select against serotiny and that the strength of selection increases with increasing squirrel density.” However, despite making it easier for squirrels to access their seeds, lodgepole pines maintain a degree of serotinous cones, since clearly their main advantage is retaining a canopy-level seed bank from which seeds are released after a fire and by which a new generation of lodgepole pines is born.

open cones of lodgepole pine (Pinus contorta)

Further Reading and Viewing:

Book Review: In Defense of Plants

Many of us who are plant obsessed didn’t connect with plants right away. It took time. There was a journey we had to go on that would ultimately bring us to the point where plants are now the main thing we think about. After all, plants aren’t the easiest things to relate to. Not immediately anyway. Some of us have to work up to it. Once there, it’s pretty much impossible to go back to our former lives. What was once just a background of green hues is now a rich cast of characters, each with their own name, unique features, and distinct story to tell. Essentially, we went through what Matt Candeias refers to as our ” green revolution.” Candeias – author and host of the long-running blog and podcast, In Defense of Plants – shares his story of learning to love plants and offers a convincing arguement for why you should love them too in his new book, aptly titled, In Defense of Plants.

It’s hard to picture Candeias as anything but a plant lover. If you’ve been following his work, you’ll know he makes it a point to put plants at center stage. It seems that much of the popular content available about plants focuses on the usefulness of plants as they pertain to humans. In many cases it can be easier to find out how to grow a certain plant species than to learn about where it’s from and what it’s like in the wild. Candeias let’s the plants speak for themselves by giving them a voice through his blog, podcast, and now his book. Through the stories he shares we get a peek into the way Candeias sees plants, with the hope being that others might also “be bitten by the botanical bug.”

One of the first plants that captured the attention of Candeias was perennial blue lupine (Lupinus perennis). While assisting with a habitat restoration project at a sand and gravel quarry, Candeias was tasked with improving the establishment of lupine, which is the host plant for the caterpillars of an endangered species of butterfly called Karner blue. The work he did at the quarry and the botanical research that went into it helped Candeias realize that plant’s aren’t at all boring, but are “incredibly interesting organisms worthy of respect and admiration” and that “plants can be both surprisingly relatable and incredibly alien all at once.” His “green revolution” had begun.

The seeds of lupine are dispersed ballistically. As the seed pods dry, tension builds. Then, as Matt Candeias writes in In Defense of Plants, “with an audible pop, the pods eventually explode, catapulting the seeds out into the environment.”

In each chapter of In Defense of Plants we get a peak into the experiences that brought Candeias to where he is now as he discovers the wonder of plants. His personal stories help introduce the main topic of each chapter. Topics include plant sex, plant dispersal, plant defenses, carnivorous plants, and parasitic plants. From countless possible examples, Candeias selects a few of his favorite plant species to help illustrate each topic. Along the way, the reader is presented with various other interesting plant-related facts as Candeias discusses the behaviors of some of the world’s most fascinating plants. In the chapter on dispersal, for example, unlikely agents of seed dispersal (like catfish!) are introduced, as well as phenomena like geocarpy, in which plants are essentially planting themselves.

Carnivorous plants provide an excellent gateway into convincing people who claim to have no interest plants that they actually do. It’s difficult to deny the impressive nature of a meat-eating plant. In the carnivorous plant chapter, Candeias introduces us to the various ways such plants capture and consume their prey, and even wonders if some of these plants should be considered omnivores. After all, certain butterworts digest pollen that falls onto their sticky leaves, and some bladderworts suck in plenty of algae and possibly gain nutrients from the act. If capturing insects inside leaves modified to look like pitchers or on leaves covered in digestive enzyme-producing glands doesn’t impress you, consider the carnivorous actions of corkscrew plants, which drill their leaves into the soil to go after soil-dwelling organisms like protozoans and worms.

Parasitic plants should also excite a reluctant plant lover. These are plants that take all or most of what they need to survive from another plant or host organism. Mistletoes are one of the more familiar parasitic plants, and Candeias describes several, including one that lives almost entirely within the stems of cacti. In fact, “you would never know a cactus had been infected until the mistletoe living within decides to flower,” at which point the flowers push their way out through the sides of the cactus. Dodder is another fairly common, highly specialized, and easy to identify parasitic plant. It basically looks like “a tangled pile of orange spaghetti tossed over the surrounding vegetation.” Orchids, a favorite of Candeias, are known for being mycoheterotrophs, which essentially means they parasitize fungi. Their seeds come unequipped with the energy stores needed to get going, so they borrow resources from mycorrhizal fungi in order to get their start. Years pass before the orchid can offer anything in return.

Datura is a genus of plants that produces toxic compounds like scopolamine and atropine. In his book, In Defense of Plants, Matt Candeias warns, “it would only take a small amount of these chemicals to completely ruin your week and slightly more to put you in a grave.”

After spending more than 200 pages celebrating plants and their amazing abilities and diversity, it’s fitting that Candeias spends the final chapter of his book mourning some of the ways the actions of humans threaten the existence of so many plants. He remarks how unfortunate it is that “plants with their unseeing, unhearing, unfeeling ways of life usually occupy the lowest rung of importance in our society.” Many of us barely notice the loss, yet “plants are the foundation of functioning ecosystems.” Due to that fact, “destroying plant communities causes disastrous ripples that reverberate throughout the entire biosphere of our planet.” Everything suffers when plants are lost. Fortunately, the book doesn’t end on this dark note. Candeias’s overall message is hopeful. When we learn to understand, appreciate, and care about plants, we will want to do everything we can to protect and restore them. With any luck, after reading this book, you too will want to offer your time, energy, and resources in defense of plants.

Listen to Matt talk about his new book on this episode of his podcast.

More Book Reviews on Awkward Botany

Weeds of Boise: Railroad Tracks Between Kootenai Street and Overland Road

Walking along railroad tracks is a pretty cool feeling. It’s also a good place to look for weeds. Active railroad tracks are managed for optimum visibility and fire prevention, which means that trees and shrubs near the tracks are removed creating plenty of open space on either side. Open areas in full sun are ideal places for a wide variety of weed species to grow. Trains passing through can also be sources or dispersal agents of seeds, so there’s a chance that you may see things growing alongside railroad tracks that you don’t often see elsewhere. All this means that railroad tracks in urban areas are great locations to familiarize yourself with your city’s wild urban flora.

I visited a small section of railroad tracks between Kootenai Street and Overland Road in Boise. At one point, this was a pretty active railroad. Passenger trains once moved along these tracks, and the Boise Depot, which is less than a mile from this location, was one of several stops between Portland, OR and Salt Lake City, UT. Unfortunately, those services ended in 1997 and have yet to resume, despite continued support for bringing passenger rail back to the region. Still, freight trains pass by with some frequency.

Managing weeds along railroad tracks in urban areas can be tricky. There is little else in the way of vegetation to compete with the weeds. The tracks are also adjacent to parks, homes, schools, gardens, and other locations that make herbicide applications complicated. The species of weeds can also vary widely from one mile to the next, so management decisions must also vary. It’s especially important that the ballast directly beneath and on either side of the tracks is kept weed free in order to prevent fires and improve visibility. All of this and more makes weed control along railroad tracks one of the most challenging jobs around. Luckily, for someone that likes to look at weeds, it means there will always be interesting things to see near the tracks, including for example this colony of harvester ants that I came across while identifying weeds. I was happy to see that they were collecting the samaras of Siberian elm (Ulmus pumila), one of several weedy trees in the Treasure Valley.

What follows are a few images of some of the weeds I encountered along the railroad tracks between Kootenai Street and Overland Road, as well as a list of the weeds I was able to identify. The list will grow as I identify the mystery weeds and encounter others that I missed, as is the case with all posts in the Weeds of Boise series.

Virginia creeper (Parthenocissus quinquefolia)
blue mustard (Chorispora tenella)
cleavers (Galium aparine)
whitetop (Lepidium sp.)
Himalayan blackberry (Rubus bifrons)
bush honeysuckle (Lonicera sp.)
Siberian elm (Ulmus pumila)
English ivy (Hedera helix)
kochia seedlings (Bassia scoparia)
  • Arctium minus (common burdock)
  • Bassia scoparia (kochia)
  • Bromus diandrus (ripgut brome)
  • Bromus tectorum (cheatgrass)
  • Chorispora tenella (blue mustard)
  • Conium maculatum (poison hemlock)
  • Convolvulus arvensis (field bindweed)
  • Cirsium arvense (creeping thistle)
  • Dactylis glomerata (orchardgrass)
  • Descurainia sophia (flixweed)
  • Elaeagnus angustifolia (Russian olive)
  • Epilobium ciliatum (northern willowherb)
  • Equisetum sp. (horsetail)
  • Erodium cicutarium (redstem filaree)
  • Galium aparine (cleavers)
  • Hedera helix (English ivy)
  • Hordeum murinum (wild barley)
  • Lactuca serriola (prickly lettuce)
  • Lepidium sp. (whitetop)
  • Lonicera sp. (bush honeysuckle)
  • Parthenocissus quinquefolia (Virginia creeper)
  • Poa bulbosa (bulbous bluegrass)
  • Poa pratensis (Kentucky bluegrass)
  • Rubus bifrons (Himalayan blackberry)
  • Rumex crispus (curly dock)
  • Secale cereale (feral rye)
  • Taraxacum officinale (dandelion)
  • Ulmus pumila (Siberian elm)

Do you live near railroad tracks? What weeds are growing there, and do you feel as cool as I do when you walk the tracks?

When Acorn Masts, Rodents, and Lyme Disease Collide

“‘Mast years’ is an old term used to describe years when beeches and oaks set seed. In these years of plenty, wild boar can triple their birth rate because they find enough to eat in the forestes over the winter… The year following a mast year, wild boar numbers usually crash because the beeches and oaks are taking a time-out and the forest floor is bare once again.” — The Hidden Life of Trees by Peter Wohlleben

———————

When a plant population’s annual production of seeds is highly variable and synchronous, it is considered a masting or mast seeding species. Why and how masting happens is a bit of a mystery, and efforts are underway to better understand this phenomenon. One thing is clear, boom and bust cycles can have dramatic effects on animals that use the fruits and seeds of these plants for food. Acorn production in oaks provides a stark example. As Koenig, et al. describe in Ecology (2015), a “variable acorn crop initiates a ‘chain reaction’ of responses that cascades through the ecosystem, affecting densities of deer, mice, ground-nesting birds, gypsy moths, and the tick vectors of Lyme disease.” The connection between mast seeding oaks and the prevalence of tick-borne pathogens is of particular interest considering the risks posed to humans.

Lyme disease is an infectious diesease caused by a bacterium vectored by ticks in the genus Ixodes. The life-cycle of a tick is generally 2 to 3 years, beginning after a larva hatches from an egg. From there the larva develops into a nymph and later an egg-laying adult, taking a blood meal each step of the way. Tick larvae feed on the blood of small rodents and birds, which is where they can pick up the bacterium that causes Lyme disease. After feeding, they develop into nymphs and go in search of another blood meal, perhaps another rodent or maybe something larger like a deer or a human. It is in their nymphal and adult stages that ticks transmit Lyme disease to humans. Nymphs tend to transmit the disease more frequently, partly because they go undetected more easily.

The risk to humans of being infected with Lyme disease varies year to year and is dependent largely on how many infected ticks are present. For this reason, it is important to understand the factors affecting the density of infected nymphs. In a study published in PLoS Biology (2006), Ostfeld, et al. collected data over a 13 year period in plots located in deciduous forests in the state of New York, a hotspot for Lyme disease. The predictors they considered included temperature, precipitation, acorn crop, and deer, white-footed mouse, and chipmunk abundances. Deer abundance and weather conditions had long been considered important in predicting the prevalence of ticks, but little attention had been paid to small mammals – the larval hosts for ticks – and the variability of acorn crops – an important food source for rodents.

deer tick (Ixodes scapularis) — via PhyloPic; user Mathilde Cordellier

The results of their study revealed a clear pathway – more acorns leads to more rodents which leads to more Lyme disease carrying nymphs. The process takes a couple of years. First, oak trees experience a mast year, flooding rodent populations with food. In the following year, the numbers of mice and chipmunks is unusually high. The year after that, there are lots and lots of nymphal ticks infected with Lyme-disease. The relationship is so direct that Richard Ostfeld claims, based on his research, that he can predict the incidence of Lyme disease among residents of New York and Connecticut based on when a mast year occurs. In a summer when there is an abundance of 2 -year-old oak seedlings in the surrounding forests, expect the infection rate of Lyme disease to be high.

Lyme disease also occurs in regions where oak trees are not present or are uncommon, so variability in acorn crops isn’t always the best predictor. The researchers acknowledge that acorn abundance is not going to be “a universal predictor of risk;” instead, anything that leads to an increase in rodent populations, whether it is some other food source or a lack of predators, may be a key indicator since rodents are reservoir hosts of Lyme disease.

A study published in Parasites and Vectors (2020) looked at the effects of rodent density on a number of tick-borne pathogens. They confirmed that an “increase in rodent density positively affects populations of nymphal ticks in the following year;” yet, they could not confirm that rodent density is the sole predictor of disease risk. Other factors come into play depending on the disease in question, and further research is needed to improve models that predict tick-borne diseases. They did, however, confirm that, by flooding the food supply with acorns, mast years can boost populations of a variety of rodents.

white-footed mouse (Peromyscus leucopus) — via wikimedia commons; USGS

A fear of ticks is justified. They suck your blood after all, and besides that, they can transmit some pretty serious diseases. Arm yourself by educating yourself. One place to do that is with The Field Guides podcast. Their tick two-parter is well worth the listen (part one and part two). Not only will it give you valuable information in protecting yourself against ticks, it may also give you an appreciation for their prowess. Just maybe. See also their You Tube video demonstrating how to sample for ticks.

The Weeds in Your Bird Seed

With February comes the return of the Great Backyard Bird Count, a weekend-long, worldwide, bird counting event that Sierra and I have enjoyed participating in for the past few years. While you can choose to count birds anywhere birds are found, part of the appeal of the event is that it can be done from the comfort of one’s own home simply by watching for birds to appear right outside the window. If there are bird feeders in your yard, your chances of seeing birds are obviously improved. Watch for at least fifteen minutes, record the number and species of birds you see, then report your sightings online. It’s for science!

Feeding and watching birds are popular activities. In the United States alone, as many as 57 million households put out food for birds, spending more than $4 billion annually to do so. While there are a variety of things one can purchase to feed birds – suet, berries, mealworms, etc. – the bulk of that money is likely spent on bags of bird seed (also referred to as bird feed). Bird seed is a relatively cheap and easy way to feed a wide variety of birds. Unfortunately, it’s also a great way to introduce new weeds to your yard.

Bird seed contaminated with noxious weed seeds is not a new problem. It has been a concern for decades, and some countries have taken regulatory steps to address the issue. In the United States, however, there are no governmental regulations that address weed seed contamination in bird seed.  With this thought in mind, researchers at the University of Missouri screened a large sampling of bird seed mixes to determine the number and species of weed seeds they harbored, as well as their viability and herbicide resistance. Their results were published last year in Invasive Plant Science and Management.

The researchers examined 98 different bird seed mixes purchased from retail locations in states across the eastern half of the U.S. The seeds of 29 weed species were recovered from the bags, including at least eight species of grasses and several annual and perennial broadleaf weeds. 96% of the mixes contained one or more species of Amaranthus, including Palmer’s amaranth (Amaranthus palmeri), which was found in 27 mixes and which the researchers refer to as “the most troublesome weed species in agroecosystems today.” About 19% of amaranth seeds recovered germinated readily, and five of the seed mixes contained A. tuberculatus and A. palmeri seeds that, once grown out, were found to be resistant to glyphosate, the active ingredient in a commonly used herbicide.

Redroot pigweed (Amaranthus retroflexus) is one of several weedy amaranth species commonly found in bird seed mixes (illustration credit: wikimedia commons)

The seeds of grass weeds were found in 76% of the bird seed mixes and included three species of foxtail (Setaria spp.), as well as other common grasses like large crabgrass (Digitaria sanguinalis) and barnyardgrass (Echinochloa crus-galli). Bird seed ingredients that seemed to favor grass seed contamination included wheat, grain sorghum, and proso millet, three crops that are also in the grass family. No surprise, as grass weeds are difficult to control in crop fields when the crop being grown is also a grass.

After amaranths and grasses, ragweed (Ambrosia artemisiifolia) was the third most common weed found in the mixes. This was a troubling discovery since populations of this species have shown resistance to a number of different herbicides. Moving ragweed to new locations via bird seed could mean that the genes that give ragweed its herbicide resistance can also be moved to new locations. Kochia (Bassia scoparia), another weed on the Weed Science Society of America’s list of top ten most troublesome weeds, was also found in certain bird seed mixes, particularly when safflower was an ingredient in the feed.

A similar study carried out several years earlier at Oregon State University found the seeds of more than fifty different weed species in ten brands of bird feed commonly sold at retail stores. Ten of the weeds recovered from the mixes are on Oregon’s noxious weed list. Both studies demonstrate how bird seed can be a vector for spreading weed seeds – and even new weed species and herbicide-resistant genes – to new locations. Weeds found sprouting below bird feeders can then potentially be moved beyond the feeders by wind and other dispersal agents. Weed seeds might also be moved to new locations inside the stomachs of birds.

Addressing this issue can be tackled from several different angles. Growers and processors can improve their management of weed species in the fields where bird seed is grown and do a better job at removing weed seeds from the mixes after they are harvested. Government regulations can be put in place that restrict the type and quantity of weed seeds allowed in bird feed. Further processing of ingredients such as chopping or shelling seeds or baking seed mixes can help reduce the presence and viability of weed seeds.

Processed bird feed like suet is less likely to harbor viable weed seeds (photo credit: wikimedia commons)

Consumers can help by choosing bird feed that is processed or seedless like sunflower hearts, dried fruit, peanuts, suet cakes, and mealworms, and can avoid seed mixes with a large percentage of filler ingredients like milo, red millet, and flax. Attaching trays below feeders can help collect fallen seeds before they reach the ground. Bird seed can also be avoided all together, and feeding birds can instead be done by intentionally growing plants in your yard that produce food for birds. By including bird-friendly plants in your yard, you will also have a better chance of seeing a wide variety of birds during the Great Backyard Bird Count.

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Winter Trees and Shrubs: Northern Catalpa

The names of plants often contain clues that can either help with identification or that tell something about the plant’s history or use. The name, catalpa, is said to be derived from the Muscogee word, katałpa, meaning “winged head,” presumably referring to the tree’s winged seeds. Or maybe, as one writer speculates, it refers to the large, heart-shaped, floppy leaves that can make it look like the tree is “ready to take flight.” Or perhaps it’s a reference to the fluted, fused petals of the tree’s large, tubular flowers. I suppose it could mean any number of things, but I’m sticking with its seeds, which are packed by the dozens in the tree’s long, slender, bean-like fruits. The seeds are flat, pale brown, and equipped with paper thin, fringed appendages on either side that assist in wind dispersal – wings, in other words.

winged seeds of northern catalpa (Catalpa speciosa)

Catalpa speciosa, or northern catalpa, is a relatively fast growing, short-lived tree native to the Midwest and one of only two species in the genus Catalpa found in the United States. Its distribution prior to the arrival of Europeans appears to have been restricted to a portion of the central Mississippi River valley, extending west into Arkansas, east into Tennessee, and north into Illinois and Indiana. It has since been widely planted outside of its native range, naturalizing in areas across the Midwest and eastern US. Early colonizers planted northern catalpa for use as fence posts, railroad ties, and firewood. Its popularity as an ornamental tree is not what it once was a century ago, but it is still occasionally planted in urban areas as a shade tree. Its messiness – littering the ground below with large leaves, flowers, and seed capsules – and its tendency to spread outside of cultivation into natural areas are reasons why it has fallen out of favor with some people.

The oval to heart-shaped, 8 to 12 inch long leaves with long petioles rotting on the ground below the tree are one sure sign that you’ve encountered a catalpa in the winter time. The leaves are some of the first to fall at the end of the growing season, briefly turning an unmemorable yellow before dropping.

leaf of northern catalpa (Catalpa speciosa) in the winter with soft hairs on the underside still visible

The leaf arrangement on northern catalpa is whorled and sometimes opposite. The twigs are easy to identify due to several unique features. They are stout, round, and grayish brown with prominent lenticels. The leaf scars are large, rounded, and raised up on the twig, looking a bit like little suction cups. They are arranged in whorls of three, with one scar considerably smaller than the other two. A series of bundle traces inside the scar form an ellipse. The leaf buds are tiny compared to the scar and are protected by loose, pointed, brown bud scales. Northern catalpa twigs lack a terminal bud. In the winter, seed capsules or the stalk of an old inflorescence often remain attached to the terminal end of the twig. The pith inside of the twig is thick, white, and solid.

twig of northern catalpa (Catalpa speciosa)

pith inside twig of northern catalpa (Catalpa speciosa)

Another common name for Catalpa speciosa is cigar tree, a name that comes from its up to 18 inch long, cigar-like seed capsules that hang from the otherwise naked tree throughout the winter. The sturdy, cylindrical pod starts out green in the summer and turns dark brown by late fall. Seed pods that haven’t fallen or already split open will dehisce in the spring time, releasing their papery seeds to the wind.

fruits of northern catalpa (Catalpa speciosa) hanging from the tree in the winter

The young bark of northern catalpa is thin and easily damaged. As it matures, it becomes furrowed with either scaly ridges or blocky plates. Mature trees are generally twisted at the base but otherwise grow straight, reaching 30 to 60 feet tall (sometimes taller) with an open-rounded to narrow-oval crown.

maturing bark of northern catalpa (Catalpa speciosa)

Northern catalpa is one of the last trees to leaf out in the spring. In late spring or early summer, 10 inch long clusters of white, tubular flowers are produced at the tips of stems. Before the flowers open, they look a bit like popped popcorn, reminding me of a song from my childhood (which I will reluctantly leave right here). The margins of its trumpet-shaped petals are ruffled and there is yellow, orange, and/or purple spotting or streaking on the inside of the tubes.

flower of northern catalpa (Catalpa speciosa) just before it opens

More Winter Trees and Shrubs on Awkward Botany:

Seed Shattering Lost – The Story of Foxtail Millet

For a plant to disperse its seeds, it must first let go of them. Sounds obvious, but it is a key step in the dispersal process and an act that is actually coded in a plant’s DNA. As fruits ripen and seeds mature, an abscission layer is formed that separates the seed-bearing fruits from the plant. No longer attached to their parents, seeds are left to their own devices. If all goes well, they will find themselves in a suitable location where they can germinate and grow into a whole new plant, fully equipped to make seed babies of their own.

The releasing of mature seeds is known as shattering, a term most commonly used in reference to grasses and plants with dehiscent seed pods (i.e. fruits that split open when ripe, such as those in the bean and mustard families). In grasses, seeds form along a central stem called a rachis. As the seeds ripen, they separate from the rachis and drop from the plant. In some cases, the rachis is brittle and a section of it breaks off with each seed that falls.

Seed shattering is not a desirable trait when it comes to food crops. It’s easy to see how yields can be poor if seeds disperse before they are harvested. Thus, an essential step in domesticating certain agricultural crops was selecting plants that lacked this particular trait. Instead of dropping mature seeds, such plants hold on to them, making them easy to collect. A simple and naturally occurring mutation in the genes of these plants allowed early farmers to select varieties that were more ideal for agriculture than their wild progenitors.

Genetic studies of agricultural crops have located genes in a number of species that code for seed shattering, confirming that domestication in many cases involved selecting plants with this gene turned off. A recent study, published in Nature Biotechnology (October 2020), took a different route in locating this gene, looking instead at a weedy, wild relative of a crop that was domesticated at least 8000 years ago. Green foxtail (Setaria viridis) is the wild antecedent of foxtail millet (Setaria italica), a crop that, while still commonly grown for food in parts of Asia, is mostly grown for hay, silage, and bird seed in North America. Recently, interest in foxtail millet and other millets (a term used to refer to the grains of several different species of grasses) is on the rise due to the ability of these crops to tolerate drought and heat.

Illustration of three Setaria species from Selected Weeds of the United States (Agriculture Handbook No. 366) published in 1970

Setaria viridis is an abundant, widespread weed adapted to human disturbance. It’s of Eurasian origin but has been present in North America since the early 1800’s and was likely introduced both intentionally and accidentally. It’s an annual grass with prominent, bristly flowerheads that are easily recognizable and the reason for its common name, green foxtail. A handful of other closely related, similar-looking species are also common weeds in North America. Due to useful traits including its short life cycle, small genome, and self-fertility, S. viridis has been used frequently as a model species to carry out a variety of scientific studies. The study referred to above aimed to further enhance the use of green foxtail, particularly when it comes to crop science.

Researchers traveled across the United States collecting nearly 600 samples of green foxtail in order to better understand its genome. They found that the North American population of green foxtail is composed of multiple introductions and that, as the species has followed humans around, it has quickly adapted to diverse climates found across the continent. In examining the genome, they were able to identify the genetic underpinnings for three traits that have importance to agriculture: response to climate, leaf angle (which is used as a predictor of yield in grain crops), and seed shattering.

foxtail millet (Setaria italica) via wikimedia commons

The seed shattering gene – which the researchers named Less Shattering 1 (SvLes1) – was an especially interesting discovery. When compared to the orthologous gene found in foxtail millet, they found that a frameshift mutation had caused a disruption in the gene, turning it off. Using CRISPR (a gene editing tool) they were able to recreate a similar interruption in green foxtail, which resulted in a loss of seed shattering similar to that of foxtail millet. It became clear that selecting plants with this mutation was an essential step in the domestication of this ancient grain.

An excerpt about seed shattering from Fruit from the Sands by Robert N. Spengler III: 

In many of the world’s domesticated grains, especially those from the founder crops of southwest Asia (i.e. wheat and barley), the earliest phenotypical trait of domestication that archaeobotanists look for is a tough rachis, the small stem by which an individual grain or small cluster of grains is attached to the ear. In their wild form, most grains are programmed to detach easily after the grain ripens; however, in domesticated cereals, the grains remain attached to the ear throughout the harvesting process. This change is an inadvertent result of human harvesting with sickles: as people reap their harvest, the grains with a brittle rachis are dropped and those with a tough rachis are collected, stored, and replanted for successive harvests.

Further Reading:

The Hidden Flowers of Viola

Violas keep a secret hidden below their foliage. Sometimes they even bury it shallowly in the soil near their roots. I suppose it’s not a secret really, just something out of sight. There isn’t a reason to show it off, after all. Showy flowers are showy for the sole purpose of attracting pollinators. If pollinators are unnecessary, there is no reason for showy flowers, or to even show your flowers at all. That’s the story behind the cleistogamous flowers of violas. They are a secret only because unless you know to look for them, you would have no idea they were there at all.

Cleistogamy means closed marriage, and it describes a self-pollinating flower whose petals remain sealed shut. The opposite of cleistogamy is chasmogamy (open marriage). Most of the flowers we are familiar with are chasmogamous. They open and expose their sex parts in order to allow for cross-pollination (self-pollination can also occur in such flowers). Violas have chasmogamous flowers too. They are the familiar five-petaled flowers raised up on slender stalks above the green foliage. Cross-pollination occurs in these flowers, and seed-bearing fruits are the result. Perhaps as a way to ensure reproduction, violas also produce cleistogamous flowers, buried below their leaves.

an illustration of the cleistogamous flower of Viola sylvatica opened to reveal its sex parts — via wikimedia commons

Flowers are expensive things to make, especially when the goal is to attract pollinators. Colorful petals, nectar, nutritious pollen, and other features that help advertise to potential pollinators all require significant resources. All this effort is worth it when it results in the ample production of viable seeds, but what if it doesn’t? Having a method for self-pollination ensures that reproduction will proceed in the absence of pollinators or in the event that floral visitors don’t get the job done. A downside, of course, is that a seed produced via self-pollination is essentially a clone of the parent plant. There will be no mixing of genes with other individuals. This isn’t necessarily bad, at least in the short term, but it has its downsides. A good strategy is a mixture of both cross- and self-pollination – a strategy that violas employ.

The cleistogamous flowers of violas generally appear in the summer or fall, after the chasmogamous flowers have done their thing. The fruits they form split open when mature and deposit their seeds directly below the parent plant. Some are also carried away by ants and dispersed to new locations. Seeds produced in these hidden flowers are generally superior and more abundant compared to those produced by their showy counterparts. People who find violas to be a troublesome lawn weed – expanding far and wide to the exclusion of turfgrass – have these hidden flowers to blame.

That being said, there is a defense for violas. In the book The Living Landscape by Rick Darke and Doug Tallamy, Tallamy writes: “Plants such as the common blue violet (Viola sororia), long dismissed by gardeners as a weed, can be reconstituted as desirable components of the herbaceous layer when their ecosystem functionality is re-evaluated. Violets are the sole larval food source for fritillary butterflies. Eliminating violets eliminates fritillaries, but finding ways to incorporate violets in garden design supports fritillaries.”

sweet violet (Viola odorata)

In my search for the cleistogamous flowers of viola, I dug up a sweet violet (Viola odorata). I was too late to catch it in bloom, but the product of its flowers – round, purple, fuzzy fruits – were revealed as I uprooted the plant. Some of the fruits were already opening, exposing shiny, light brown seeds with prominent, white elaiosomes, there to tempt ants into aiding in their dispersal. I may have missed getting to see what John Eastman calls “violet’s most important flowers,” but the product of these flowers was certainly worth the effort.

Fruits formed from the cleistogamous flowers of sweet violet (Viola odorata)

Up close and personal with the fruit of a cleistogamous flower

The seeds (elaiosomes included) produced by the cleistogamous flower of sweet violet (Viola odorata)

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