The Carnivorous Plant Guild Welcomes a New Member

Photo by Michael Kauffmann (www.backcountrypress.com)

It is a rare but special day when we can add a new plant to the relatively small list of carnivorous plants. It is even more exciting when that plant has been “hiding” in plain sight all this time. Meet the western false asphodel (Triantha occidentalis), a lovely monocot native to nutrient-poor wetlands in western North America.

Triantha occidentalis may seem like an odd carnivorous plant. At first glance, it doesn’t have much in the way of carnivorous adaptations; there are not pitfall traps, no sticky leaves, no snap traps, and no bladders anywhere on the plant. However, if you were to examine this species during its flowering season, you would notice that a lot of small insects seem to get stuck to its flowering stem.

Indeed, the ability of this species to trap insects has been known for quite some time. Even old herbarium collections of T. occidentalis are chock full of insect remains stuck to the scape. Magnify the flowering stem and you will see that it is covered in sticky hairs or trichomes that look a lot like miniature versions of those covering the leaves of more obvious carnivores like sundews (Drosera spp.). Observations such as these led scientists to investigate whether this wonderful little wetland monocot actually benefits from trapping all those arthropods.

Via a series of experiments using isotopes of nitrogen, scientists have revealed that T. occidentalis really does obtain a nutritional boost from the insects it traps. This isn’t a passive process on the part of the plant either. It was also discovered that the plant also secrets the digestive enzyme phosphatase, which helps break down the trapped insects. When the team examined what was going on within the tissues of the plant, they found even more evidence of its carnivorous nature.

Look closely and you can see sticky glands and trapped insects just below the flowers! Photo by Michael Kauffmann (www.backcountrypress.com)

It turns out that 64% of the nitrogen within the plant is obtained via insect digestion, which is comparable to that of other known carnivorous plants such as the aforementioned sundews. Interestingly, it appears that the insect nitrogen the plant obtains is first stored in the flowering stem and fruits but is then transported down into the roots and rhizome underground to be utilized in the following growing season. Why exactly the plant does this requires further investigations. Perhaps by using its flowering stems to obtain nutrients that are in short supply in its wetland habitat, the plant is able to better offset the cost of flowering each year.

By far the most remarkable aspect of this discovery is where carnivory occurs on the plant. With few exceptions, the vast majority of carnivorous plants keep their feeding organs away from their flowers. The leading hypothesis on this suggests that separating feeding and reproduction in space (and sometimes time) helps carnivorous plants avoid catching and digesting their pollinators. However, T. occidentalis does the opposite. It produces all of its sticky hairs very close to its blooming flowers.

Large floral visitors like butterflies appear to be the main pollinators and are too large to get stuck, whereas smaller insects like midges do. Photo by Michael Kauffmann (www.backcountrypress.com)

The key to this apparent morphological contradiction may lie in the stickiness of those hairs. It has been observed that the vast majority of insects trapped on the flowering stems of T. occidentalis are mostly midges and other small insects that don’t function as pollinators for the plant. It is possible that the larger bees and butterflies that could function as true pollinators are simply too large and strong to be trapped. Again, more research is needed to say for sure.

All in all, T. occidentalis represents a unique carnivorous plant whose true nature required solid natural history knowledge and observation to reveal. The fact that we are just learning about its carnivorous habit after all this time suggests that many more potentially carnivorous plants may also be “hiding” in plain sight (I’m looking at you, Silene), waiting for curious minds to collect the necessary data. This is also an exciting discovery from a taxonomic perspective as well. Up until now, all of the known carnivorous monocots hail from the order Poales. Therefore, T. occidentalis represents the first non-Poalean carnivorous monocot! For all these reasons and more, I am excited about future research on this plant and others like it.

Further Reading: [1]

Roadside Seeding and Bluebonnet Genetics

Photo by Adam Baker licensed under CC BY-NC 2.0

The mass blooming of bluebonnets (Lupinus texensis) is truly one of southern North America’s most stunning natural spectacles. Celebrated across its native range, the bluebonnet has greatly benefited from supplemental planting by humans. Indeed, in states like Texas, hundreds of miles of roadsides are seeded with bluebonnets every year and the end result can be spectacular. The popularity of mass seeding of this wonderful species has led some to ask how the practice may be affecting the genetic diversity of the species throughout its range.

Before we get into population genetics, it is worth getting to know this plant a bit better. Bluebonnets are a type of winter annual lupine endemic to southern Texas and northern Mexico. Their highly camouflaged seeds usually begin to germinate late in the fall after enough weathering has weakened the hard seed coat the protects the embryo. Seedlings remain small throughout fall and winter, rarely growing more than a few tiny, palmate leaves. Once spring arrives, growth accelerates.

Within a few short weeks, most individuals will have already pushed up a spike chock full of their characteristic blue and white flowers. Their main pollinators are bumblebees such as the American bumblebee (Bombus pensylvanicus). Once pollinated, plants don’t waste any time producing seeds. Bluebonnets utilize an explosive seed dispersal mechanism, which can be pretty fun to witness in person. As the pods mature, they gradually dry out, creating a lot of tension. Eventually, the tension within the pod becomes so great that the whole structure gives in and explodes, launching seeds as far as 13 feet (4 m) away from the parent plant where they will wait until fall returns.

Photo by Danny Barron licensed under CC BY-NC-ND 2.0

Although 13 feet may sound like a decent distance for a plant the size of a bluebonnet to launch its seeds, it pales in comparison to many other forms of seed dispersal. As such, one would expect bluebonnets within any given population to be more closely related to one another than they would be to bluebonnets growing in other, more distant populations. It is this assumption that led scientists to ask how intentional seeding of bluebonnets may be affecting the genetics of these plants. Before we jump into their findings, I first want to make one thing very clear.

I am in no way disparaging intentional seeding of native plants, especially not by municipalities! I think the practice of seeding with native plants is vital to any environmental management practice we humans undertake. That being said, it is important that we try to understand how any of our actions may be impacting any aspect of biodiversity. Now, onto the research.

By sampling the DNA of both natural and intentionally planted populations across a wide swath of bluebonnet’s endemic range, scientists revealed an intriguing picture of their genetic structure. Simply put, there is surprisingly little. Where they expected to find genetic differences among populations, they instead found a lot of uniformity. It is almost as if populations were mixing their genetic material across the range of the species.

There are a few possible explanations that could explain this pattern. For one, it is possible that estimates of seed dispersal in this species are vastly underestimated. Perhaps seed dispersal events regularly exceed previous estimates of around 13 feet. Along a similar line of reasoning, it is also possible that bluebonnets don’t rely solely on ballistics to get their seeds out into the environment. If birds or mammals occasionally move seeds long distances, this could eventually lead to genetic mixing among different populations. However, such possibilities are unlikely given the nature of bluebonnet seeds and the fact that animals are far more likely to act as seed predators for bluebonnets than seed dispersers.

Scientists have also put forth the possibility that bluebonnets in both natural and cultivate populations simply haven’t been isolated long enough for genetic differences to emerge among populations. However, this does not explain why there is so few genetic differences among widely separated natural populations.

The most likely reason why bluebonnets are so alike genetically is intentional planting. Though plenty of effort is put into ensuring that bluebonnet plantings are done using seeds sourced within 124 miles (200 km) from the planting site, we simply can’t rule out the idea that genes from individuals sourced from cultivation are not completely swamping the gene pools of wild populations as they are sowed along roadsides and into other planting projects.

To be fair, though these findings are compelling, we can’t necessarily jump to any conclusions as to whether such genetic swamping is a net negative or net positive for bluebonnets across their range. The scientists involved with the study do mention that swamping of fractured wild bluebonnet populations with genes of cultivated individuals could prove beneficial for the species, especially as the impact of human development continues to increase. It is possible that cultivated individuals that are selected because they perform well in human-dominated environments are introducing genes into wild populations that may allow them to cope with the increased human disturbances.

The alternative argument to that point is that we are swamping wild populations with potentially deleterious alleles at a faster rate than natural selection can purge them from the population. If this is the case, we may see a gradual decline in some populations that grow more and more out of sync with their local environment.

Though it is far too early to draw any hard fast conclusions about the impacts of genetic swamping, the genetic patterns that have been uncovered among bluebonnets are important to document. Now that we know that genetic diversity is low across populations, we can begin to dive deeper into both the mechanisms that created said patterns and their impacts on various populations. Once again, this is not an argument against intentional seeding and planting of native plants. Instead, it is a nice reminder that even the best intentions can have vast and unintended consequences that we need to study in more detail.

Further Reading: [1]

Some Magnolia Flowers Have Built-In Heaters

Magnolia denudata. Photo by 阿橋 HQ licensed under CC BY-SA 2.0

There are a lot of reasons to like magnolias and floral thermogenesis is one of them. That’s right, the flowers of a surprising amount of magnolia species produce their own heat! Although much more work is needed to understand the mechanisms involved in heat generation in these trees, research suggests that it all centers on pollination.

Magnolias have a deep evolutionary history, having arose on this planet some 95+ million years ago. Earth was a very different place back then. For one, familiar insect pollinators like bees had not evolved yet. As such, the basic anatomy of magnolia flowers was in place long before bees could work as a selective pressure in pollination. What were abundant back then were beetles and it is thought that throughout their history, beetles have served as the dominant pollinators for most species. Indeed, even today, beetles dominate the magnolia pollination scene.

Magnolia sprengeri. Photo by Aleš Smrdel licensed under CC BY-NC 2.0

Beetles are generally not visiting flowers for nectar. They are instead after the protein-rich pollen within each anther. It seems that when the anthers are mature, beetles are very willing to spend time munching away within each flower, however, keeping their attention during the female phase of the flower is a bit trickier. Because there are no rewards for visiting a magnolia flower during its female phase, evolution has provided some species with an interesting trick. This is where heat comes in.

Though it varies from species to species, thermogenic magnolias produce combinations of scented oils that various beetles species find irresistible. That is, if they can pick up the odor against the backdrop of all the other enticing scents a forest has to offer. By observing floral development in species like Magnolia sprengeri, researchers have found that as the flowers heat up, the scented oils produced by the flower begin to volatilize. In doing so, the scent is dispersed over a much greater area than it would be without heat.

Magnolia tamaulipana. Photo by James Gaither licensed under CC BY-NC-ND 2.0

Unlike some other thermogenic plants, heat production in magnolia flowers doesn’t appear to be constant. Instead, flowers experience periodic bursts of heat that can see them reaching temperatures as high as 5°C warmer than ambient temperatures. These peaks in heat production just to happen to coincide with the receptivity of male and female organs. Also, only half of the process is considered an “honest signal” to beetles. During the male phase, the beetles will find plenty of pollen to eat. However, during the female phase, the scent belies the fact that beetles will find no reward at all. This has led to the conclusion that the non-rewarding female phase of the magnolia flower is essentially mimicking the rewarding male phase in order to ensure some cross pollination without wasting any energy on additional rewards.

The timing of heat production also changes depending on the species of beetle and their feeding habits. For species like the aforementioned M. sprengeri, which is pollinated by beetles that are active during the day, heat and scent production only occur when the sun is up. Alternatively, for species like M. tamaulipana whose beetle pollinators are nocturnal, heat and scent production only occur at night. Researchers also think that seasonal climate plays a role as well, suggesting that heat itself may be its own form of pollinator reward in some species. Many of the thermogenic magnolias bloom in the early spring when temperatures are relatively low. It is likely that, aside from pollen, beetles may also be seeking a warm spot to rest.

Personally, I was surprised to learn just how many different magnolias are capable of producing heat in their flowers. When I first learned of this phenomenon, I thought it was unique to M. sprengeri but I was wrong. We still have a lot to learn about this process but research like this just goes to show you that even familiar genera can hold many surprises for those curious enough to seek them out.

Further Reading: [1] [2] [3] [4] [5]

How Fungus Gnats Maintain Jack-in-the-pulpits

[SOURCE]

There are a variety of ways that the boundaries between species are maintained in nature. Among plants, some of the best studied examples include geographic distances, differences in flowering phenology, and pollinator specificity. The ability of pollinators to maintain species boundaries is of particular interest to scientists as it provides excellent examples of how multiple species can coexist in a given area without hybridizing. I recent study based out of Japan aimed to investigate pollinator specificity among fungus gnats and five species of Jack-in-the-pulpit (Arisaema spp.) and found that pollinator isolation is indeed a very strong force in maintaining species identity among these aroids, especially in the wake of forest disturbance.

Fungus gnats are the bane of many a houseplant grower. However, in nature, they play many important ecological roles. Pollination is one of the most underappreciated of these roles. Though woefully understudied compared to other pollination systems, scientific appreciation and understanding of fungus gnat pollination is growing. Studying such pollination systems is not an easy task. Fungus gnats are small and their behavior can be very difficult to observe in the wild. Luckily, Jack-in-the-pulpits often hold floral visitors captive for a period of time, allowing more opportunities for data collection.

By studying the number and identity of floral visitors among 5 species of Jack-in-the-pulpit native to Japan, researchers were able to paint a very interesting picture of pollinator specificity. It turns out, there is very little overlap among which fungus gnats visit which Jack-in-the-pulpit species. Though researchers did not analyze what exactly attracts a particular species of fungus gnat to a particular species of Jack-in-the-pulpit, evidence from other systems suggests it has something to do with scent.

Like many of their aroid cousins, Jack-in-the-pulpits produce complex scent cues that can mimicking everything from a potential food source to a nice place to mate and lay eggs. Fooled by these scents, pollinators investigate the blooms, picking up and (hopefully) depositing pollen in the process. One of the great benefits of pollinator specificity is that it greatly increases the chances that pollen will end up on a member of the same species, thus reducing the chances of wasted pollen or hybridization.

Still, this is not to say that fungus gnats are solely responsible for maintaining boundaries among these 5 Jack-in-the-pulpit species. Indeed, geography and flowering time also play a role. Under ideal conditions, each of the 5 Jack-in-the-pulpit species they studied tend to grow in different habitats. Some prefer lowland forests whereas others prefer growing at higher elevations. Similarly, each species tends to flower at different times, which means fungus gnats have few other options but to visit those blooms. However, such barriers quickly break down when these habitats are disturbed.

Forest degradation and logging can suddenly force many plant species with different habitat preferences into close proximity with one another. Moreover, some stressed plants will begin to flower at different times, increasing the overlap between blooming periods and potentially allowing more hybridization to occur if their pollinators begin visiting members of other species. This is where the strength of fungus gnat fidelity comes into play. By examining different Jack-in-the-pulpit species flowering in close proximity to one another, the team was able to show that fungus gnats that prefer or even specialize on one species of Jack-in-the-pulpit are not very likely to visit the inflorescence of a different species. Thanks to these preferences, it appears that, thanks to their fungus gnat partners, these Jack-in-the-pulpit species can continue to maintain species boundaries even in the face of disturbance.

All of this is not to say that disturbance can’t still affect species boundaries among these plants. The researchers were quick to note that forest disturbances affect more than just the plants. When a forest is logged or experiences too much pressure from over-abundant herbivores such as deer, the forest floor dries out a lot quicker. Because fungus gnats require high humidity and soil moisture to survive and reproduce, a drying forest can severely impact fungus gnat diversity. If the number of fungus gnat species declines, there is a strong change that these specific plant-pollinator interactions can begin to break down. It is hard to say what affect this could have on these Jack-in-the-pulpit species but a lack of pollinators is rarely a good thing. Certainly more research is needed.

Photo Credit: [1]

Further Reading: [1]

Krassilovia: An Amazing Cretaceous Conifer

Reconstructing extinct organisms based on fossils is no simple task. Rarely do paleontologists find complete specimens. More often, reconstructions are based on fragments of individuals found either near one another or at least in similar rock formations. This is especially true for plants as their growth habits frequently result in fragmentary fossilization. As such, fossilized plant remains of a single species are often described as distinct species until subsequent detective work pieces together a more complete picture.

Such was the case for the fossil remains of what were described as Krassilovia mongolica and Podozamites harrisii. Hailing from the Early Cretaceous (some 100-120 million years ago), Krassilovia was only known from oddly spiny cone scales and Podozamites was only known from strap-shaped leaves found in a remote region of Mongolia. Little evidence existed to suggest they belonged to the same plant. That is, until these structures were analyzed using scanning electron micrographs.

(A–C) Articulated seed cones, (D) Isolated cone axis, (E) Incomplete leafy shoot showing a cluster of three attached leaves, (F) Three detached strap-shaped leaves, G) Detail of A showing tightly imbricate interlocking bract-scale complexes, (H) Detail of leaf apex showing converging veins, (I) Three isolated bract-scale complexes showing abaxial (top) and adaxial (bottom) surfaces, (J) Two isolated seeds showing narrow wings. [SOURCE]

These fossilized plant remains were preserved in such detail that microscopic anatomical features such as stomata were visible under magnification. By studying the remains of these plants as well as others, scientists discovered some amazing similarities in the stomata of Krassilovia and Podozamites. Unlike other plant remains associated with those formations, the Krassilovia cone scales and Podozamites leaves shared the exact same stomate morphology. Though not without some uncertainty, the odds that these two associated structures would share this unique morphological trait by chance is slim and suggests that these are indeed parts of the same plant.

The amazing discoveries do not end with stomata either. After countless hours of searching, fully articulated Krassilovia cones were eventually discovered, which finally put the strange spiky cone scales into context. It turns out those spiked scales interlocked with one another, with the two bottom spikes of one scale interlocking with the three top spikes of the scale below it. In life, such interlocking may have helped protect the developing seeds within until they had matured enough to be released. Also, the sheer volume of cone scales coupled with other minute anatomical details I won’t go into here indicate that, similar to Abies and Cedrus cones, Krassilovia cones completely fell apart when fully ripe.

Though not related, the cone scales of the extinct Krassilovia (left) show similarities with the cone scales of modern day Cryptomeria species (right).

Interestingly, the ability to resolve microscopic structures in these fossils has also provided insights into some modern day taxonomic confusion. It turns out that Krassilovia shares many minute anatomical similarities with present day Gnetales. Gnetales really challenge our perception of gymnosperms and their superficial resemblance to angiosperms have led many to suggest that they represent a clade that is sister to flowering plants. However, more recent molecular work has placed the extant members of Gnetales as sister to the pines. Evidence of shared morphological features between extinct conifers like Krassilovia and modern day Gnetales add some interesting support to this hypothesis. Until more concrete evidence is described and analyzed, the true evolutionary relationships among these groups will remain the object of heated debate for the foreseeable fture.

What we can say is that Krassilovia mongolica was one remarkable conifer. Its unique morphology clearly demonstrates that conifers were once far more diverse in form and function than they are currently. Even the habitat in which Krassilovia once lived is not the kind of place you can find thriving conifer communities today. Krassilovia once grew in a swampy habitat. However, whereas only a few extant conifers enjoy swamps, Krassilovia once shared its habitat with a wide variety of conifer species, the likes of which we are only just beginning to appreciate. I for one am extremely excited to see what new fossil discoveries will uncover in the future.

LISTEN TO EPISODE 300 OF THE IN DEFENSE OF PLANTS PODCAST TO LEARN MORE ABOUT THIS FOSSIL AND THE ECOSYSTEM IN WHICH IT ONCE EXISTED.

Photo Credits: [1] [2] [3]

Further Reading: [1]

Insect Egg Killers

© Copyright Walter Baxter and licensed under CC BY-SA 2.0

Plants and herbivores are engaged in an evolutionary arms race hundreds of millions of years in the making. As plants evolve mechanisms to avoid being eaten, herbivores evolve means of overcoming those defenses. Our understanding of these dynamics is vast but largely focused on the actual act of an organism consuming plant tissues. However, there is growing evidence that plants can take action against herbivores before they are even born.

Taking out herbivores before they even have a chance to munch on a plant seems like a pretty effective means of defense. Indeed, for a growing number of plant species, this starts with the ability to detect insect eggs deposited on or in leaves and stems. As Griese and colleagues put it in their 2020 paper, “Every insect egg being detected and killed, is one less herbivorous larva or adult insect feeding on the plant in the near future.” Amazingly, such early detection and destruction has been found in a variety of plant lineages from conifers to monocots and eudicots.

Gumosis in cherries is a form of defense. Photo by Rosser1954/Public Domain

There are a few different ways plants go about destroying the eggs of herbivores. For instance, upon detecting eggs on their leaves, some mustards will begin to produce volatile compounds that attract parasitoid wasps that lay their eggs on or in the herbivore’s eggs. For other plants, killing herbivore eggs involves the production of special egg-killing compounds. Research on cherry trees (Prunus spp.) has shown that as cicadas push their ovipositor into a twig, the damage induces the production of a sticky gum that floods the egg chamber and prevents the eggs from hatching. Similarly, resin ducts full of insect-killing compounds within the rinds of mangoes will rupture when female flies insert their ovipositor, killing any eggs that are deposited within.

One of the coolest and, dare I say, most badass ways of taking out herbivore eggs can be seen in a variety of plants including mustards, beans, potatoes, and even relatives of the milkweeds and involves a bit of sacrifice on the plant end of things. Upon detecting moth or butterfly eggs, leaf cells situated directly beneath the eggs initiate a defense mechanism called the “hypersensitive response.” Though normally induced by pathogenic microbes, the hypersensitive response appears to work quite well at killing off any eggs that are laid.

“Leaves from B. nigra treated with egg wash of different butterfly species and controls inducing or not a HR-like necrosis. Pieris brassicae (P. b.), P. mannii, (P. m.), P. napi (P. n.), and P. rapae (P. r.) and Anthocharis cardamines (A. c.) induce a strong HR-like necrosis. Egg wash of G. rhamni (G. r.) and Colias sp. (C. sp.) induces a very faint response resembling a chlorosis and does not fit into the established scoring system (faintness indicates 1, but showing up on both sides of the leaf indicates 2).” [SOURCE]

Once eggs are detected, a signalling pathway within the leaf ramps up the production of highly reactive molecules called reactive oxygen species. These compounds effectively kill all of the cells upon which the butterfly eggs sit. The death of those plant cells is thought to change the microclimate directly around the eggs, causing them to either dry up or fall off. These forms of plant defense don’t stop once the eggs have been killed either. There is evidence to suggest that the hypersensitive response to insect eggs also induces these plants to begin producing even more anti-feeding compounds, thus protecting the plants from any herbivores that result from any eggs that weren’t killed.

Plants may be sessile but they are certainly not helpless. Defense mechanisms like these just go to show you how good plants can be at protecting themselves. Certainly, the closer we look at interactions like these, the more we will discover about the amazing world of plant defenses.

Photo Credits: [1] [2] [3]

Further Reading: [1] [2]

How Overharvesting is Changing an Alpine Plant in China

[Source]

We are increasingly becoming aware of the importance of camouflage in the plant kingdom. By blending in with their surroundings, some plants are able to avoid attention from hungry herbivores. Amazingly, a recent investigation in Hengduan Mountains of southwest China has revealed that it can also help plants avoid being harvested for the herbal trade.

Fritillaria delavayi is a tiny, alpine plant that grows among the rocky scree at high elevations in the Hengduan Mountains. It is a slow growing plant that can take upwards of 5 years to produce its first flower. It is also variable in its overall coloration. Some populations consist of plants with green leaves and bright yellow flowers, whereas other populations consist of plants with leaves and flowers in various shades of brown that cause them to blend in with the surrounding rock.

Variation in plant coloration is not terribly novel. Many plant populations can differ from one another in their overall appearance, however, there seems to be a pattern among F. delavayi. It would seem that plants growing in more accessible places tend to be brown whereas plants growing in less accessible places tend to be green and yellow. Interestingly, herbivores don’t seem to explain these differences. Indeed, F. delavayi is chock full of toxic alkaloids that deter what few herbivores exist at such high elevations. Nonetheless, it is the presence of those alkaloids that explain why populations differ so much from one area to the next.

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(A and B) Normal green individuals in populations with low harvest pressure. (C and D) Camouflaged individuals in populations with high harvest pressure [SOURCE]

Because of their purported medicinal value, the demand for F. delavayi bulbs has greatly increased over time. Each year more and more people are heading to these mountains to harvest the plants to sell them in herbal markets. This led a team of researchers to investigate if harvesting by humans could explain color variations among populations.

Amazingly, it did! By looking at ease of access and harvesting, researchers found that plants that were in hard to reach areas or places where harvesting is difficult were more likely to have bright green leaves and yellow flowers. By contrast, plants in easy to reach locations that were not difficult to harvest were more likely to be brown. The researchers even went the extra mile and tested how easily plants of each coloration could be found by humans. Not surprisingly, it took humans much longer to find brown plants that it did for them to find green plants.

Based on their findings, researchers have concluded that harvesting pressures are changing F. delavayi populations in the Hengduan Mountains. Because they are harder to detect and therefore less likely to be harvested, plants sporting the brown coloration are far more likely to survive and reproduce in highly trafficked areas, resulting in an increase in camouflaged offspring. Alternatively, populations growing in hard to reach areas do not experience such heavy selection pressures and can continue to safely sport bright green leaves and yellow flowers. It just goes to show you that human activities can often have unintended consequences for other species. This research also raises the question of how humans have shaped the defensive strategies of other highly sought after plant species.

Photo Credits: [1] [2]

Further Reading: [1]

A Remarkable Floral Radiation on Hawai'i

Ohaha (Brighamia rockii)

Hawai’i is home to so many interesting species of plants, many of which are found nowhere else in the world. One group however, stands out among the rest in that it represents the largest plant radiation not just in Hawai’i, but on any island archipelago!

I am of course talking about the Hawaiian lobelioids (Campanulaceae). Many of you will be familiar with members of the genus Lobelia, which include the lovely cardinal flower (Lobelia cardinalis) and the great blue lobelia (Lobelia siphilitica), but the 6 genera that comprise the Hawaiian radiation are something quite different altogether.

'Oha Wai (Clermontia samuelii). Photo by Forest and Kim Starr licensed under CC BY 2.0

Numbering roughly 125 species in total (in addition to many extinct species), it was long thought that the diversity of Hawaiian lobelioids were the result of at least 3 separate dispersal events. Thanks to recent DNA analysis, it is now believed that all 6 genera are the result of one single dispersal event by a lobelia-like ancestor. This may seem ridiculous but when you consider the fact that this invasion happened back when Gardner Pinnacles and French Frigate Shoals were actual islands and none of the extant islands even existed, then you can begin to grasp the time scales involved that produced such a drastic and varied radiation.

Delissea sp.

Sadly, like countless Hawaiian endemics, the invasion of the human species has spelled disaster. Hawaiian endemics are declining at an alarming rate due to threats like introduced pigs and rats that eat seeds, devour seedlings, and even go as far as to chew right through the stems of adult plants. To make matters worse, many species evolved to a specific suite of pollinators.

ʻŌlulu (Brighamia insignis)

Take, for instance, the case of the ʻŌlulu (Brighamia insignis). It is believed to have evolved a pollination syndrome with a species of sphinx moth known as the fabulous green sphinx moth (Tinostoma smaragditis), which is also believed to be extinct. Similarly, the ʻŌhā wai nui (Clermontia arborescens) evolved for pollination by the island's endemic honey creepers. Due to avian malaria and other human impacts, many honey creepers are endangered and some have already gone extinct. Without their pollinators, many of these lobelioids are doomed to slow extinction if they haven't disappeared already.

While it may be too late to bring back species that have likely gone extinct, that doesn’t mean conservation of these incredible plants is off the table. Indeed, many efforts are being put forth by institutions like the National Tropical Botanical Garden and the Chicago Botanic Gardens to help conserve and restore some of these species. Along the way, the Hawaiian lobelioids are teaching us important and timely lessons on the need for understanding and protecting all pieces of Earth’s ecosystems, rather than individual parts in isolation.

LISTEN TO EPISODE 291 OF THE IN DEFENSE OF PLANTS PODCAST TO LEARN MORE ABOUT LOBELIOID CONSERVATION IN HAWAI’I

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2]

Floral Trickery of the Bat Plants

Photo by Geoff McKay licensed under CC BY 2.0

Bat plants (genus Tacca) are bizarre-looking plants. Their nondescript appearance when not in flower enshrouds the extravagant and, dare I say macabre appearance of their blooms. The inflorescence of this genus is something to marvel at. The flowers are borne above sets of large, conspicuous bracts and numerous whisker-like bracteoles. Despite their unique appearance and popularity among plant collectors, the pollination strategies utilized by the roughly 20 species of bat plants have received surprisingly little attention over the years.

Bat plants are most at home in the shaded, humid understories of tropical rainforests around the globe (though there are a couple exceptions to this rule). Amazingly, these plants are members of the yam family (Dioscoreaceae) and are thought to be closely related to the equally bizarre Burmanniaceae, a family comprised entirely of oddball parasites. Taxonomic affinities aside, there is no denying that bat plants produce truly unique inflorescences and many a hypotheses has been put forth to explain the function of their peculiar floral displays.

The white bat plant (Tacca integrifolia). Photo by MaX Fulcher licensed under CC BY-NC-SA 2.0

The black bat plant (Tacca chantrieri). Photo by Hazel licensed under CC BY-SA 2.0

The most common of these is that the flowers are an example of sapromyiophily and thus mimic a rotting corpse in both smell and appearance as a means of attracting carrion flies. However, despite plenty of speculation, such hypotheses have largely gone untested. It wasn’t until fairly recently that anyone put forth an attempt to observe pollination of these plants in their natural habitats.

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A) T. leontopetaloides; (B) T. plantaginea; (C) T. parkeri; (D) T. palmatifida; (E) T. palmata; (F) T. subflabellata; (G) T. integrifoliafrom; (H) T. integrifoliafrom; (I) T. ampliplacenta; (J) T. chantrieri. [SOURCE]

A 2005 study done in South Yunnan province, China found that almost nothing visited the flowers of Tacca chantrieri. Despite the presence of numerous potential pollinators, only a handful of small, stingless bees paid any attention to these obvious floral cues. This led the authors to suggest that most bat plants are self-pollinated. Indeed, genetic analysis of different populations of T. chantrieri helped bolster this conclusion by demonstrating that there is very little evidence of genetic transfer between T. chantrieri populations. Yet, this is far from a smoking gun. Strong genetic structuring among populations could simply mean that pollinators aren’t moving very far. Also, if most bat plants simply opt for fertilizing their own blooms, why has this genus maintained such elaborate floral morphology? Needless to say, more work was needed.

Luckily, a recent study from Malaysia has made great strides in our understanding of the sex lives of these plants. By observing 7 different species of bat plant in the wild, researchers were able to collect plenty of data on bat plant pollination. It turns out that the flowers of these 7 species are quite popular with insects. Bat plant floral visitors in their study included everything from tiny, stingless bees to ants, beetles, and weevils. However, the most common floral visitors for most bat plant species were small, biting midges. This is where things get very interesting.

(A–C) Female Forcipomyia biting midge. Arrows indicating pollen grains. [SOURCE]

As their common name suggests, biting midges are most famous for biting other animals. Though they will drink nectar, female biting midges need lots of protein to successfully produce eggs. They meet their protein needs by drinking the blood of insects and mammals. Of the biting midges that most frequently visited bat plant flowers, the most common hail from two groups known to feed exclusively on mammalian blood. Finding these biting midges in high numbers on bat plant flowers raises the question of what they stand to gain from these strange-looking blooms.

The conclusion the authors came to was that bat plant blooms are using a bit of trickery to lure in female midges. They hypothesize that the color patterns of the bracts and flickering motion of whisker-like bracteoles simulates the movements of mammals that the midges normally feed on. It is also possible that bat plant flowers emit volatile scents that enhance this mimicry, though more work is needed to say for sure. What the researchers do know is that the behavior of female biting midges upon visiting a flower is enough to pick up and deposit plenty of pollen as they search for a blood meal that doesn’t exist. How common this floral ruse is among the remaining species is yet to be determined but the similarities in inflorescence structure among members of this genus suggest similar tricks are being played on pollinators wherever bat plants grow.

Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2]

Floral Pigments in a Changing World

Photo by moggafogga licensed under CC BY-NC-ND 2.0

Flowers paint the world in a dazzling array of colors. Some of these we can see and others we cannot. Many plants paint their blooms in special pigments that absorb ultraviolet light, revealing intriguing patterns to pollinators like bees and even some birds that can see well into the UV part of the electromagnetic spectrum. UV absorbing pigments do more than attract pollinators. They can also protect sensitive reproductive organs from UV radiation. By studying these pigments, scientists are finding that many different plants are changing their floral displays in response to changes in their environment.

Growing up I heard a lot about the hole in the ozone layer. Prior to the 1980’s humans were pumping massive quantities of ozone-depleting chemicals such as halocarbon refrigerants, solvents, and chlorofluorocarbons (CFCs) into the atmosphere, creating a massive hole in the ozone layer. Though ozone depletion has improved markedly thanks to regulations placed on these chemicals, it doesn’t mean that life has not had to adapt. As you may remember from your grade school science class, Earth’s ozone layer helps protect life from the damaging effects of ultraviolet radiation. UV radiation damages sensitive biological molecules like DNA so it is in any organisms best interest to minimize its impacts.

UV absorbing pigments in floral tissues can do just that. In addition to attracting pollinators, these pigments act as a sort of sun screen, reducing the likelihood of damaging mutations. By studying 1,238 herbarium specimens collected between 1941 and 2017 representing 42 different species, scientists discovered a startling change in the amount of UV pigments produced in their flowers.

Exemplary images for a species with anthers exposed to ambient conditions, Potentilla crantzii (A–C) and a species with anthers protected by floral tissue Mimulus guttatus (D–F). Darker petal areas possess UV-absorbing compounds whereas lighter areas are UV reflective and lack UV-absorbing compounds. (B) and (E) display a reduced area of UV-absorbing pigmentation on petals compared to (C) and (F). Arrows in (E) and (F) highlight differences in pigment distribution on the lower petal lobe of M. guttatus. [SOURCE]

Across North America, Europe, and Australia, the amount of UV pigments produced in the flowers tended to increase by an average of 2% per year from 1941 to 2017. These increases in UV pigments occurred in tandem with decreases in the ozone layer. It would appear that, to protect their reproductive organs from harmful UV rays, many plants were increasing these protective pigments.

However, changes in UV pigments were not uniform across all the species they examined. Plants that produce saucer or cup-shaped flowers experienced the greatest increases in UV pigments. This makes complete sense as this sort of floral morphology exposes the reproductive organs directly to the sun’s rays. The pattern reversed when scientists examined flowers whose petals enclose the reproductive organs such as those seen in bladderworts (Utricularia spp.). UV pigments in flowers that conceal their reproductive organs actually decreased over this time period.

The reason for this comes down to a trade off inherent in UV pigments. Absorbing UV radiation is a great way to reduce its impact on sensitive tissues but it also leads to increased temperatures. For plants that enclose their reproductive organs within their petals, this can lead to overheating. Heat can also be very damaging to floral structures so it makes complete sense that species with this type of floral morphology would demonstrate the opposite pattern. By reducing the amount of UV absorbing pigments in their flowers, plants like bladderworts are able to minimize the effect of increased radiation and temperatures that occurred over this time period.

How changes in floral pigments are affecting pollination rates for these plants is another story entirely. Because UV pigments also help attract certain pollinators, there is always a chance that the appearance of some of these flowers may also be changing over time. Now that we know this is occurring across a wide range of unrelated plants, research can now be aimed at tackling questions like this.

Photo Credits: [1] [2]

Further Reading: [1]

What's the deal with nodding flowers?

While working in the garden the other day, I noticed that some of the nodding onion (Allium cernuum) we planted last year had finally come into bloom. I must have spent the good part of an hour watching bees pay a visit to their downward pointing flowers. I have seen a lot of onion species in bloom before, but this particular native is the only one that I know of personally that orients its flowers facing the ground. This got me to thinking about floral orientation. A lot of plants produce flowers that face the ground but many more do not. Why is there such variety among the orientation of flowers?

As always, I hit the literature. It turns out, many scientists have set out to investigate the function of floral orientation. What immediately stuck out to me is just how many different flowering plant lineages boast species whose flowers face down rather than out or up. I knew instantly that with so much variety in lineage, habitat, and pollination strategies, the answer wasn’t going to be simple or straight forward. Indeed, each investigation I read about seemed to end in a slightly different conclusion. Still, there were enough patterns among the results and conclusions to make some general statements about the subject.

The nodding flowers of the Michigan lily (Lilium michiganense)

We often find plants with downward facing flowers in harsh climates. Harsh can mean a lot of different things depending on the plant and region in question but take, for instance, the case of the genus Cremanthodium. This interesting group of asters resemble sunflowers in the basic appearance of their flowers but the plants themselves are vastly different in overall growth habit. Many hail from alpine environments in Asia and possess a short stature and flowers that face the ground instead of the sun. Research on the reproductive habits of these plants has revealed that the downward orientation of their flowers helps protect the sensitive reproductive parts from solar radiation and rain.

Growing at high elevations exposes these plants to lots of UV radiation and plenty of storms. If flowers were to orient towards the sky, rain could wash away pollen and UV radiation could really hinder successful reproduction. By facing the ground, the flowers are able to avoid these potentially harmful effects altogether. Similar results have been found for other members of the aster family in the genus Culcitium growing in alpine habitats in the Andes. Here again we see that downward pointing flowers help protect the sensitive reproductive parts from rain, snow, and too much sun.

The recently described Cremanthodium wumengshanicum growing at elevation in Yunnan, China. [SOURCE]

However, its not just the elements that have selected for downward pointing flowers. As you can probably imagine, pollinators also play a role in floral orientation. While watching bee visit our nodding onions, I noticed that they seem to be much better able to land on and collect pollen and nectar from downward pointing flowers than any of the flies I see attempting visits. Indeed, floral orientation can have a massive impact on what kinds of pollinators are able to effectively visit a flower.

A great example of this can be seen in members of the genus Zaluzianskya. Some species present their flowers horizontally or vertically while others present their flowers facing the ground. By comparing the visitors that frequent each species, researchers found that orientation matters. Upright or horizontally facing flowers were mostly visited by hawkmoths. Hawkmoths hover while they feed, which means they have a much harder time visiting downward facing flowers. By presenting their flowers in different orientations, the various species of Zaluzianskya ensure that only specific pollinators are able to access their rewards and thus achieve pollination. As such, upright, horizontal, and downward flowering species remain reproductively isolated from one another. Similar results have also been found in genera such as the afore mentioned Culcitium as well as some Commelina and Nicotiana.

Investigating pollinator visitation among different species of Zaluzianskya. [SOURCE]

I am sure many more examples exist out there but alas, I only have so much time to pursue my random curiosities these days. Nonetheless, what started as a fun observation in the garden turned into an entertaining dive into ideas that I had not given too much thought to before. What seems like a funny quirk of anatomy turns out to have massive implications for where plants are able to grow and how they are able to reproduce and all of these factors and more have shaped flowering plant evolution over time. Not bad for a few hours in the garden.

Photo Credits: [1] [2] [3]

Further Reading: [1] [2] [3] [4] [5] [6]

Drunken Pollinators & Chemical Trickery: Musings on the Complex Floral Chemistry of a Generalist Orchid

There was a time when I thought that all orchids were finicky botanical jewels, destined to perish at the slightest disturbance. Certainly many species fit this description to some degree, but more often these days I am appreciating the role disturbance can play in maintaining many orchid populations. Seeing various genera like Platanthera or Goodyera thriving along trails and old dirt roads, lawn orchids (Zeuxine strateumatica) growing in manicured lawns, or even various Pleurothallids growing on water pipes in the mountains of Panama has opened my eyes to the diversity of ecological strategies this massive family of flowering plants employs.

Of the examples mentioned above, none can hold a candle to the hardiness of the broad-leaved helleborine orchid (Epipactis helleborine) when it comes to thriving in disturbed habitats. Originally native throughout much of Europe, North Africa, and Asia, this strangely beautiful orchid can now be found growing throughout many temperate and sub-tropical regions of the world. Indeed, this is one species of orchid that has greatly benefited from human disturbance. In fact, I more often see this orchid growing in and around cities and along roadsides than I do in natural settings (not to say it isn’t there too). In many areas here in North America, the broad-leaved helleborine orchid has gone from a naturalized oddity into a full blown invasive.

Much of its success in conquering new and often highly disturbed territory has to do with its relationship with mycorrhizal fungi. Like all orchids, the broad-leaved helleborine orchid requires fungi for germination and growth, relying on the symbiotic relationship into maturity. Without mycorrhizal fungi, these orchids could not survive. However, while many orchids seem to be picky about the fungi they will partner with, the broad-leaved helleborine is something of a generalist in this regard. At least one study in Europe was able to demonstrate that over 60 distinct groups of mycorrhizal fungi were able to partner with this orchid. By opening itself up to a wider variety of fungal partners, the broad-leaved helleborine orchid is able to live in places where pickier orchids cannot.

Another key to this orchids success has to do with its pollination strategy. Here again we see that being a generalist comes with serious advantages. Though wasps are thought to be the most effective pollinators, myriad other insects from various kinds of flies to beetles and butterflies will visit these blooms. How is it that this orchid has become to appealing to such a wide variety of insects? The answer is chemistry.

The broad-leaved helleborine orchid is something of a skilled chemist. When scientists analyzed the nectar produced in the cup-shaped lip of the flower, they found a diverse array of chemicals, many of which lend to some incredible insect interactions. For starters, highly scented compounds such as vanillin (the compound responsible for the vanilla scent and flavor of Vanilla orchids) are produced in the nectar, which certainly attracts many different kinds of insects. There is also evidence of some floral mimicry going on as well.

Scientists found a group of chemicals called kairomones in broad-leaved helleborine nectar, which are very similar to aphid alarm pheromones. When released by aphids, they warn nearby kin that predators are in the area. In one sense, the production of these compounds in the nectar may serve to ward off aphids looking for a new place to feed. However, these chemicals also appear to function as pollinator attractants. For aphid predators like hoverflies, these pheromones act as a dinner bell, signalling good egg laying sites for gravid female hoverflies whose larvae gorge themselves on aphids as they grow. It just so happens that hoverflies also serve as important pollinators for the broad-leaved helleborine orchid.

A series of compounds broadly classified as green-leaf volatiles were found in the nectar as well. Many plants produce these compounds when their leaves are damaged by insect feeding. Like the aphid example above, green-leaf volatiles signal to nearby predatory insects that plump herbivores are nearby. For instance, when the caterpillars of the cabbage white butterfly feed on cabbage plants, green-leaf volatiles attract wasps, which quickly set to work eating the caterpillars, relieving the plant of its herbivores in the process. As previously mentioned, wasps are thought to be the main pollinators for this orchid so attracting them makes sense. However, attracting pollinators using chemical trickery can be risky. What happens when a pollinator shows up and realizes there is no plump aphid or caterpillar to eat?

The answer to this comes from a series of other compounds produced in this orchid’s nectar. Few insects will turn down a sugary meal, and indeed, many visitors end up sipping some broad-leaved helleborine nectar. Sit back and watch and it won’t take long to realize that these insects appear to quickly become intoxicated. Their behavior becomes sluggish and they generally bumble around the flowers until they sober up and fly off. This is not happenstance. This orchid actively gets its pollinators wasted, but how?

Along with the chemicals we already touched on, scientists have also found a plethora of narcotics in broad-leaved helleborine nectar. These include various types of alcohols and even chemicals similar to that of opioids like Oxycodone. Now, some have argued that the alcohols are not the product of the plant but rather the result of fermentation by yeasts and bacteria living within the nectar. However, the presence of different antimicrobial compounds coupled with the sheer concentrations of alcohols within the nectar appear to discount this hypothesis and point to the plant as the sole creator. Nonetheless, after a few sips of this narcotic concoction, insects like wasps and flies spend a lot more time at each flower than they would if they remained sober the whole time. This has led to the suggestion that narcotics help improve the likelihood of successful pollination.

Indeed, the broad-leaved helleborine orchid seems to have no issues with sex. Most plants produce a bountiful crop of seed-laden fruits each summer. In fact, it has been found that plants growing in areas of high human disturbance tend to set more seed than plants growing in natural areas. Scientists suggest this is due to the wide variety of pollinators that are attracted to the complex nectar. Human environments like cities tend to have a different and sometimes more varied suite of insects than more rural areas, meaning there are more opportunities for run ins with potential pollinators.

The broad-leaved helleborine orchid stands as an example of the complexities of the orchid family. Few orchids are as generalist in their ecology as this species. Its ability to grow where others can’t while taking advantage of a variety of pollinators has lent to the extreme success of this species world wide.

Photo Credit: [1]

Further Reading: [1] [2] [3] [4] [5] [6]

How Trees Are Shaping Treehoppers

Photo by Judy Gallagher licensed under CC BY-ND 2.0.

The sessile nature of plants means that they are strongly shaped by their environment. Natural selection is constantly at work on plants but that doesn’t mean that plants don’t shape their environment as well. When I think about the impact of plants on resident animal communities, I am always reminded of a quote by artist Terence McKenna, “Animals are something invented by plants to move seeds around.” Now, I realize that the animal kingdom got its start long before plants came onto the scene but there are many threads of truth to this quote.

Take, for instance, the case of the two-marked treehopper (Enchenopa binotata). This wonderful little insect enjoys a distribution that encompasses much of North and Central America, ranging from Canada down into Panama. Not only do these treehoppers look cool with their intriguing color pattern and that thorny pronatum, but their ecology and evolutionary history is absolutely fascinating as well. The existence of these treehoppes is entirely tied to the trees on which they live and breed. Moreover, while the two-marked treehopper may look like a single species, it is actually a complex of multiple cryptic “species” whose entire identity is owed to their preferred host tree.

Photo by Katja Schulz licensed under CC BY-ND 2.0.

The two-marked treehopper is not a species that moves around the landscape very much. While males will venture out into the environment in search of mates, females tend to live out their whole lives feeding and breeding on the tree upon which they were born. After mating, a female will lay her eggs within the stem of the host tree. The eggs overwinter in a sticky secretion called “egg froth.” This egg froth not only protects the eggs, it is also full of pheromones that signal to other females in the area to lay their eggs near by. The nymphs of the two-marked treehopper are gregarious. There is safety in numbers and the more nymphs hanging out on a branch, the less likely any single individual will be attacked by a predator.

Come spring, as trees begin to break dormancy, eggs laid the previous summer get the cue to hatch as sap begins to flow. Since treehoppers are sap feeders, this signal is essentially a ringing dinner bell. Apparently the specificity of this sap feeding habit is one of the reasons these treehoppers are so specific about their host.

As I mentioned earlier, the two-marked treehopper is not a single species but rather a complex of distinct taxonomic units. All of this cryptic diversity has to do with their preferred trees as each species within the complex feeds and breeds on a specific genus of tree/shrub: Carya, Celastrus, Cercis, Juglans, Liriodendron, Ptelea, Robinia, and Viburnum. Because no two tree species are alike, each has its own phenology. Different trees leaf out and begin growth at different times. Different tree species have different chemicals and nutrients in their sap. Also, different tree species have different wood densities. All of these factors and more have left their mark on the evolution of two-marked treehoppers.

Because females generally don’t leave the trees on which they were born, their offspring will inevitably be born on the same species of tree. This means they will be raised on a diet of the same sap as their mother. As mentioned, different trees produce different kinds of sap, which means that the digestive systems of these insects become highly tuned to their specific host tree. By experimentally moving two-marked treehopper nymphs to different host trees and tracking their development, scientists have also been able to demonstrate that host switching does not work well for the treehoppers. Nymphs raised on species different than the tree on which their eggs were laid do not develop as well or at all. It appears that their specific feeding habits are entirely tuned to the chemical composition of their host sap.

Additionally, the phenology of their host tree life cycle means that species raised on different trees rarely sync up in nature. Some trees force their resident treehoppers to emerge and mate earlier than others and vice versa. Evidence for this was made even stronger by studying these dynamics in the human environment.

The preferred hosts of two-marked treehoppers rarely grow in the same habitats in nature. However, thanks to our gardening and landscaping efforts, it isn’t hard to find these species in close proximity in the human environment. In cases where different host trees are found only a few meters from one another, the specific feeding requirements of each species means that species barriers among different treehopper populations are maintained. However, even before offspring enter into the picture, host trees also seem to have an effect on two-marked treehopper mating habits.

Waveforms of male signals for nine species in the Enchenopa binotata complex based on host tree identity [SOURCE].

Treehoppers are surprisingly musical creatures. Though we can’t hear them without the help of microphones, treehoppers utilize different types of vibrational calls to communicate with one another. This is especially true during mating. Males make repeated vibrations on the stems that the females will then respond to. By studying variations in these calls, scientists have found that two-marked treehoppers living on different trees produce vastly different calls. They key to this appears to lie in the ability of vibrations to travel through wood. Just as different types of wood work well for different types of instruments, the differences in wood density of their host trees affect how their mating calls travel and are eventually perceived. In other words, with a bit of training and some good recordings, you could identify the tree on which a two-marked treehopper lives just by its calls.

The ecological barriers between these insects are maintained no matter how close they are to one another and it is all thanks to the biology of the trees on which they live. Keep an eye out for these wonderful little insects. They are a joy to watch and offer us plenty of examples of evolution in action.

Photo Credits: [1] [2] [3]

Further Reading: [1] [2] [3] [4] [5]

Buzzing Bees Make Evening Primrose Flowers Sweeter

Photo by Guy Haimovitch licensed under CC BY-ND 2.0.

Plants, like all living organisms, must be able to sense and respond to their environment. The more we look at these sessile organisms, the more we realize that plants are far from static in their day to day lives. Recent evidence even suggests that some plants may be able to “hear” their pollinators and react accordingly.

I place the word “hear” in quotes because I want to make sure that we are not talking about hearing in an animalistic sense. Plants do not have ears, a nervous system, or anything like a central processing unit to make sense of such stimuli. What they do have are mechanoreceptors that can sense vibrations and those are what are likely at work in this example.

The beach evening primrose (Oenothera drummondii) is native to southeastern North America. It is pollinated by bees during the day and by moths at night. Like most members of its genus, O. drummondii produces relatively large, showy flowers. That doesn’t mean it steals all of the attention though. Competition for pollinators can be stiff among flowering plants. To sweeten the deal a bit, O. drummondii also produces a fair amount of nectar.

Nectar is costly for plants to produce and maintain. Not only does it take water and carbohydrates away from the rest of the plant, it also puts the reproductive structures at risk of degradation by microbes feeding on sugars as well as nectar thieves who end up drinking the nectar without pollinating the flower. It stands to reason that a plant that can modulate the quality of its nectar reward in response to pollinator availability could potentially increase its fitness. If the plant doesn’t always have to present sugar-rich nectar then why bother? It appears that selective nectar production is exactly the strategy O. drummondii employs.

Photo by Yu-Ju Chang licensed under CC BY-ND 2.0.

Researchers have discovered that individual O. drummondii flowers can rapidly increase the sugar content of their nectar after being exposed to the sound of a visiting bee. Within 3 minutes of being exposed to playbacks of bee wings, the flowers of O. dummondii increased the sugar content of their nectar by 20%. What’s more, flowers that had sensed the vibrations and increased their sugar content were more likely to be visited by bees. This is because bees are really good at sensing the sugar content of nectar.

This is pretty remarkable. Not only does this enable the plant to respond to the availability of pollinators and reduce the chances of nectar spoilage and theft, it significantly increases their chances of pollination. The fact that the response is so rapid (~3 mins) likely stems from the foraging habits of bees, who prefer to limit the amount of time between floral visits. Thus, the faster the plant can respond, the more likely that bees are willing to stick around and visit more flowers.

In terms of a mechanism, researchers believe the flower itself is the main sensory organ involved in the response. As mentioned, plants do produce mechanoreceptor proteins, which can sense physical vibrations. The presence of these proteins within the petals likely plays a role in sensing bee vibrations. Moreover, the bowl-shape of the flower itself may be under some selective pressures that favor the ability of the flower to sense its pollinators. More work is needed to better understand exactly how the signal pathways play out. Also, the question remains as to how wide spread this phenomenon is and how it differs between different plants and floral shapes.

Photo Credits: [1] [2]

Further Reading: [1]

How radioactive carbon from nuclear bomb tests can tell us what parasitic orchids are eating

Yoania japonica. Photo by Qwert1234 licensed by CC BY-NC-SA 4.0

Historically, non-photosynthetic plants were defined as saprotrophs. It was thought that, like fungi, such plants lived directly off of decaying materials. Advances in our understanding have since revealed that parasitic plants don’t do any of the decaying themselves. Instead, those that aren’t direct parasites on the stems and roots of other plants utilize a fungal intermediary. We call these plants mycoheterotrophs (fungus-eaters). Despite recognition of this strangely fascinating relationship, we still have much to learn about what kinds of fungi these plants parasitize and where most of the nutritional demands are coming from.

It is largely assumed that most mycoheterotrophic plants are parasitic on mycorrhizal fungi. This would make them indirect parasites on other photosynthetic plants. The mycorrhizal fungi partner with photosynthetic plants, exchanging soil nutrients for carbon made by the plant during photosynthesis. However, this is largely assumed rather than tested. New research out of Japan has shown a light on what is going on with some of these parasitic relationships and the results are a bit surprising. What’s more, the methods they used to better understand these parasitic relationships are pretty clever to say the least.

Cyrtosia septentrionalis Photo by Qwert1234 licensed by CC BY-NC-SA 4.0

Photosynthesis involves the uptake of and subsequent breakdown of CO2 from the atmosphere. The carbon from CO2 is then used to build carbohydrates, which form the backbone of most plant tissues. Not all carbon is created equal, however, and by looking at ratios of different carbon isotopes in living tissues, scientists can better understand where the carbon came from. For this research, scientists utilized an isotope of carbon called 14C.

Eulophia zollingeri photo by Vinayraja licensed by CC BY-NC-SA 3.0

14C is special because it is not as common in our atmosphere as other isotopes of carbon such as 12C and 13C. One of the biggest sources of 14C in our atmosphere were nuclear bomb explosions. From the 1950’s until the Partial Nuclear Test Ban in 1963, atomic bomb tests were a regular occurrence. During that time period, the concentration of 14C in our atmosphere greatly increased. Any organism that was fixing carbon into its tissues during that span of time will contain elevated levels of 14C compared to the other carbon isotopes. Alternatively, anything fixing carbon today, say via photosynthesis, will have considerably reduced levels of 14C in its tissues.

Gastrodia elata Photo by Qwert1234 licensed by CC BY-NC-SA 4.0

By looking at the ratios of 14C in the tissues of parasitic plants, scientists reasoned that they could assess the age of the carbon being utilized. If more 14C was present, the source of the carbon could not come from today’s atmosphere and therefore not from recent photosynthesis. Instead, it would have to come from older sources like decaying wood of long-dead trees. In other words, if parasitic plants were high in 14C, then the scientists could reasonably conclude that they were parsitizing wood-decaying saprotrophic fungi. If the plants were high in 12C or 13C, then they could conclude that they were partnering with mycorrhizal fungi instead, which were obtaining carbon from present-day photsynthesis.

The researchers looked at 10 different species of parasitic plants across Japan, most of which were orchids. They analyzed their tissues and ran analyses on the carbon molecules within. What they found is that 6 out of the 10 plants contained much higher levels of 12C and 13C in their tissues, which points to mycorrhizal fungi as their host. However, for the 4 remaining species (Gastrodia elata, Cyrtosia septentrionalis, Yoania japonica and Eulophia zollingeri), the ratios of 14C were considerably higher, meaning their host fungi were eating dead wood, not partnering with photosynthetic plants near by.

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Indeed, it appears that at least some mycoheterotrophic plants are benefiting from saprotrophic rather than mycorrhizal fungi. Those early assumptions into the livelihood of such plants were not as far off the mark after all. This is very exciting research that opens the door to a much deeper understanding of some of the strangest plants on our planet.

LEARN MORE ABOUT MYCOHETEROTROPHIC PLANTS IN EPISODE 234 OF THE IN DEFENSE OF PLANTS PODCAST

Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2]

How a cactus from the Andes may be using hairs to attract its bat pollinators

Source: Merlin Tuttle’s Bat Conservation

Plants go to great lengths to attract pollinators. From brightly colored flowers to alluring scents and even some sexual deception, there seems to be no end to what plants will do for sex. Recently, research on the pollination of a species of cactus endemic to the Ecuadorian Andes suggests that even plant hairs can be co-opted for pollinator attraction.

Espostoa frutescens is a wonderful columnar cactus that grows from 1,600 ft (487 m) to 6,600 ft (2011 m) in the Ecuadorean Andes. Like many other high elevation cacti, this species is covered in a dense layer of hairy trichomes. These hairs serve an important function in these mountains by protecting the body of the plant from excessive heat, cold, wind, and UV radiation. Espostoa frutescens takes this a step further when it comes time to flower. It is one of those species that produces a dense layer of hairs around its floral buds called a cephalium. Cacti cephalia are thought to have evolved as a means of protecting developing flowers and fruits from the outside elements. What scientists have now discovered is that, at least for some cacti, the cephalium may also serve an important role in attracting bats.

Bats are famous for their use of echolocation. Because they mainly fly at night, bats rely on sound and scent, rather than sight to find food. More and more we are realizing that a lot of plants have taken advantage of this by producing structures that reflect bat sonar in such a way that makes them more appealing to bats. Some plants, like Mucuna holtonii and Marcgravia evenia, do this for pollination. Others, like Nepenthes hemsleyana, do this to obtain a nitrogen-rich meal.

Espostoa frutescens apparently differs from these examples in that its not about reflecting bat sonar, but rather absorbing it at specific frequencies. Close examination of the hairs that comprise the E. frutescens cephalium revealed that they were extremely well adapted for absorbing ultrasonic frequencies in the 90 kHz range. This may seem arbitrary until you look at who exactly pollinates this cactus.

The main pollinator for E. frutescens is a species of bat known as Geoffroy’s tailless bat (Anoura geoffroyi). It turns out that Geoffroy’s tailless bat happens to echolocate at a frequencies right around that 90 kHz range. Whereas the rest of the body of the cactus reflects plenty of sound, bat calls reaching the cephalium of E. frutescens bounced back an average of 14 decibels quieter.

Essentially, the area of floral reward on this species of cactus presents a much quieter surface than the rest of the plant itself. It is very possible that this functions as a sort of calling card for Geoffroy’s tailless bats looking for their next meal. This makes sense from a communication standpoint in that it not only saves the bats valuable foraging time, it also increases the chances of cross pollination for the cactus. To obtain enough energy from flowers, bats must travel great distances. Anything that helps them locate a meal faster will increase visitation to that flower. By changing the way in which the flowers “appear” to echolocating bats, the cacti thus increase the amount of visitation from bats, which brings pollen in from cacti located over the bats feeding range.

It is important to note that, at this point in time, research has only been able to demonstrate that the hairs surrounding E. frutescens flowers are more absorbent to the ultrasonic frequencies used by Geoffroy’s tailless bat. We still have no idea whether bats are more likely to visit flowers borne from cephalia or not. Still, this research paves the way for even more experiments on how plants like E. frutescens may be “communicating” with pollinators like bats.

Photo by Merlin Tuttle’s Bat Conservation. Please Consider supporting this incredible conservation group!

Further Reading: [1]

Buckthorns Gone Wild

Colletia paradoxa photo by James Gaither licensed by CC BY-NC-ND 2.0

When I think of the buckthorn family (Rhamnaceae), my mind conjures up images of battling with Rhamnus invasions around the Great Lakes or the amazing diversity of Ceanothus in western North America. Never have my thoughts drifted to the bizarre and wonderful genus Colletia. Native to temperate regions of South America, this strange group of spiny shrubs is certainly worth a closer look.

Though new to me, the genus Colletia has been known to science and horticulture since at least the late 1700’s. Hailing from temperate climates, at least two of the five known species of Colletia have found there way into temperate gardens elsewhere. Who could blame gardeners for their fascination with these shrubs. Close inspection of Colletia reveals surprisingly complex morphological features.

Colletia paradoxa

For starters, those large, thick, leaf-like thorns are not leaves at all. They are flattened extensions of the stem called cladodes. Instead of relying on leaves for most of their photosynthetic needs, the various Colletia instead produce chlorophyll in their stems. The cladodes function in much the same way as leaves in that their increased surface area maximizes photosynthetic potential. It is likely that cladodes are a means of conserving valuable resources for the plant.

Instead of producing vulnerable leaves that are subject to plenty of damage, these shrubs simply utilize stem tissues. Stems don’t need to be regrown year after year and by adorning the tips of the cladodes with spines, the plant is better able to protect its photosynthetic tissues. That is not to say that Colletia produce no leaves at all. Colletia will produce leaves near the base of each cladode, especially on younger tissues. Leaves, however, are deciduous and don’t stick around long enough to do much photosynthesizing.

Colletia ulicina with its red, tubular flowers. Photo by FarOutFlora licensed by CC BY-NC-ND 2.0

The flowers of Colletia ulicina are pollinated by hummingbirds. photo by James Gaither licensed by CC BY-NC-ND 2.0

Colletia are made all the more noticeable when they come into flower. For most species, clusters of lightly-scented, white flowers are produced at the base of the cladodes. For these species, insects are thought to be the predominant pollinators. Such is not the case for Colletia ulicina. This species produces sprays of bright red, tubular flowers along its stems. In the wild, these are pollinated by the green-backed firecrown hummingbird (Sephanoides sephaniodes).

Another interesting aspect of Colletia ecology is that they are all nitrogen fixers. To be fair, the plants themselves don’t do any of the fixing. Instead, they produce tiny structures on their roots called “nodules,” and those nodules house specialized bacteria collectively referred to as actinomycetes. In exchange for carbohydrates produced via photosynthesis, these bacteria fix nitrogen from the air. This extra boost of nitrogen allows Colletia to survive and excel in the nutrient-poor soils they call home.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] [3]

Salty Succulents

Photo by Leoboudv licensed by CC BY 2.5

Succulent plants come in a variety of shapes, sizes, and colors. They also hail from a variety of plant families. If there is one thing that unites these plants (other than their succulent habit) its that the vast majority of them around found growing in dry places. Whether its the heart of a desert or up in the canopy of a tree, succulence has evolved as a means of storing water. However, those of you living near salt marshes may recognize that a handful of salt marsh plants are succulent as well. How is it that plants so frequently found growing in standing water have evolved a succulent habit? The answer lies in salt.

Salt water is pretty bad for most plants. Just like we get dehydrated from drinking or eating high amount of salt, so too do plants. In general, salt both dehydrates plants and causes issues with nutrient uptake. Such is not the case for genera like Salicornia. Commonly referred to as glassworts, pickleweeds, or picklegrass, the various Salicornia are true salt-lovers.

Photo by OliBac licensed by CC BY 2.0

Taxonomically speaking, the genus Salicornia has been called a “taxonomic nightmare.” Thanks to their highly reduced morphology and extreme phenotypic plasticity, delineating species among the genus is something best left to Salicornia experts. What we do know is that they all belong in the amaranth family, Amaranthaceae. All of this confusion should not take away from your enjoyment of Salicornia. Indeed, there is a lot worth appreciating in this family, including their ability to grow in conditions that would kill most other plants.

Salicornia are not simply salt tolerators that can hang on under saline conditions. They are true salt lovers or ‘halophytes.’ In fact, experiments have shown that various Salicornia grow much better when salt levels are high. This all has to do with the way in which these plants deal with their salty environment. Like all succulents, Salicornia have enlarged vacuoles that store water. However, these large vacuoles store more than good ol H2O. They also store salts and lots of them.

Photo by S.Ahmadihayeri licensed by CC BY-SA 3.0

The secret to Salicornia’s salty success has to do with osmosis. As you may remember from science class, substances in our universe like to move from areas of high concentration to areas of low concentration. In the case of water within the tissues of an organism, this often occurs between biological membranes. As you add salt to water, it actually displaces water molecules such that the more salt you add, the less concentrated the water becomes. That is why salt water dehydrates us. When you surround a cell with salt, water will diffuse out of the cell to balance out the concentrations on both sides of the cell membrane. Salicornia use this to their advantage.

These plants actively take up salt from their environment and dump it into their vacuoles. This means that the concentration of water within the vacuole is less than the concentration of water outside of the cell. Osmosis then takes over and water rushes into the plant’s cells. By concentrating salt in their vacuoles, Salicornia are always ensuring that they are on the receiving end of the water gradient. Water is always moving into these salty plants and not the other way around. By co-opting morphological adaptation to drought, Salicornia are able to conquer a niche that is largely unavailable to most other plant species. It also means that, despite all of the water in their environment, these plants maintain a pleasingly succulent habit.

Photo Credits: [1] [2] [3]

Further Reading: [1] [2] [3] [4]

To grow or to flower, that is the cactus conundrum

Melocactus intortus

Flowers are costly structures for plants to produce. In the flowering plant world, there is always a trade-off between growth and reproduction. Flowers are produced from tiny structures called axillary buds, and many plants can only produce one flush of flowers per bud. Cacti are no exception to this rule and their amazing morphological adaptations to harsh climates has forced them into quite a conundrum when it comes to reproduction.

The axillary buds of cacti are located at the base of their spines in little structures called areoles. This is where the flowers will eventually emerge. However, unlike plants that can produce cheap stems and branches, cacti must produce a whole new chunk of stem or internode before they can produce more axillary buds. Think of it this way, if a cactus wants to produce 10 flowers, it must produce ten internodes to do so. This means producing all of the expensive cortex and epidermis along with it. Their harsh environments have forced most cacti into an extremely tight relationship between growth, water storage, photosynthesis, and flowering that is potentially very limiting from a reproductive standpoint.

Micranthocereus estevesii with lateral cephalium

Amazingly, some cacti have managed to break from this evolutionary relationship and they have done so in a bizarre way. Take a look at all of the cacti pictured here. Each has developed a strange looking structure called a cephalium. Essentially, you can think of the cephalium of a cactus as its “adult” reproductive form whereas the rest of the body consists of non-reproductive, photosynthetic “juvenile” form.

The cephalium is a unique and fascinating structure. It differs from the rest of the cactus body in that it is not photosynthetic. It also produces no chlorophyll and no stomata. In fact, it does not form anything like the epidermis of the rest of the plant. Instead, the cephalium produces dense clusters of short spines and trichomes. Most importantly, it produces tightly packed axillary buds in high abundance. These are the buds that will produce the flowers. The end result is a wacky looking structure that has the ability to produce far more flowers than that of cacti that do not grow a cephalium.

Facheiroa tenebrosa with lateral cephalium

Obviously not all cacti produce cephalia but it is common in genera such as Melocactus, Backebergia, Espostoa, Discocactus, and Facheiroa (this is not a complete list). What the cephalium has done for genera like these is decouple the afore mentioned relationships between growth and reproduction. For a period of time (often many years) following germination, these cacti grow the typical succulent, photosynthetic stems we are accustomed to seeing.

At some point in their development, something triggers these plants to switch to their adult forms. Axillary buds within either lateral or apical meristems switch their growth habit and begin forming the cephalium. It is worth mentioning that no one yet knows what triggers this switch. If the cephalium is produced from axillary buds in the apical meristem like we see in Melocactus, the plant will no longer produce photosynthetic tissues. This represents another major trade-off for these cacti. Such species must rely on the photosynthetic juvenile tissues for all of their photosynthetic needs for the rest of their lives (unless the cephalium is damaged or lost). Backebergia have managed to get around this trade-off by not only growing multiple stems, they will also shed their apical cephalia after a few years, thus re-initiating photosynthetic juvenile growth.

Backebergia militaris with bizarre apical cephalia reminiscent of the bearskin hats of the Queen’s guard.

Things are a bit different for cacti that produce lateral cephalia. Genera such as Espostoa, Facheiroa, and Buiningia are less limited by their cephalia because they are produced along the ribs of the stem, thus leaving the apical meristem free to continue more typical photosynthetic growth. Nonetheless, the process is much the same. Dense clusters of spines, trichomes, and most importantly, axillary buds are produced along the rib, giving each stem a lovely, lopsided appearance.

There are other benefits to growing cephalia in addition to simply being able to produce more flowers. The densely packed spines and trichomes offer the developing flowers and fruits ample protection from both the elements and herbivores. Floral buds are free to develop deep within the interior of the cephalium until they are mature. At that point, the cells will begin to swell with water, pushing the flower outward from the cephalium where it will be exposed to pollinators. As the petals curl back, they offer a safe spot for visiting pollinators that is free from menacing spines. Once pollination has been achieved, the flower wilts and the deeply inferior ovaries are then free to develop within the safety of the cephalium. Once the fruits are mature, they too will begin to swell with water and be pushed out from the cephalium where they will attract potential seed dispersers.

Melocactus violaceus with fruits emerging from the cephalium

I hope that I have convinced you of just how awesome this growth form can be. I will never forget the first time I saw a cactus topped with a cephalium. It was a mature Melocactus growing in a cactus house. Sticking out of the odd “cap” on top was a ring of bright pink fruits. I knew nothing of the structure at that time but it was incredible to see. Now that I know what it is and how it functions, I am all the more appreciative of these cacti.

This post was inspired by the diligent work of Dr. Jim Mauseth. Click here to learn more about cacti.

Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2] [3]

Your string of pearls (and its cousins) are all members of the daisy family

Photo by LynnK827 licensed by CC BY-NC-ND 2.0

I love the spike in popularity of houseplants. The more popular indoor gardening becomes, the more plants become available for obsessive growers such as myself. If you are like me, then learning about the ecological and evolutionary history of the plants you keep makes them all the more special. Take, for instance, a small group of scrambling succulents affectionately referred to as “string of pearls,” “string of bananas,” and “string of tears.” These all make incredible houseplants if given the proper care, but they become all the more interesting when you realize that they are distant cousins of the dandelions growing in your yard.

That’s right, each of these species are highly derived members of the daisy family (Asteraceae). Their taxonomy has been a bit wonky over the years. When I first took interest in these succulents, they resided in the genus Senecio. Some authors have suggested moving them into the genus Kleinia or Cacalia, but current systematics suggests they belong in a genus of their own - Curio. Inspection of the relationships within this group reveals that closely related species have evolved slightly different growing habits. The plants I will be focusing on for this article each resemble creeping vines but many of their close relatives are less vine-like but nonetheless still creep along the ground. For the sake of this piece, I am going to stick with the genus Senecio because, regardless of their taxonomic placement, the “sting of” clade is super fascinating from an ecological standpoint.

Senecio citriformis photo by Salchuiwt licensed by CC BY-SA 2.0

Senecio radicans photo by KENPEI licensed by CC BY-SA 3.0

All of these stringy plants hail from arid regions of South Africa. In the wild, they mostly scramble over rocks and bushes, often emerging out of cracks in rock in search of the right microclimate. Their oddly shaped, succulent leaves are an evolutionary adaptation to the tough conditions in which they evolved. The most leaf-like anatomy belongs to that of the string of bananas (S. radicans). Each leaf of S. radicans is shaped like a tiny green banana. More extreme versions of leaf morphology are found in the string of tears (S. citriformis) and string of pearls (S. rowleyanus & S. herreianus). The leaves of these three species resemble peas in shape, size and color. The leaves of S. rowleyanus are more spherical in shape (pearls), whereas the leaves of S. citriformis taper towards the tip (tears).

Senecio herreianus photo by Frank Vincentz licensed by CC BY-SA 3.0

Though all of these species grow in dry habitats, the more spherical shaped leaves of S. rowleyanus and S. citriformis are thought to be best adapted for drought. In growing spherical leaves, these plants are taking advantage of the surface area to volume ratio of a sphere. The benefit of this is that these species are able to maximize water storage while minimizing the amount of leaf surface exposed to the blistering sun. This way the leaves are able to maintain high levels of photosynthesis without overheating, all the while reducing leaf temperature.

In each of these species, the surface or adaxial side of the leaf exhibits a translucent window that runs the length of the leaf. It has long been hypothesized that leaf windows allow light to transmit into deep into the interior of the leaf where the photosynthetic machinery resides. More recent experiments on window-leaved succulents suggests that reality is not that simple. Instead, these windowed surfaces appear to allow the plant to maintain healthy levels of photosynthesis without the damaging their leaves via overheating.

Photo by Frank Vincentz licensed by CC BY-SA 3.0

When plants reach maturity, flowering can be prolific. Thin stems topped with tiny composite heads of cream-colored flowers erupt from the mat of vegetation. Then and only then do these plants readily reveal their placement within the daisy family. The inflorescence is made up entirely of discoid flowers. There are no rays like that of a sunflower. The flowers themselves are said to produce a pleasant odor frequently described as sweet and spicy. After pollination, the flowers give way to seeds topped with a parachute-like pappus that will carry them far and wide on the wind.

Learning about the natural history of these plants has given me a whole new appreciation of these strange, succulent members of the daisy family. What’s more, there is a whole world of succulent asters out there (a post for a later time) and many of them are equally as fascinating and beautiful.

Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2] [3] [4]