My Unforgettable Encounter with a Fevertree

When someone asks you if you would like to see a wild fever tree, you have to say yes. As a denizen of cold climates defined by months of freezing temperatures, I will never miss an opportunity to encounter any species in its native habitat that cannot survive frosts. This was the scenario I found myself in last week as friend and habitat restoration specialist for the Atlanta Botanical Garden, Jeff Talbert, was showing us around a wonderful chunk of Florida scrubland he has been managing over the last few years.

He drove our small group over to an area that, up until a year or two ago, was completely choked with swamp titi (Cyrilla racemiflora). Like many habitats throughout southeastern North America, this patch of Florida scrub is dependent on regular fires to maintain ecological function. Without it, aggressive shrubs like titi completely take over, choking out much of the amazing biodiversity that makes this region unique. Jeff and his team have been very busy restoring fire to this ecosystem and the results have been impressive to say the least.

We walked off the two-track, down into a wet depression and were greeted by an impressive population of spoon-leaf sundews (Drosera intermedia), which is a good sign that water quality on the site is improving. After a few minutes of sundew admiration, Jeff motioned for us to look upward towards the surrounding tree line. That’s when we saw it. Growing up out of the small seep that was feeding this wet depression was a spindly tree with bright pink splotches decorating its canopy. This was to be my first encounter with a fevertree (Pinckneya bracteata).

A few of us were willing to get our feet wet and were rewarded with a close look at the growth habit of this incredible tree. Clustered at the end of its spindly branches are dark green, ovate leaves that give the tree a tropical appearance. Erupting from the middle of some of those leafy branches were the inflorescences. These are what produce the pink splotches I could see in the canopy of larger individuals. They remind me a lot of a poinsettia and at first, I thought this tree might be a member of the genus Euphorbia. Indeed, the pink coloration comes from a handful of rather large, leaf-like sepals attached to the base of each inflorescence.

Upon seeing the flowers, I instantly knew this was not a member of Euphorbiaceae. Each flower was long and tubular ending in five reflexed lobes. They are colorful structures in and of themselves, adorned with splashes of pink and yellow. After a bit of scrutiny, our group was finally able to place this within its true taxonomic lineage, the coffee family (Rubiaceae).

Within the coffee family, fevertree is closely related to the genus Cinchona. Like Cinchona, the fevertree produces quinine and other alkaloids that are effective in treating malaria. Fevertree has been used for millennia to do just that, hence the common name. It also seems fitting that fevertrees tend to grow in wetland habitats where mosquitos can be abundant. However, this is by no means an obligate wetland species. Those who have grown fevertree frequently succeed in establishing plants in dry, upland habitats as well. Perhaps highly disturbed wetlands are some of the few places where this spindly tree can avoid intense competition from other forms of vegetation.

Fevertrees do need regular disturbance to persist. They are not a large, robust tree by any means and can easily get outcompeted by more aggressive vegetation. However, this species does have a trick that enables individuals to persist when disturbances don’t come frequent enough. Fevertree is highly clonal. Instead of producing a single trunk, it sends out numerous stems in all directions in search of a gap in the canopy. This clonal habit allows it to eek out an existence in the gaps between its more robust neighbors until disturbances return and clear things out.

This clonal habit is also very important when it comes to reproduction. Fevertree requires a decent amount of sunlight to successfully flower and set seed. By using its clonal stems to find light gaps, it can at least guarantee some level of reproduction until fires, floods, or some other form of canopy clearing disturbance frees up enough space for it to prosper and its seeds to germinate. However, its clonal habit can also hurt its reproductive capacity over the long term if recruitment of new individuals does not occur.

Fevertree is considered self-incompatible. In other words, its flowers cannot be pollinated via pollen from a genetically identical individual. As more and more clonal shoots are produced, the tree effectively increases the chances that its own pollen will end up on its own flowers. This is yet another important reason why regular disturbance favors fevertree reproduction. Fevertree seeds need light and bare ground to germinate, which is usually provided as fires and other disturbances clear the canopy and open up bare ground. Only then can enough unrelated individuals establish to ensure plenty of successful pollination opportunities.

With its long, tubular flowers and bright pink sepals, fevertrees don’t seem to have any trouble attracting pollinators, which mainly consist of ruby-throated hummingbirds and bumblebees. Only these organisms have what it takes to successfully access the pollen and nectar rewards of this plant and travel the distances necessary to ensure pollen ends up on unrelated individuals. The seeds that result from pollination are winged and can travel a decent distance with a decent wind. With any luck, a few seeds will end up in another disturbance-cleared wet area and usher in the next generation of fevertrees.

I am so happy that restoration activities at this site are making more suitable habitat for this unique tree. Looking around, we saw many more small individuals starting to emerge where there was once a dense canopy of titi. Hopefully with ongoing management, this population will continue to grow and spread, securing the a future for this species in a region with an ever-growing human presence. If you ever find the opportunity to see one of these trees in person, do yourself a favor and take it!

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

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]

Meet the Golden Lotus Banana

Photo by Linda De Volder licensed under CC BY-NC-ND 2.0

While perusing the internet the other day, I scrolled past an image of what looked like the physical manifestation of the sun emoji on my phone. The bright yellow flash was so striking that it caused me to pause and scroll back to the source. I was pleasantly surprised to see that the sun-like object belonged to something botanical. I was even more surprised to find out that it was produced by a unique cousin of the banana called the golden lotus banana (Musella lasiocarpa).

The golden lotus banana is an oddball in many ways. For starters, it has a confusing taxonomic history. For many years, this odd plant has bounced back and forth between what was originally the only two genera in the banana family (Musaceae). Indeed, it has many outward characteristics that could firmly land it in either the genus Musa or the genus Ensete. Still, this plant is strange enough that numerous taxonomists have taken their own stab at narrowing down its correct placement. It wasn’t until DNA analyses revealed it to be so distinct from either of these genera that it warranted its own unique taxonomic placement. Thus, the monotypic genus Musella was born.

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

The plant itself is well known and widely cultivated throughout its home range in the Yunnan province of China. In fact, the golden lotus banana is so widely cultivated in this region as food for both humans and cattle alike, that experts couldn’t quite figure out if there were any wild populations left. It wasn’t until relatively recently that some wild populations were found. Sadly, these populations are under threat of being completely extirpated as much of the conifer-oak forests it calls home have been highly fragmented and degraded due to human activities. At least its popularity in cultivation means this species is not likely to go completely extinct any time soon.

The golden lotus banana is rather interesting in form. When you look for pictures of this species around the web, you are likely to pull up images of a stubby, nearly leafless stalk tipped with the bright yellow bracts that look like the rays of a cartoonish sun. Apparently, plants can lose many of their leaves in cultivation around the time the inflorescence matures, giving the impression that it never had any to begin with. Of course, the plant does produce typical banana-like leaves for most of the year. As mentioned, the amazing inflorescence is borne at the tip of what looks like a small, woody trunk, but in reality is actually the fused petioles of their leaves. All members of the banana family are, after all, overgrown herbs, not trees.

As is typical with this family, the flowers don’t all ripen at once. Instead, they begin at the base and gradually ripen over time, revealing consecutive whirls of tubular flowers surrounded by bright yellow bracts, though a variant population that produces red bracts was recently described as well. Interestingly, the golden lotus banana differs from its banana cousins in that its flowers are not pollinated by bats or birds. Instead, bees and wasps comprise the bulk of floral visitors, at least among cultivated populations. The first flowers to mature are male flowers that produce a small amount of nectar and copious amounts of pollen. Only the flowers near the base of the inflorescence are female and they produce a lot more nectar than the male flowers.

Photo by Linda De Volder licensed under CC BY-NC-ND 2.0

Research has shown that bees are far more likely to visit female over male flowers and their visits to female flowers last much longer. This is likely due to the differences in nectar production, but the end result is that by encouraging bees to spend less time on male flowers and more time on female flowers, each plant greatly increases the chances that pollen of unrelated individuals will end up on the stigma. After pollination, tiny fruits are formed, however, from what I have read they are largely inedible to humans. Once the fruits ripen and seeds are dispersed, the flowering stalk dies back and is replace by a fresh new growth stalk from the underground rhizome.

The next time you find yourself at a botanical garden with a decent tropical plant collection, keep an eye out for the golden lotus banana. Outside of China, this species has gained some popularity among specialist plant growers and you just might be lucky to stumble across one in the process of blooming.

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

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]

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]

The Amazing Pollination Strategy of Bellflowers

Harebell (Campanula rotundifolia). Photo by H. Zell licensed under CC BY-SA 3.0

Pollination is the key to success for any sexually reproducing plant. The movement of pollen grains from one flower to another is a way of ensuring genetically diverse offspring. Plants have many different ways of maximizing the chances that their pollen will end up on an unrelated individual rather than their own flowers. There is no one size fits all strategy after all. I only recently learned of an incredible pollination mechanism that is used by bellflowers in the genera Campanula and Campanulastrum and it involves moving hairs.

The bellflowers utilize a strategy called secondary pollen presentation to minimize the chances of pollinating their own flowers. What this means is that pollen is locked up in the anthers until the style elongates and drags the pollen with it. The process is aided by the fact that bellflowers styles are covered in hairs that collect the pollen as it elongates. Essentially, the style acts like a tiny pipe cleaner, emptying the anthers of the pollen they contain. The stigma itself will not become receptive to pollen until most of its own pollen has been removed. But how does the plant “know” when this happens? The key to this lies again in those hairs.

(1) Immature stamen surround the style; (2) elongation of the style by which the anthers dehisce and pollen grains are swept on the stylar hairs of the immature style; and (3) further outgrowth of the style, anthers are withered. [SOURCE]

The hairs that cover the style are sensitive to touch. When an insect lands on the style and begins collecting pollen, its movements send a signal to cells at the base of each hair that causes a change in how they store water. When triggered, these cells expel water, causing them to shrink. As they shrink, the hairs are gradually drawn down into pockets or cavities within the style. As they do this, pollen either drops off or is taken down into the cavities with the hairs.

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Pollen collecting hairs on 1) Campanula barbata; 2) Campanula kremeri; 3) Campanula dichotoma; 4 - 6) Cavities in which pollen collecting hairs have retreated. [SOURCE]

Only after the hairs have retracted will the stigma become receptive to pollen. In doing so, the plant minimizes the chances that its own pollen will end up on the receptive stigma. That is not to say this works 100% of the time. Research has found that the rate at which the hairs retract is a function of how often the flowers are visited. Flowers that receive numerous pollinator visits in a short period of time will retract their hairs much faster than plants that receive fewer visits. If a flower is not visited, the style will eventually become receptive regardless if pollen has been removed or not. In a pinch, even self-pollination will ensure a continuation of that individuals genes. Not ideal, but this backup plan certainly works, especially for annual species like American bellflower (Campanulastrum americanum) that usually have only one season for reproduction.

American bellflower (Campanulastrum americanum) with its elongated, receptive style. Photo by Joshua Mayer licensed under CC BY-SA 2.0

I have always enjoyed bellflowers. They are beautiful plants with lots of ecological value. Learning about this interesting and surprisingly complex pollination mechanism only makes me appreciate them more. I only wish you could see the process happening with the naked eye.

Photo Credits: [1] [2] [3] [4] All images licensed under CC BY-ND 2.0.

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

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]

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]

A Rare Case of Ant Pollination in Australia

Photo by Nicola Delnevo [SOURCE]

Ants have struck up a lot of interesting and important relationships with plants. They disperse seeds, protect plants from herbivores and disease, and can even help acquire nutrients. For all of the beneficial ways in which ants and plants interact, pollination rarely enters into the equation. More often than not, ants are actually detrimental to the sex lives of flowering plants. Such is not the case for a rare species of protea endemic to Western Australia called the smokebush (Conospermum undulatum).

The reason ants usually suck at pollination is thanks to a tiny organ called the metapleural gland. For many ant species, this gland secretes special antimicrobial fluids that the ants use to groom themselves. Because ants tend to live in high densities in close quarters, this antimicrobial fluid helps keep their little bodies clean of any pathogens that might threaten their existence. For as good as these fluids are for ants, they destroy pollen grains, rendering them useless for pollination.

Leioproctus conospermi. Photo by Sarah McCaffrey licensed under CC BY-ND 2.0.

As is so often the case in nature, there are always exceptions to the rule and it seems that one such exception is playing out in Western Australia. While investigating the reproductive ecology of the smokebush, researchers noted that ants were regular visitors to their small flowers. They knew that in drier climates, some ant species have evolved to produce considerably less antimicrobial fluids. The thought is that drier climates tend to harbor fewer microbial pathogens and thus ants don’t need to waste as much energy protecting themselves from such threats. If this was the case in Western Australia then it was entirely possible that ants could potentially serve as pollinators for this plant. Armed with this hypothesis, they decided to take a closer look.

It turns out that the floral morphology of the smokebush lends well to visiting ant anatomy. The tiny flowers produce a small amount of nectar at the base. As ants shove their heads down into the flower to get a drink, it triggers an explosive mechanism that causes the style the smack down onto the back of the ant. In doing so, it also mops up any pollen the ant may be carrying. At the same time, the anthers explosively dehisce, coating the visitor with a fresh dusting of pollen. During their observations, researchers noted that ants weren’t the only insects visiting smokebush blooms. They also noted lots of visitation from invasive honeybees (Apis mellifera) and a tiny native bee called Leioproctus conospermi.

(A) White flowers of Conospermum undulatum. (B) Floral details. (C–H) Insects visiting flowers of C. undulatum: (C) Leioproctus conospermi; (D) Camponotus molossus; (E) Camponotus terebrans; (F) Iridomyrmex purpureus; (G) Myrmecia infima; (H) Apis mellifera. [SOURCE]

After recording visits, researchers needed to know whether any of these floral visitors resulted in successful pollination. After all, just because something visits a flower doesn’t mean it has what it takes to get the job done for the plant. By looking at differences in seed set between ant and bee visitors, they were able to paint a fascinating picture of the pollination ecology of the rare smokebush.

It turns out that ants are indeed excellent pollinators of this shrub, contributing just as much to overall seed set as the tiny native Leioproctus conospermi. Alternatively, invasive honeybees barely functioned as pollinators at all. Their heads were too big to effectively trigger the pollination mechanism of the flowers but nonetheless were able to access the nectar within. As such, honeybees are considered nectar thieves for the smokebush, harming its overall reproductive effort rather than helping.

Amazingly, the effectiveness of ants as smokebush pollinators is not because they produce less antimicrobial fluids. In fact, these ants were fully capable of producing ample amounts of these pollen-killing substances. Instead, it appears that the plant itself has evolved to tolerate ant visitors. Smokebush pollen is resistant to the toxic effects of the metaplural gland fluids. With plenty of hungry ants always on the lookout for food, the smokebush has managed to tap in to an abundant and reliable vector for pollination. No doubt other examples exist, we simply have to go looking.

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

Further Reading: [1]

Deer Skew Jack-in-the-Pulpit Sex Ratios

Photo by Michael Janke licensed under CC BY-ND 2.0.

Deer populations in North America are higher than they have been at any point in history. Their explosion in numbers not only leads to series health issues like starvation and chronic wasting disease, it has also had serious impacts on regional plant diversity. Wherever hungry herds of deer go, plants disappear from the landscape. However, the impacts of deer on plants aren’t limited to species they can eat. Research on Jack-in-the-Pulpit (Arisaema triphyllum) has shown that deer can have plenty of surprising indirect impacts on plants as well.

Though I wouldn’t put anything past a hungry deer, plants like Jack-in-the-Pulpit aren’t usually on the menu for these ungulates. Their leaves, stems, and flowers are chock full of raphide crystals that will burn the mouths and esophagus of most herbivores. Still, this doesn’t mean deer aren’t impacting these plants in other ways. Because deer are congregating in high abundance in our ever-shrinking natural spaces, they are having serious impacts on local growing conditions. Wherever deer herds are at high numbers, forests are experiencing soil compaction, soil erosion, and a disappearance of soil leaf litter (also due in part to invasive earthworms). Thanks to issues like these, plants like Jack-in-the-Pulpit are undergoing some serious changes.

Like many aroids, sex expression in the genus Arisaema is fluid and relies on energy stores. Smaller plants store less energy and tend to only produce male flowers when they bloom. Pollen, after all, is cheap compared to eggs and fruit. Only when a plant has stored enough energy over the years will it begin to produce female flowers in addition to males and only the largest, most robust plants will switch over entirely to female flowers. As you can imagine, the ability of a plant to acquire and store enough energy is dependent on the quality of the habitat in which it grows. This is where deer enter into the equation.

High densities of deer inevitably cause serious declines in habitat quality of plants like Jack-in-the-Pulpit. As leaf litter disappears and soil compaction grows more severe, individual plants have a much harder time storing enough energy each growing season. In places where deer impacts are heaviest, the sex ratios of Jack-in-the-Pulpit populations begin to skew heavily towards males because individual plants must grow much longer before they can store enough energy to produce female flowers. It doesn’t end there either. Not only does it take longer for a plant to begin producing female flowers, individual plants must also reach a much larger size in order to produce female flowers than in areas with fewer deer.

Photo by Charles de Mille-Isles licensed under CC BY-ND 2.0.

As mentioned, seed production takes a lot of energy and any plant that is able to produce viable fruits will have less energy stores going into the next season. This means that even if a plant is able to produce female flowers and successfully set seed, they will have burned through so much energy that they will likely revert right back to producing only male flowers the following year, further skewing the sex ratios of any given population towards males. Interestingly, this often results in more individuals being produced via clonal offshoots. The more clones there are in a population, the less diverse the gene pool of that population becomes.

Without actually eating the plants, deer are having serious impacts on Jack-in-the-Pulpit population dynamics. I am certain that this species isn’t alone either. At least Jack-in-the-Pulpit is somewhat flexible in its reproductive behaviors. Other plants aren’t so lucky. I realize deer are a hot button issue but there is no getting around the fact that our mismanagement of their natural predators, habitat, and numbers are having serious and detrimental impacts on wild spaces and all the species they support.

Photo Credits: [1] [2]

Further Reading: [1]

Bees Bite Leaves to Induce Flowering

Photo by Ivar Leidus licensed under CC BY-ND 2.0.

Imagine spending all winter sleeping underground, living off of the energy reserves you accumulated the previous year. By the time spring arrived and you started waking up, your need to eat would be paramount to all other drives. Such is the case for emerging queen bumblebees. Food in the form of nectar and pollen is their top priority if they are to survive long enough to start building their own colony, but flowers can be hard to come by during those first few weeks of spring.

Spring can be very unpredictable. If bees emerge from their slumber too early or too late, they can miss the flowering period of the plants they rely on for food. By the same token, the plants themselves then miss out on important pollination services. Mismatches like this are becoming more common as climate change continues to accelerate. However, not all bees are helpless if they emerge onto a landscape devoid of flowers. It turns out that, with a little nibble, some bees are able to coax certain plants into flowering.

Over a series of experiments, scientists were able to demonstrate that at least three species of bumblebee (Bombus terrestris, B. lapidarius, and B. lucorum) were able to induce early flowering in tomatoes (Solanum lycopersicum) and mustards (Brassica nigra) simply by nibbling on their leaves. The queens would land on the leaf and make a series of small holes with their mandibles before flying off. The bees did not appear to be feeding on any of the sap, nor were they carrying chunks of leaf when they flew away. Amazingly, the act of nibbling on the leaves in each experiment resulted in earlier flowering times across both species of plant.

(A) Sequential images of a worker penetrating a leaf with its proboscis. (B) A worker cutting into a leaf with its mandibles. (C) Characteristic bee-inflicted damage. [SOURCE]

The results were not minor either. Flowers on bee-nibbled plants were produced an average of 30 days earlier than non-nibbled plants. Amazingly, when scientists tried to simulate bee nibbles using tweezers and knives, they were only able to coax flowering an average of 8 days earlier than non-damaged plants. What this means is that there is something about the bite of a bee that sends a signal to the plant to start flowering. Perhaps there’s a chemical cue in the bee’s saliva. Indeed, this is not unheard of in the plant kingdom. Some trees have shown to respond to the detection of deer saliva, ramping up defense compounds in their leaves only once they have detected deer. More work is needed before we can say for sure.

Through a complex series of experimental trials, scientists were also able to demonstrate that this behavior was the result of pollen limitation rather than nectar. As pollen availability increased both artificially (by adding already flowering plants) or naturally (as time wore on, more plants came into bloom), the leaf biting behavior declined. Such was not the case when only nectar was available. Pollen is a protein-rich food source for bees and is especially important for their developing larvae. By inducing plants to flower early, the bees are ensuring that there will be a ready supply of pollen when they and their developing larvae need it the most.

Considering the role bees play in pollination of plants like tomatoes and mustards, it is likely that this interaction benefits both players to some degree; bees are able to coax floral resources much sooner than they would normally become available while the plants are flowering when effective pollinators are present in the area. These exciting results open yet another window into the multitude of ways in which plants and their pollinators interact. Given that plants have been known to skew the caste systems in eusocial bees, it should come as no surprise to learn that some bees have a few tricks up their sleeves as well.

Photo Credits: [1] [2]

Further Reading: [1]

The Deceptive Ways of the Calypso Orchid

Photo by Murray Foubister licensed under CC BY-ND 2.0.

Behold the Calypso orchid, Calypso bulbosa. This circumboreal orchid exists as a single leaf lying among the litter of dense conifer forests. They go virtually unnoticed for most of the year until it comes time to flower.

In early spring, the extravagant blooms open up and await the arrival of bumblebees. Calypsos go to great lengths to attract bumblebees. The flower is said to have a sweet scent. Also, the lip sports small, yellow, hair-like protrusions that are believed to mimic anthers covered in pollen. Finally, within the pouch formed by the lip are two false nectar spurs. All of these are a ruse. The Calypso offers no actual rewards to visiting bumblebees.

Not just any bumblebee will do. For the ruse to work, it requires freshly emerged workers that are naive to the orchid’s deception. Bumblebees are not mindless animals. They quickly learn which flowers are worth visiting and which are not. Because of this, the Calypso has only short window of time in which bumblebees in the vicinity are likely to fall for its tricks. As a result, pollination rates are often very low for this orchid.

The most interesting aspect of all of this is that the so-called "male function" of the flower - pollinia removal - is more likely to occur than the "female function" - pollen deposition. The reason for this makes a lot of sense in context; male function requires a bumblebee to be fooled only once whereas female function requires a bumblebee to be fooled at least twice.

The caveat to all of this deception is that a single Calypso, like all other orchids, can produce tens of thousands of seeds. Each orchid therefore has tens of thousands of potential propagules to replace itself in the next generation. Despite that fact, the Calypso orchid is on the decline. Habitat destruction, poaching, deer, and invasive species are taking their toll. If you care about orchids like the Calypso, please consider supporting organizations like the North American Orchid Conservation Center.

Photo by Murray Foubister licensed under CC BY-ND 2.0.

Photo Credit: [1] [2]

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

Opossum Pollination of a Peculiar Parasite

Floral traits can provide us with insights into the types of pollinators most suited for the job. For many flowering plants, the relationship is relatively easy to understand, but check out the flowers of Scybalium fungiforme. You would be completely excused for not even realizing that these bizarre structures belonged to a plant. The anatomy of those flowers would leave most of asking “what on Earth do they attract?” The answer to this are opossums!

Scybalium fungiforme hails from a peculiar family of parasitic plants called Balanophoraceae and is native to the Atlantic forests of Brazil. Members of this family can be found in tropical regions around the globe and all of them are obligate root holoparasites. Essentially this means that all one ever sees of these plants are their strange flowers. The rest of the plant lives within the vascular system of a host plant’s roots.

The adorable big-eared opossums (Didelphis aurita).

Scybalium fungiforme is particularly strange in that its flowers are covered in scale-like bracts. As such, accessing the flowers would be difficult for most animals. Because its strange blooms superficially resemble some marsupial and rodent pollinated Proteaceae in Australian and South Africa, predictions of a non-flying mammal pollination syndrome were about the only explanations that made sense. Now, with the help of night vision cameras, this prediction has been vindicated.

They key to this unique pollination syndrome lies in those bracts and an interesting aspect of opossum anatomy. Until the scale-like bracts are removed, not much is able to access the floral parts inside. Luckily big-eared opossums (Didelphis aurita) come equipped with opposable toes on their back feet. Upon locating the flowers of S. fungiforme, the opossum uses its back feet to remove the bracts. This unveils a bounty of nectar within. As the opossum feeds, its furry little snout gets covered in pollen. When the opossum visits subsequent flowers throughout the night, pollination is achieved.

Floral visitors of Scybalium fungiforme. b) The big-eared opossum, Didelphis aurita drinking nectar on a plant with five inflorescences (one male and four females). c) The montane grass mouse, Akodon montensis, visiting a plant with about 10 inflorescences and drinking nectar on a female one. d) The Violet-capped Woodnymph hummingbird, Thalurania glaucopis visiting a male and e) a female inflorescence. f) detail of an A. angulata wasp manipulating a male flower to eat pollen. g) Agelaia angulate visiting a female inflorescence with the head inserted among flowers to reach the nectar secreted in the inflorescence receptaculum.

Interestingly, activity doesn’t end when the opossums are done. Enough nectar often remains by the next day that a suite of other animals come to pay a visit to these strange blooms. Day time visitation of S. fungiforme consisted largely of wasps, bees, and even a mouse or two. Researchers were also lucky enough to witness Violet-capped Woodnymph hummingbirds (Thalurania glaucopis) repeatedly visit the flowers for a sip of nectar. It would appear that although the main pollinators of this strange parasite are opossums, the removal of the bracts opens up the flowers for plenty of secondary pollinators as well.

Though this is by no means the only plant to be pollinated by non-flying mammals, this pollination syndrome certainly broadens our understanding of the evolution of pollination syndromes.

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

Further Reading: [1]

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]

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]

What an orchid that smells like rotting meat can tell us about carrion flies

Satyrium pumilum Photo by Bernd Haynold licensed by CC BY-SA 3.0

Orchids are really good at tricking pollinators. Take, for instance, this strange looking orchid from South Africa. Satyrium pumilum is probably obscure to most of us but it is doing fascinating things to ensure its own reproductive success. This orchid both smells and kind of looks like rotting meat, which is how it attracts its pollinators.

It is a bit strange to think of orchids living in arid climates like those found in South Africa but this family is defined by exceptions. That is not to say that Satyrium pumilum is a desert plant. To find this orchid, you must look in special microclimates where water sticks around long enough to support its growth. Populations of S. pumilum are most often found clustered near small streams or hidden under bushes throughout the western half of the greater Cape Floristic Region.

Satyrium pumilum blooms from the beginning of September until late October. As is typical in the orchid family, S. pumilum produces rather intricate flowers. Whereas the sepals are decked out in various shades of green, the interior of the flower is blood red in color. Also, unlike many of its cousins, S. pumilum doesn’t throw its flowers up on a tall stalk for all the world to see. Instead, its flowers open up at ground level and give off an unpleasant smell of rotting meat.

This is where pollinators enter into the picture. It has been found that carrion flies are the preferred pollinator for S. pumilum. By producing flowers at ground level that both look and smell like rotting meat, the plants are primed to attract these flies. The plants are tapping into the flies’ reproductive habits, a biological imperative so strong that they simply do not evolve a means of discriminating a rotting corpse from a flower that smells like one. This is the trick. Flies land on the flower thinking they have found a meal and a place to lay their eggs. They go through the motions as expected and pick up or deposit pollen in the process. Unfortunately for the flies, their offspring are doomed. There is not food to be found in these flowers.

What is most remarkable about the reproductive ecology of S. pumilum is that not just any type of fly will do. It appears that only a specific subset of flies actually visit the flowers and act as effective pollinators. Amazingly, this provides insights into some long-running hypotheses regarding carrion fly ecology.

(A) The habitat of S. pumilum (B) Satyrium pumilum in situ (scale bar = 1 cm). (C–E) Pollination sequence of a S. pumilum flower by a sarcophagid fly in an arena (scale bar for all three photos = 0·5 cm); (C) the fly carrying five pollinaria from other S. pumilum flowers enters an unpollinated flower (D) as the fly moves deeper into the flower towards the right-hand spur, it presses an attached pollinium against the stigma, and its thorax against the right-hand viscidium; (E) as it leaves the flower, the fly has deposited two massulae on the stigma (1), and removed a pollinarium (2) – it now carries six pollinaria. [SOURCE]

Apparently there has been a lot of debate in the fly community over why we see so many different species of carrion flies. Rotting meat is rotting meat, right? Probably not, actually. Fly ecologists have comes up with a few hypotheses involving niche segregation among carrion flies to explain their diversity on the landscape. Some believe that flies separate themselves out in time, with different species hatching out and breeding at different times of the year. Others have suggested that carrion flies separate themselves by specializing on carrion at different stages of decay. Still others have suggested that some flies specialize on large pieces of carrion whereas others prefer smaller pieces.

By studying the types of flies visiting the flowers of S. pumilum researchers did find evidence of niche segregation based on carrion size. It turns out that S. pumilum is exclusively pollinated by a group of flies known as sarcophagid carrion flies. These flies were regularly observed with orchid pollen sacs stuck to their backs and plants seemed to only set seed after they had been visited by members of this group. So, what is it about these flowers that makes them so specific to this group of flies?

The answer lies both in their size as well as the amount of scent they produce. It is likely that the quantity of scent compounds produced by S. pumilum most closely mimics that of smaller rotting corpses. The types of flies that visited these blooms were mostly females of species that lay relatively few eggs compared to other carrion flies. It could very well be that the smaller brood size of these flies allows them to effectively utilize smaller bits of carrion than other, more fecund species of fly. To date, this is some of the best evidence in support of the idea that flies avoid competition among different species by segregating out their feeding and reproductive niches.

Rotting meat smells are not uncommon in the plant world. Even within the home range of S. pumilum, there are other plants produce flowers that smell like carrion as well. It would be extremely interesting to look at what kinds of flies visit other carrion flowers and in what numbers. Like I mentioned earlier, reproductive is such a major part of any organisms life that it may simply be too costly for carrion flies to evolve a means of discriminating real and fake breeding sites. It is amazing to think of what we gain from trying to understand the reproductive biology of a small, obscure orchid growing tucked away in arid regions of South Africa.

Photo Credits: [1] [2]

Further Reading: [1]

An Intriguing Way of Presenting One's Pollen

Photo by Monteregina (Nicole) licensed by CC BY-NC-SA 2.0

Getting pollen from one flower to another is the main reason why flowers exist in the first place. It makes sense then why pollen is often made readily available to pollinators. For many flowering plants, this means directing the pollen-filled anthers outward where they are ready to take advantage of floral visitors. The sunflower family (Asteraceae) does this a bit differently than most. They utilize a technique called secondary pollen presentation.

Though secondary pollen presentation is not unique to the sunflower family, their abundance on the landscape makes it super easy to observe. For the sunflower family, what looks like a single flower is actually an inflorescence made up of dense clusters of individual flowers. Each individual flower is roughly tubular in shape and, oddly enough, the anthers are tucked inside the tube facing the interior of the flower. It may seem odd to hide the anthers and their pollen inside of a tube until you see the blooming process sped up.

Photo by László Németh licensed by CC BY-SA 3.0

The sunflower family actually relies on the female parts of the flower to bring the pollen out from the floral tube and into the environment where pollinators can access it. Members of the sunflower family are protandrous, meaning the male parts mature before the female parts. What this means is that the style of the flower can be involved in presenting pollen before it becomes receptive to pollen. This allows enough time for pollen presentation and reduces the likelihood of self pollination.

As the style elongates within the floral tube, one of two things can happen with the pollen inside. In some cases, the style acts like a tiny piston, literally pushing the pollen out into the world. In other cases, the style is covered in tiny, brush-like hairs that rake the pollen from the sides of the floral tube and carry it out as it emerges. In both cases, the style remains closed until enough time has passed for pollen to be taken away from the inflorescence.

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After a period of time (which varies from species to species), the style splits at the tip and each side curls back on itself to reveal the stigmatic surface. Only at this point in time is are the female parts of the flower mature and ready to receive pollen. With any luck, much of the flowers own pollen would have been collected and taken away to other plants.

The combination of sequential blooming of individual flowers and protandry mean that members of the sunflower family both maximize their chances of pollination and reduce the likelihood of inbreeding. Add to that their ability to disperse their seeds great distances and myriad defense strategies and it should come as no surprise that this family is so darn successful. Get outside and try to witness secondary pollen presentation for yourself. Armed with a hand lens, you will unlock a world of evolutionary wonders!

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

Further Reading: [1] [2]

Of Bladderworts & Birds

Photo by Jean and Fred licensed by CC BY 2.0

Bladderworts are as beautiful as they are deadly. Though they are known the world over for their carnivorous bladder traps, their flowers are something to marvel at as well. Bladderworts flower in a range of colors from yellows to whites, purples to reds. What’s more, the variety of shapes and sizes among bladderwort flowers are incredible. Though the vast majority of bladderwort species rely on insects for pollination, at least one species appears to have co-opted a bird for its reproductive needs.

Red coats (Utricularia menziesii) are endemic to a few coastal regions of western Australia. They are not floating aquatic plants like many of their North American cousins, nor do they grow epiphytically like many tropical bladderworts. Red coats are terrestrial in their habit. Moreover, they live in habitats that dry up for good portions of the year. As the soils dry out, red coats die back into tiny corms in which they store energy during their dry dormancy that will fuel growth as soon as rains return and the surrounding soils are once again saturated.

Photo by Jean and Fred licensed by CC BY 2.0

When conditions are right, red coats produce some of the most spectacular flowers of the entire genus. Though other species also produce red flowers, few produce such outlandishly bright blossoms. Moreover, the flowers themselves are rather robust structures complete with a long, tough nectar spur. Their color, form, and proximity to the ground has led more than one author to suggest that birds, not insects, are the main pollinators of this species.

Indeed, it appears that birds are what these flowers are attracting. Not just any bird will do either. It seems that the western spinebill (Acanthorhynchus superciliosus) is wonderfully primed to pollinate this lovely little carnivore. Red is a major attractant for birds and the fact that red coat flowers are presented so close to ground level places at the perfect height for ground-foraging spinebills. Also, the length, curvature, and nectar content of the nectar spur fits the spinebill beak nicely. Birds approach the plants on the ground and dip their long, curved beaks into the flower, picking up and depositing pollen as they go.

The western spinebill (Acanthorhynchus superciliosus). Note the curved beak. Photo by Jean and Fred licensed by CC BY 2.0

This isn’t the only bladderwort to be suspected of bird pollination. At least two others (Utricularia quelchii & Utricularia campbelliana) have been hypothesized to utilize hummingbirds for pollination. However, there is scant evidence for this. Pollination studies can be tricky like that. Without proper observation and study, one simply can’t confirm a particular pollination syndrome.

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

Further Reading: [1]