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]

My New Book Has Arrived!

The time has finally come! In Defense of Plants: An Exploration into the Wonder of Plants is now in stores. I thank everyone who pre-ordered a copy of the book. They should be on their way! I still can’t believe this is a reality. I always knew I wanted to write a book and I am eternally grateful to Mango Publishing for giving me this opportunity.

In Defense of Plants is a celebration of plants for the sake of plants. There is no denying that plants are extremely useful to humanity in many ways, but that isn’t why this exist. Plants are living, breathing, self-replicating organisms that are fighting for survival just like the rest of life on Earth. And, thanks to their sessile habit, they are doing so in remarkable and sometimes alien ways.

One of the best illustrations of this can be found in Chapter 3 of my new book: “The Wild World of Plant Sex.” Whereas most of us will have a passing familiarity with the concept of pollination, we have only really scratched the surface of the myriad ways plants have figured out how to have sex. Some plants go the familiar rout, offering pollen and nectar to floral visitors in hopes that they will exchange their gametes with another flower of the same species.

Others have evolved trickier means to get the job done. Some fool their pollinators into thinking they are about to get a free meal using parts of their anatomy such as fake anthers or by offering nectar spurs that don’t actually produce nectar. Some plants even pretend to smell like dying bees to lure in scavenging flies. Still others bypass food stimuli altogether and instead smell like receptive female insects in hopes that sex-crazed males won’t know the difference.

Pollination isn’t just for flowering plants either. In In Defense of Plants I also discuss some of the novel ways that mosses have converged on a pollination-like strategy by co-opting tiny invertebrates that thrive in the humid microclimates produced by the dense, leafy stems of moss colonies.

This is just a taste of what is printed on the pages of my new book. I really hope you will consider picking up a copy. To those that already have, I hope you enjoy the read when it arrives! Thank you again for support In Defense of Plants. You are helping keep these operations up and running, allowing me to continue to bring quality, scientifically accurate botanical content to the world. Thank you from the bottom of my heart.

Click here if you would like to order a copy!

You can also purchase a copy directly from the publisher

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]

Orchid Booby Traps

Pterostylis coccina. Photo by BerndH licensed under CC BY-SA 3.0

Looking as if they have escaped from some sort of modern art exhibit, the flowers of the various greenhood orchids (genus Pterostylis) are as complex as they are beautiful. Native throughout Australia, New Zealand, New Guinea, New Caledonia, and Indonesia, greenhood orchids number around 300 species, all of which are terrestrial. As more attention is paid to their ecology, we are also discovering that many of those 300 species utilize seriously complex trickery to increase their chances of being pollinated.

Pterostylis metcalfei. Photo by Geoff Derrin licensed under CC BY-SA 4.0

Though they vary in shape, size, and color, the flowers of greenhorn orchids roughly conform to a similar morphological theme. The dorsal sepal and two lateral petals are fused, forming a good-like structure, hence the hooded reference in the common name. On the front of the flower, the two lateral sepals also fuse near their base and taper into two points or wings at the top that give the flowers even more charisma. The whole structure forms a sort-of pitfall trap around the sexual organs. In many species, the lower petal or labellum often sticks up and out of the mouth of the floral tube and is frequently dressed in hairs or other protuberances.

Pterostylis turfosa. Photo by Geoff Derrin licensed under CC BY-SA 4.0

Greenhood orchid flowers are true marvels of evolution. Not only are they structurally complex, they are also painted in various shades of greens, whites, reds, and browns. Of course, all of this intricate beauty serves a single function for these orchids, sex. Essentially, greenhood orchid flowers are pollinator booby traps. Like so many other orchids, the greenhoods are tricksters, luring in their pollinators with the promise of food or even sex but offering nothing in return. Though we still have a lot to learn about pollination in this genus, what evidence we have compiled indicates that the promise of sex is the main ruse the greenhoods employ.

Pterostylis baptistii. Photo by Melburnian licensed under CC BY 3.0

The prevalence of male insects visiting the flowers of many greenhood species tells us these orchids achieve pollination via sexual deception. Lured in by scents that precisely mimic the pheromones of receptive female insects, the males land on the flower and begin searching for a mate. They inevitably begin exploring the labellum, which leads them down into the floral tube. At a certain point in their journey, the male insect will reach a tipping point on the labellum. Like an unbalanced seesaw, the labellum snaps backwards as the insect’s weight shifts, slamming the visitor into the column where it comes into contact with the reproductive organs.

The whole process seems very alarming for the unsuspecting victim. The male insect will struggle quite a bit before it finds a single escape route provided by the floral anatomy that ensures both pollen acquisition and deposition. Experiments have shown that this lever mechanism can be repeated upwards of 3 times within a few hours, so each flower has at least a few attempts to get the process right.

Pterostylis alpina. Photo by Melburnian licensed under CC BY 3.0

So, who are the insects that fall victim to the greenhood ruse? It turns out that its mostly small flies like fungus gnats and mosquitoes. The few detailed investigations that have been made into the pollination syndromes of these orchids has revealed surprisingly complex and often species-specific relationships between the plants and their pollinators. This makes sense from a chemical standpoint. The mating pheromones of one species of fly or mosquito are unlikely to attract males of different species. As such, the orchids trickery only works on one or possibly even a couple closely related species. Still, many mysteries abound in this diverse and widespread group of orchids and it will take a new generation of curious botanists and ecologists to uncover them.

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

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]

The Heartleaf Twayblade Orchid

Photo by Cptcv licensed under CC BY-ND 2.0.

The heartleaf twayblade is truly a sight for sore eyes.... that is, if you can find it. This diminutive orchid stands no more than 30 cm tall when in bloom and, for much of its life, exists as a single pair of tiny, heart-shaped leaves. Finding this species in bloom has been one of the major highlights of the last few years of botanizing. Getting to see it up close makes me wonder how many times I may have passed it over completely.

A closeup examination of the flowers will reveal what looks like tiny little humanoids. Indeed, the flowers are complex little structures. Tiny trigger hairs located at the base of the pollinia squirt glue on the back of visiting insects, which affixes the pollen sacs or pollinia. One to two days after the pollinia have been removed the stigmas become receptive to pollen. Though this orchid can self fertilize, differential ripening of sexual parts like this helps ensure cross pollination between different individuals.

With flowers so small, it is a wonder that insects can even find them. As it turns out, the flowers emit a foul smelling odor, though one would be hard pressed to detect it having to bend down so close to the forest floor. This attracts a wide variety of small insects like wasps and flies. The most common visitors, however, are fungus gnats. Ever abundant in the moist duff of the forest, these tiny dipterids offer plenty of opportunity for pollination. The orchid even sweetens the deal a bit by producing a small amount of nectar.

Being so small it is quite easy to overlook this plant. One must put in a bit of searching to find them. Their tiny size also means that they are often under-represented in conservation efforts as well. Entire populations can exist in only a few square meters of forest and thus are quite sensitive to disturbance. Timber harvesting and sprawl represent the largest threats the this species but luckily it has a surprisingly large geographic distribution. Still, keep an eye out for this lovely little species. They may be hard to find but they are well worth the effort!

Photo Credit: [1]

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

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]

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]

The Round Leaved Orchid

In the northern temperate regions of North America, late June marks the beginning of what I like to call orchid season. If you're lucky you may stumble across one of these rare beauties in full bloom. Their diversity in shape and size are mainly a result of the intricate evolutionary relationships they have formed with their pollinators. I spend much of my time botanizing trying to locate and photograph these botanical curiosities and any time I get to meet a new species is a very special time indeed. 

Take the round leaved orchid (Platanthera orbiculata) for example. For years I have only known this species as two round leaves that are slightly reminiscent of the phaleanopsis orchids you see for sale in nurseries and grocery stores. The leaves can be quite large too. With their glossy appearance, they are the easiest way to locate this plant.

When conditions are right and the plants have enough stored energy they will begin to flower. Rising from the middle of the pair of leaves is a decent sized inflorescence loaded with greenish white flowers. The flowers are interesting structures. Not particularly colorful, they have a long white lip and considerable green nectar spurs. There are said to be two varieties of this species, each being characterized by the length of the nectar spur. Unlike many orchids that offer no reward to pollinators, P. orbiculata produces nectar. The flowers are pollinated by noctuid moths, which is probably why they are white in color. Whereas most lepidopteran pollinated orchid attach their pollinia to the proboscis of the butterfly or moth, P. orbiculata attaches its pollinia to the eyes of visiting moths. 

If this isn't strange enough, the pollinia themselves have some of their own intriguing adaptations. Visiting moths take a certain amount of time to successfully access the nectar from the nectar spur. If the plant is to avoid wasting precious pollen on itself, then it must find a way to delay this process. The pollinia are the solution to this. When first attached to the eyes, the pollinia stick straight up. This keeps them away from the female parts of the plant as the moth feeds. Only after enough time has elapsed will the stalks of the pollinia begin to bend forward. At this point the moth will hopefully have moved on to the flowers of an unrelated individual. Pointing straight forward, they are now perfectly positioned to transfer pollen. 

Like all orchids, P. orbiculata relies on specialized mycorrhizal fungi for germination and survival. At the beginning of its life, P. orbiculata relies solely on the fungi for sustenance. Once it has enough energy to produce leaves it will repay the fungi by providing carbohydrates. However, the relationship is not over at this point. Every spring, P. orbiculata produces a new set of leaves as well as a whole new root system. The fungi supply a lot of energy for this process and if the plant is disturbed (ie. dug up by greedy poachers) or browsed upon, it is likely that it will not recover from the stress and it will die. The mycorrhizal fungi it relies on live on rotting wood so finding well rotted logs is a good place to start searching for this species. With declining populations throughout much of its range, it is important to remember to enjoy it where it grows. Leave wild orchids in the wild!

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

Twinspurs & Their Pollinators

© El Grafo / CC-BY-SA-3.0 (via Wikimedia Commons)

Garden centers and nurseries always have something to teach me. Though I am largely a native plant gardener, the diversity of plant life offered up for sale is always a bit mind boggling. Perusing the shelves and tables of myriad cultivars and varieties, I inevitably encounter something new and interesting to investigate. That is exactly how I came to learn about the twinspurs (Diascia spp.) and their peculiar floral morphology. Far from being simply beautiful, these herbaceous plants have evolved an interesting relationship with a small group of bees.

Diascia whiteheadii. Photo by Ragnhild&Neil Crawford licensed under CC BY-SA 2.0

The genus Diascia comprises roughly 70 species and resides in the family Scrophulariaceae. They are native to a decent chunk of southern Africa and have adapted to a range of climate conditions. Most are annuals but some have evolved a perennial habit. The reason these plants caught my eye was not the bright pinks and oranges of their petals but rather the two spurs that hang off the back of each bloom. Those spurs felt like a bit of a departure from other single-spurred flowers that I am used to so I decided to do some research. I fully expected them to be a mutation that someone had selectively bred into these plants, however, that is not the case. It turns out, those two nectar spurs are completely natural and their function in the pollination ecology of these plants is absolutely fascinating.

Diascia rigescens photo by Dinkum licensed under CC BY-SA 3.0

Not all Diascia produce dual spurs on each flower but a majority of them do. The spurs themselves can vary in length from species to species, which has everything to do with their specific pollinator. The inside of each spur is not filled with nectar as one might expect. Instead, the walls are lined with strange trichomes and that secrete an oily substance. It’s this oily substance that is the sole reward for visiting Diascia flowers.

Diascia megathura (a) inflorescenc with arrows indicating spurs and (b) cross sectioned spur showing the trichomes secreting oil (Photos: G. Gerlach).

If you find yourself looking at insects in southern Africa, you may run into a genus of bees called Rediviva whose females have oddly proportioned legs. The two front legs of Rediviva females are disproportionately long compared to the rest of their legs. They look a bit strange compared to other bees but see one in action and you will quickly understand what is going on. Rediviva bees are the sole pollinators of Diascia flowers. Attracted by the bright colors, the bees alight on the flower and begin probing those two nectar spurs with each of their long front legs.

If you look closely at each front leg, you will notice that they are covered in specialized hairs. Those hairs mop up the oily secretions from within each spur and the bee then transfers the oils to sacs on their hind legs. What is even more amazing is that each flower seems to have entered into a relationship with either a small handful or even a single species of Rediviva bee. That is why the spur lengths differ from species to species - each one caters to the front leg length of each species of Rediviva bee. It is worth noting that at least a few species of Diascia are generalists and are visited by at least a couple different bees. Still, the specificity of this relationship appears to have led to reproductive isolation among many populations of these plants, no doubt lending to the diversity of Diascia species we see today.

Diascia 'Coral Belle' Photo by KENPEI licensed under CC BY-SA 3.0

The female bees do not eat the oils they collect. Instead, they take them back to their brood chambers, feed them to their developing offspring, and use what remains to line their nests. At this point it goes without saying that if Diascia were to disappear, so too would these bees. It is incredible to think of the myriad ways that plants have tricked their pollinators into giving up most, if not all of their attention to a single type of flower. Also, I love the fact that a simple trip to a garden center unlocked a whole new world of appreciation for a group of pretty, little bedding plants. It just goes to show you that plants have so much more to offer than just their beauty.

Photo Credits: [1] [2] [3] [4] [5] [6]

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

The Upside Down World of Orchid Flowers

Did you know that most orchid flowers you see are actually blooming upside down? That's right, referred to as "resupination," the lower lip of many orchid flowers is actually the top petal and, as the flower develops inside the bud, the whole structure makes a 180° rotation. How and why does this happen?

The lip of an orchid flower usually serves to attract pollinators as well as function as a landing pad for them. The flower of an orchid is an incredibly complex organ with an intriguing evolutionary history. Basically, the lip is the most derived structure on the flower and, in most cases, it is the most important structure in initiating pollination.

The non-resupinate flowers of the grass pink (Calopogon tuberosus) showing the lip on top.

As an orchid flower bud develops, it begins to exhibit gravitropic tendencies, meaning it responds to the pull of gravity. By removing specific floral organs like the column and pollinia, researchers found that they produce special hormones called auxins that tell the developing bud to begin the process of resupination. The ovary starts to twist, causing the flower to stand on its head.

Not all orchids exhibit resupinate flowers. Grass pinks (Calopogon tuberosus) famously bloom with the lip pointing up as it does in the early stages of bud development. It is an interesting mechanism and serves to demonstrate the stepwise tendencies that the forces of natural selection and evolution can manifest. But why does it occur at all? What is the evolutionary advantage of resupinate flowers?

Not only are Dracula flowers resupinate, many species also face them towards the ground.

The most likely answer to this biological twist is that, for orchids, resupination places the lip in such a way that facilitates pollination by whatever the flowers are attracting. For many orchids, this means providing an elaborate landing strip in the form of the lip. For the grass pinks, which operate by slamming visiting bees downward onto the column to achieve pollination, placing the lip at the top makes more mechanical sense. When a bee visits the upward pointing lip thinking it will find a pollen-rich meal, the lip bend at the base like a hinge. Anything goes in evolution provided the genes are present for selection to act upon and nowhere is this fact more beautifully illustrated than in orchids.

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

Viper's Bugloss

Photo by Derek Parker licensed under CC BY-NC-ND 2.0

Throughout much of North America, brown fields, roadsides, and other waste places occasionally take on a wonderful blue hue. Often time the cause of this colorful display is none other than Echium vulgare, or as its commonly referred to, viper's bugloss. Viper’s bugloss is a member of the borage family and was originally native to most of Europe and Asia. However, humans introduced it to North America some time ago. It has since naturalized quite well and is even considered invasive in parts of Washington. No matter your views on this plant, the reproductive ecology of this species is quite interesting.

Viper's bugloss produces its flowers on spikes. Starting off pink and gradually changing to blue as they mature, the flowers ripen their male portions on their first day and ripen their female portions on the second day. This is known as "protandry." Plants that exhibit this lifestyle offer researchers a window into the advantages and disadvantages with regards to the fitness investment of each sex. What they have found in viper's bugloss is that there are clearly distinct strategies for each type of flower.

Male flowers are pollinator limited. They must hedge their bets towards increasing the number of visitors. Bees are the main pollinators of this species and the more bees that visit, the more pollen can be disseminated. Unlike female flowers, which are resource limited, male flowers can produce pollen and nectar quite cheaply. Because of this, male flowers produce significantly more nectar than female flowers to bring in more bees. As the anthers senesce and give way to receptive styles, things begin to change. The plant now has to redirect resources into producing seed. At this point, resources are everything. The plant produces considerably less nectar resources than pollen but the bees can’t know that without visiting.

Photo by BLMIdaho licensed under CC BY 2.0

The other interesting aspect its reproductive ecology has to do with population size. Bees are notorious for favoring plants that are more numerous on the landscape. This makes a lot of sense. Why spend time looking for uncommon plants when they can go for easier, more numerous targets. This can be very detrimental to the fitness of rare plant species. However, plants like viper's bugloss don't seem to fall victim to this.

By looking at large and small populations, researchers found that pollination success pretty much evens out for viper's bugloss no matter how numerous it is in a given area. Large populations receive many more visits from bees but the bees spend less time on each flower. When viper's bugloss populations are small, flowers receive fewer visits but bees spend more time at each flower. This results is no significant difference in the reproductive fitness of either population.

Considering how efficient this plant is reproductively, it is no wonder it has done so well outside of its native range. Add to this its ability to grow in some of the worst soil conditions, it goes without saying that viper's bugloss is here to stay. If you find this species growing, certainly take time to get up close with the flowers. You will be glad you did.

Photo Credits: [1] [2]

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