Apocynaceae Ant House

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The dogbane family, Apocynaceae, comes in many shapes, sizes, and lifestyles. From the open-field milkweeds we are most familiar with here in North America to the cactus-like Stapeliads of South Africa, it would seem that there is no end to the adaptive abilities of this family. Being an avid gardener both indoors and out, the diversity of Apocynaceae means that I can be surrounded by these plants year round. My endless quest to grow new and interesting houseplants was how I first came to know a genus within the family that I find quite fascinating. Today I would like to briefly introduce you to the Dischidia vines.

Bullate leaves help the vine clasp to the tree as well as house ant colonies.

Bullate leaves help the vine clasp to the tree as well as house ant colonies.

The genus Dischidia is native to tropical regions of China. Like its sister genus Hoya, these plants grow as epiphytic vines throughout the canopy of warm, humid forests. Though they are known quite well among those who enjoy collecting horticultural curiosities, Dischidia as a whole is relatively understudied. These odd vines do not attach themselves to trees via spines, adhesive pads, or tendrils. Instead, they utilize their imbricated leaves to grasp the bark of the trunks and branches they live upon.

The odd, bulb-like leaves of the urn vine ( Dischidia rafflesiana )

The odd, bulb-like leaves of the urn vine (Dischidia rafflesiana)

One thing we do know about this genus is that most species specialize in growing out of arboreal ant nests. Ant gardens, as they are referred to, offer a nutrient rich substrate for a variety of epiphytic plants around the world. What's more, the ants will visciously defend their nests and thus any plants growing within.

The flowers of   Dischidia ovata

The flowers of Dischidia ovata

Some species of Dischidia take this relationship with ants to another level. A handful of species including D. rafflesiana, D. complex, D. major, and D. vidalii produce what are called "bullate leaves." These leaves start out like any other leaf but after a while the edges stop growing. This causes the middle of the leaf to swell up like a blister. The edges then curl over and form a hollow chamber with a small entrance hole.

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These leaves are ant domatia and ant colonies quickly set up shop within the chambers. This provides ample defense for the plant but the relationship goes a little deeper. The plants produce a series of roots that crisscross the inside of the leaf chamber. As ant detritus builds up inside, the roots begin to extract nutrients. This is highly beneficial for an epiphytic plant as nutrients are often in short supply up in the canopy. In effect, the ants are paying rent in return for a place to live.

Growing these plants can take some time but the payoff is worth. They are fascinating to observe and certainly offer quite a conversation piece as guests marvel at their strange form.

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

Further Reading: [1]

The Extraordinary Catasetum Orchids

Male  Catasetum osculatum

Male Catasetum osculatum

Orchids, in general, have perfect flowers in that they contain both male and female organs. However, in a family this large, exceptions to the rules are always around the corner. Take, for instance, orchids in the genus Catasetum. With something like 166 described species, this genus is interesting in that individual plants produce either male or female flowers. What's more, the floral morphology of the individual sexes are so distinctly different from one another that some were originally described as distinct species. 

Female  Catasetum osculatum

Female Catasetum osculatum

In fact, it was Charles Darwin himself that first worked out that plants of the different sexes were indeed the same species. The genus Catasetum enthralled Darwin and he was able to procure many specimens from his friends for study. Resolving the distinct floral morphology wasn't his only contribution to our understanding of these orchids, he also described their unique pollination mechanism. The details of this process are so bizarre that Darwin was actually ridiculed by some scientists of the time. Yet again, Darwin was right. 

Catasetum longifolium

Catasetum longifolium

If having individual male and female plants wasn't strange enough for these orchids, the mechanism by which pollination is achieved is quite explosive... literally. 

Catasetum orchids are pollinated by large Euglossine bees. Attracted to the male flowers by their alluring scent, the bees land on the lip and begin to probe the flower. Above the lip sits two hair-like structures. When a bee contacts these hairs, a structure containing sacs of pollen called a pollinia is launched downwards towards the bee. A sticky pad at the base ensures that once it hits the bee, it sticks tight. 

Male Catasetum flower in action. Taken from BBC's Kingdom of Plants.

Male Catasetum flower in action. Taken from BBC's Kingdom of Plants.

Bees soon learn that the male flowers are rather unpleasant places to visit so they set off in search of a meal that doesn't pummel them. This is quite possibly why the flowers of the individual sexes look so different from one another. As the bees visit the female flowers, the pollen sacs on their back slip into a perfect groove and thus pollination is achieved. 

Eulaema polychroma  visiting  Catasetum integerrimum

Eulaema polychroma visiting Catasetum integerrimum

The uniqueness of this reproductive strategy has earned the Catasetum orchids a place in the spotlight among botanists and horticulturists alike. It begs the question, how is sex determined in these orchids? Is it genetic or are there certain environmental factors that push the plant in either direction? As it turns out, light availability may be one of the most important cues for sex determination in Catasetum

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A paper published back in 1991 found that there were interesting patterns of sex ratios for at least one species of Catasetum. Female plants were found more often in younger forests whereas the ratios approached an even 1:1 in older forests. What the researchers found was that plants are more likely to produce female flowers under open canopies and male flowers under closed canopies. In this instance, younger forests are more open than older, more mature forests, which may explain the patterns they found in the wild. It is possible that, because seed production is such a costly endeavor for plants, individuals with access to more light are better suited for female status. 

Catasetum macrocarpum

Catasetum macrocarpum

Aside from their odd reproductive habits, the ecology of these plants is also quite fascinating. Found throughout the New World tropics, Catasetum orchids live as epiphytes on the limbs and trunks of trees. Living in the canopy like this can be stressful and these orchids have evolved accordingly. For starters, they are deciduous. Most of the habitats in which they occur experience a dry season. As the rains fade, the plants will drop their leaves, leaving behind a dense cluster of green pseudobulbs. These bulbous structures serve as energy and water stores that will fuel growth as soon as the rains return. 

Catasetum silvestre in situ

Catasetum silvestre in situ

The canopy can also be low in vital nutrients like nitrogen and phosphorus. As is true for all orchids, Catasetum rely on an intimate partnership with special mychorrizal fungi to supplement these ingredients. Such partnerships are vital for germination and growth. However, the fungi that they partner with feed on dead wood, which is low in nitrogen. This has led to yet another intricate and highly specialized relationship for at least some members of this orchid genus. 

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Mature Catasetum are often found growing right out of arboreal ant nests. Those that aren't will often house entire ant colonies inside their hollowed out pseudobulbs. This will sometimes even happen in a greenhouse setting, much to the chagrin of many orchid growers. The partnership with ants is twofold. In setting up shop within the orchid or around its roots, the ants provide the plant with a vital source of nitrogen in the form of feces and other waste products. At the same time, the ants will viciously attack anything that may threaten their nest. In doing so, they keep many potential herbivores at bay.  

Female  Catasetum planiceps

Female Catasetum planiceps

To look upon a flowering Catasetum is quite remarkable. They truly are marvels of evolution and living proof that there seems to be no end to what orchids have done in the name of survival. Luckily for most of us, one doesn't have to travel to the jungles and scale a tree just to see one of these orchids up close. Their success in the horticultural trade means that most botanical gardens house at least a species or two. If and when you do encounter a Catasetum, do yourself a favor and take time to admire it in all of its glory. You will be happy that you did. 

Photo Credits: [1] [2] [3] [4] [5] [6] [7] [8] [9] 

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

Devil's Gardens

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Imagine, if you will, walking through the dense understory somewhere in the Amazon basin. Diversity reigns supreme here and it would seem that every few steps reveals myriad new plant species. As you walk along, something in the vegetation changes. You stumble into a clearing in the middle of the forest dominated entirely by a single species of tree. Why the sudden change? How did this monoculture develop? You, my friend, have just found yourself on the edge of a Devil's garden. 

Devil's gardens are said to be the resting place of an evil spirit known to local tribes as Chullachaki. Anyone unlucky enough to stumble into his garden is said at risk of attack or curse. In reality, these gardens have a biological origin. The real gardeners are a handful of ant species which seem to have rather specific gardening preferences. Careful inspection would reveal that the gardens largely consist of trees in one of three genera - Duroia, Tococa, or Clidemia

Tococa  sp. (Melastomataceae)

Tococa sp. (Melastomataceae)

The reason that ants are so fond of these genera has to do with housing. These plant groups contain species which produce swellings along their stems and petioles known as domatia. These domatia are hollow and are the favorite nesting spots of various ant species. Ant colonies set up shop within. As anyone who has ever blundered into an ant colony can attest, ants are quite voracious at defending their home. 

By providing ant colonies with a home base, these plants have essentially hired body guards. It is a wonderful form of symbiosis in which the ants aggressively defend against anything that might want to take a bite out of their host tree. Any herbivore trying to take up residence or lay eggs within the Devil's garden is viciously attacked. In doing so, the ants are protecting their host trees at the cost of all other plants unlucky enough to germinate within the garden. Still, this anti-herbivore behavior doesn't totally explain the monoculture status these host trees achieve within the garden itself. Why are these gardens so ominously devoid of other plant species?

To answer this, one would have to watch how the ants behave as they forage. While scouting, if ants encounter a seedling of their host tree, nothing really happens. They go about their business and let the seedling grow into a future home. When they encounter a non-host tree, however, their behavior completely changes. 

Behold - A Devil's Garden

Behold - A Devil's Garden

The ants begin biting the stem of the plant, exposing its vascular tissue. As they bite, the ants also sting the foreign seedling, injecting minute amounts of formic acid into the wound. One or two ants isn't enough to bring down a seedling but one thing ants have on their side are numbers. Soon an entire platoon of ants descend upon the hapless seedling, stinging it repeatedly. In no time at all, the seedling succumbs to the formic acid injections and dies. By repeating this process any time a foreign plant is found growing within the vicinity of the garden, the resident ants ensure that only trees that will produce domatia are allowed to grow in their garden. Thus, a Devil's garden has been formed. 

Although this relationship seems incredibly beneficial for each party, it does come at some cost to the plants themselves. Certainly forming the domatia is a costly endeavor on the part of the plant, but research has also shown that growing in such high, monoculture-like densities in the jungle has its downsides. It has been found that individual host trees can actually experience more herbivore pressures when growing within a Devil's garden than if it was growing alone, elsewhere in the forest. 

Despite their aggression towards herbivores, the ants simply cannot be everywhere at once. As such, the high densities of host tree species within a Devil's garden act like a dinner bell for any insect that enjoys feeding on that particular type of plant. Essentially, the ants are concentrating a potential food source. Experts believe that this might explain why Devil's gardens never completely take over entire swaths of forest. Essentially, there are diminishing returns to living in such high densities. Still, benefits must outweigh costs if such mutualisms are to be maintained and it is quite obvious that both plant and ant benefit from this interaction to a great degree. 

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

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

Live-In Mites

Hearing the word "mite" as a gardener instantly makes me think of pests such as spider mites. This is not fair. The family to which mites belong (Acari) is highly varied and contains many beneficial species. Many mites are important predators at the micro scale. Some are fungivorous, eating potentially harmful species of fungi. Whereas this may be lost on the majority of us humans, it is certainly not lost on many species of plants. In fact, the relationship between some plants and mites is so strong that these plants go as far as to provide them with a sort of home.

Domatia are specialized structures that are produced by plants to house arthropods. A lot of different plant species produce domatia but not all of them are readily apparent to us. For instance, many trees and vines such as red oak (Quercus rubra), sugar maples (Acer saccharum), black cherries (Prunus serotina), and many species of grape (Vitis spp.) produce tiny domatia specifically for mites. The domatia are often small, hairy, and function as shelter for both the mites and their eggs.

By housing certain species of mites, these plants are ensuring that they have a steady supply of hunters and cleaners living on their leaves. Predatory mites are voracious hunters, keeping valuable leaves free of microscopic herbivores while frugivorous mites clean the leaves of detrimental fungi that are known to cause infections such as powdery mildew. The exchange is pretty straight forward. Mites get a home and a place to breed and the plants get some protection. Still, some plants seem to want to sweeten the relationship in a literal sense.

Some plants, specifically grape vines in the genus Vitis, also produce extrafloral nectaries on their leaves. These tiny glands secrete sugary nectar. In a paper recently published in the Annals of Botany, it was found that extrafloral nectaries enhances the efficacy of these mite domatia by enticing more mites to stick around. By adding nectar to domatia-producing leaves that did not secrete it, the researchers found that nectar increases beneficial mite densities on these leaves by 60 - 80%. This translates to an increase in fitness for these plants in the long run.

I love research like this. I had no idea that so many of my favorite and most familiar tree and vine species had entered into an evolutionary relationship with beneficial mites. This adds a whole new layer of complexity to the interactions within any given environment. It just goes to show you how much is left to be discovered in our own back yards.

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

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

Carnivorous Bromeliads

Brocchinia reducta

Brocchinia reducta

I would like to introduce you to quite possibly the strangest members of the Bromeliad family - those of the genus Brocchinia. Aside from the odd appearance and habits of this particular group, researchers have learned quite a bit about the Bromeliad family as a whole from studying this group. From their origins to their impressive radiation, Brocchinia offers us a window into the history of this charismatic family.

Brocchinia are considered sister to all other bromeliads. They were the first genus to diverge some 70 million years ago. Their center of origin can be traced back to the Guayana Shield, a region in the northeast corner of what is now South America. The earliest members of this group were likely terrestrial plants growing in nutrient poor areas. Surprisingly, the epiphytic nature of many bromeliad species we know and love today evolved more recently.

Brocchinia reducta

Brocchinia reducta

Since this time, Brocchinia has undergone an impressive adaptive radiation. Because they have remained specialists on nutrient poor soils, much of this radiation has to do with the evolution of nutrient acquisiton. Like its cousins, Brocchinia utilize a "tank" formed by their tightly clasped leaves.

Interestingly, at least two species of Brocchinia, (B. reducta and B. hechtioides) have taken this to the extreme and have adopted a carnivorous lifestyle. Their tall, columnar growth form coupled with slick, waxy leaves means insects can't keep a foothold and fall down into the tank. The plants sweeten the deal by luring them in with sweet secretions.

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Whether or not this was a case of true carnivory was highly debated until 2005 when a group of researchers analyzed the chemical makeup of the liquid inside the tank. They discovered that they plant was secreting an enzyme called phosphatase, which actively digest hapless insects that fall in. A true carnivore indeed!

Others have even more peculiar evolutionary adaptations for nutrient acquisition. B. tatei, for example, was discovered to house nitrogen-fixing cyanobacteria within its tank. Another species, B. acuminata, produces hallow chambers at the base of its leaves that house ant colonies. The ants pay rent via their nutrient-rich waste and voracious defense of their bromeliad home.

Brocchinnia acuminata

Brocchinnia acuminata

In total, this group is quite amazing. The amount of information we have been able to glean from studying Brocchinia has allowed us to shine a light on the whole bromeliad family. As we have also seen, the species within this group have quite the evolutionary history to tell in their own way. Brocchinia serves as a reminder to researchers blind to organismal study. We shouldn't have to study ecology at the expense of individual organisms. There is plenty to learn from both avenues of research.

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

Further Reading: [1] [2]

Orchid Ant Farms

I am beginning to think that there is no strategy for survival that is off-limits to the orchid family. Yes, as you may have figured out by now, I am a bit obsessed with these plants. Can you really blame me though? Take for instance Schomburgkia tibicinis (though you may also see it listed under the genera Laelia or more accurately, Myrmecophila). These North, Central, and South American orchids are more commonly referred to as cow-horn orchids because they possess hollow pseudobulbs that have been said to been used by children as toy horns. What is the point of these hollow pseudobulbs?

A paper published back in 1989 in the American Journal of Botany found the answer to that question. As it turns out, ants are quite closely associated with orchids in this genus. They crawl all over the flowers, feeding on nectar. The relationship goes much deeper though. If you were to cut open one of these hollow pseudobulbs, you would find ant colonies living within them. The ants nest inside and often pile up great stores of food and eventually waste within these chambers. The walls of the chambers are lined with a dark tissue that was suspect to researchers.

Using radioactively labeled ants, the researchers found that the orchids were actually taking up nutrients from the ant middens! What's more, nutrients weren't found solely in adjacent tissues but also far away, in the actively growing parts of the roots. These orchids are not only absorbing nutrients from the ants but also translocating it to growing tissues.

While orchids without a resident ant colony seem to do okay, it is believed that orchids with a resident ant colony do ever so slightly better. This makes sense. These orchids grow as epiphytes on trees, a niche that is not high in nutrients. Any additional sources of nutrients these plants can get will undoubtedly aid in their long-term survival. Also, because the ants use the orchids as a food source and a nest site, they are likely defending them from herbivores.

Photo Credit: Scott.Zona (http://bit.ly/1hvWiGX)

Further Reading:
http://www.jstor.org/stable/2444355