How Air Plants Drink

 Tillandsia tectorum. Photo by Edu licensed under CC BY-NC-ND 2.0

Air plants (genus Tillandsia) are remarkable organisms. All it takes is seeing one in person to understand why they have achieved rock star status in the horticulture trade. Unlike what we think of as a "traditional" plant lifestyle, most species of air plants live a life free of soil. Instead, they attach themselves to the limbs and trunks of trees as well as a plethora of other surfaces. 

Living this way imposes some serious challenges. The biggest of these is the acquisition of water. Although air plants are fully capable of developing roots, these organs don't live very long and they are largely incapable of absorbing anything from the surrounding environment. The sole purpose of air plant roots is to anchor them to whatever they are growing on. How then do these plants function? How do they obtain water and nutrients? The answer to this lies in tiny structures called trichomes. 

Trichomes are what gives most air plants their silvery sheen. To fully appreciate how these marvelous structures work, one needs some serious magnification. A close inspection would reveal hollow, nail-shaped structures attached to the plant by a stem. Instead of absorbing water directly through the leaf tissues, these trichomes mediate the process and, in doing so, prevent the plant from losing more water than it gains. 

The trichomes themselves start off as living tissue. During development, however, they undergo programmed cell death, leaving them hollow. When any amount of moisture comes into contact with these trichomes, they immediately absorb that water, swelling up in the process. As they swell, they are stretched out flat along the surface of the leaf. This creates a tiny film of water between the trichomes and the rest of the leaf, which only facilitates the absorption of more water. 

Trichomes up close.  Photo by Mark Smith1989 licensed under CC BY-SA 4.0

Because the trichomes form a sort of conduit to the inside of the leaf, water and any nutrients dissolved within are free to move into the plant until the reach the spongy mesophyll cells inside. In this way, air plants get all of their water needs from precipitation and fog. Not all air plants have the same amount of trichomes either. In fact, trichome density can tell you a lot about the kind of environment a particular air plant calls home. 

Photo by Bernard DUPONT licensed under CC BY-SA 2.0

The fuzzier the plant looks, the drier the habitat it can tolerate. Take, for instance, one of the fuzziest air plants - Tillandsia tectorum. This species hails from extremely arid environments in the high elevation regions of Ecuador and Peru. This species mainly relies on passing clouds and fog for its moisture needs and thus requires lots of surface area to collect said water. Now contrast that with a species like Tillandsia bulbosa, which appears to have almost no trichome cover. This smoother looking species is native to humid low-land habitats where high humidity and frequent rain provide plenty of opportunities for a drink. 

Photo by Bocabroms licensed under CC BY-SA 3.0

Absorbing water in this way would appear to have opened up a plethora of habitats for the genus Tillandsia. Air plants are tenacious plants and worthy of our admiration. One could learn a lot from their water savvy ways. 

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

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

The Extraordinary Catasetum Orchids

Male Catasetum osculatum. Photo by Orchi licensed under CC BY-SA 3.0

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. Photo by Valdison Aparecido Gil licensed under CC BY-SA 4.0

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. Photo by Maarten Sepp licensed under CC BY-SA 4.0

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.

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. 

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. 

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

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. Photo by maarten sepp licensed under CC BY-SA 2.0

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. Photo by Antonio Garces licensed under CC BY-NC-ND 2.0

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. 

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

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. Photo by sunoochi licensed under CC BY 2.0

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]

Seed Anchor

[SOURCE]

Epiphytic plants live out their entire lives on the trunks or branches of trees. Using their roots, they attach themselves tightly to the bark. Spend any amount of time in the tropics and it will become quite clear that such a lifestyle has been very successful for a plethora of different plant families. Still, living on a tree isn't easy. Epiphytic plants must overcome harsh conditions among or near the canopy.

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

One of the biggest challenges these plants face starts before they even germinate. This is especially true for orchids. Orchid seeds are more like spores than they are seeds. They are so small that thousands could fit inside of a thimble. Upon ripening, the dust-like seeds waft away on the slightest breeze. In order for epiphytic species to germinate and grow, their seeds must somehow anchor themselves in place on a trunk or branch. Inevitably most seeds are doomed to fail. They simply will not land in a suitable location. It stands to reason then that any adaptation that increases their chances of finding the right kind of habitat will be favored. That's where the strange coils on the tip of Chiloschista seeds, a genus of leafless orchids native to southeast Asia, New Guinea, and Australia, come in. For these orchids, this process is aided by some truly unique seed morphology.

Unlike most orchid seeds that are nothing more than a thin sheath surrounding a tiny embryo, the seeds of Chiloschista have additional parts. These "appendages," which are specialized seed coat cells, are tightly wound into coils. Upon contact with water, these coils shoot out like tiny grappling hooks that grab on to moss and bark alike. In doing so, they anchor the seed in place. By securing their hold on the trunk or branch of a tree, the seeds are much more likely to germinate and grow. This is one of the most extreme examples of seed specialization in the orchid family.

Photo Credit: [1] [2]

Further Reading: [1]

A Litter Trapping Orchid From Borneo

Ch'ien C. Lee - http://www.wildborneo.com.my/

Epiphytes live a unique lifestyle that can be quite challenging. Sure, they have a relatively sturdy place on a limb or a trunk, however, blistering sun, intense heat, and plenty of wind can create hostile conditions for life. One of the hardest things to come by in the canopy is a steady source of nutrients. Whereas plants growing in the ground have soil, epiphytes must make do with whatever falls their way. Some plants have evolve a morphology that traps falling litter. There are seemingly endless litter trapping plants out there but today I want to highlight one in particular.

Meet Bulbophyllum beccarii. This beautiful orchid is endemic to lowland areas of Sarawak, Borneo. What is most interesting about this species is how it grows. Instead of forming a clump of pseudobulbs on a branch or trunk, this orchid grows upwards, wrapping around the trunk like a leafy green snake. At regular intervals it produces tiny egg-shapes pseudobulbs which give rise to rather large, cup-shaped leaves. These leaves are the secret to this orchids success.

The cup-like appearance of the leaves is indeed functional. Each one acts like, well, a cup. As leaves and other debris fall from the canopy above, the orchid is able to capture them. Over time, a community of fungi and microbes decompose the debris, turning it into a nutrient-rich humus. Instead of having to compete for soil nutrients like terrestrial species, this orchid makes its own soil buffet!

If that wasn't strange enough, the flowers of this species are another story entirely. Every so often when conditions are just right, the plant produces an inflorescence packed full of hundreds of tiny flowers. The flowers dangle down below the leaves and emit an odor that has been compared to that of rotting fish. Though certainly disdainful to our sensibilities, it is not us this plant is trying to attract. Carrion flies are the main pollinators of this orchid and the scent coupled with their carrion-like crimson color attracts them in swarms.

The flies are looking for food and a place to lay their eggs. This is all a ruse, of course. Instead, they end up visiting a flower with no rewards whatsoever. Regardless, some of these flies will end up picking up and dropping off pollinia, thus helping this orchid achieve pollination.

Epiphyte diversity is incredible and makes up a sizable chunk of overall biodiversity in tropical forests. The myriad ways that epiphytic plants have adapted to life in the canopy is staggering. Bulbophyllum beccarii is but one player in this fascinating niche.

Photo Credits:
Ch'ien C. Lee - http://www.wildborneo.com.my/

Further Reading:
http://www.orchidspecies.com/bulbbeccarrii.htm

The Strangest Spiderworts

What if I told you that what you are looking at right now is a member of the spiderwort family (Commelinaceae)? At first, I didn't believe it either. Even after seeing those magnificent blooms, it took a bit of convincing. Regardless, the genus Cochliostema represents some of the strangest members of the family.

Cochliostema can be found growing in Central and South America. There are only two species in the genus, C. odoratissimum and C. jacobianum. My introduction to this group was Cochliostema odoratissimum. It is one of those plants that you smell before you see. The fragrance of the flowers is something worth experiencing. Because I lack the descriptive vocabulary needed to convey the proper respect, I'm going to ask you to trust me when I say that its lovely. The fragrance emanates from some seriously awesome flowers. It has been suggested that they are quite possibly the most complex flowers in the entire family. They are born on a type of spike called a "thyrse." Each flower consists of 3 sepals, 3 fringed petals, 3 stamens, which are fused in the upper half of the flower, and 3 carpels fused into a single pistil. The end result, as you can clearly see, is stunning.

The plants themselves are epiphytes, though they will grow terrestrially if they happen to fall from their host tree. As evidenced by their radial growth habit, they are akin to bromeliads in their ecology with at least one species considered a tank epiphyte. As such, they provide ample habitat in the canopy for a variety of flora and fauna.

Further Reading:

http://onlinelibrary.wiley.com/doi/10.1111/j.1095-8339.2000.tb02348.x/abstract