How Air Plants Drink

  Tillandsia tectorum

 Tillandsia tectorum

Air plants (genus Tillandsia) are remarkable organisms. All it takes is seeing one in person to understand why they have achieved rock start 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. 

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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.  

Trichomes up close.  

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. 

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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. 

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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. 

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

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

The Stinging Nettles

We've all been there at some point. It's summer, it's a beautiful day, and you find yourself strolling along a trail. You are walking along, enjoying the sights, sounds, and smells of your environment when you harmlessly brush by a patch of waist-high plants. You don't think anything of it. They are herbaceous and don't readily catch the eye. A few steps later and the burning starts. It is mild at first but wherever your skin met the tissues of those plants an itchy, burning sensation starts to amplify. You have likely just encountered a species of stinging nettle. 

Nettles hail from a handful of genera. There are many different species of nettle but you are most likely to encounter either stinging nettle (Urtica dioica) or the wood nettle (Laportea canadensis), all of which belong to the nettle family (Urticaceae). A closer inspection of the plant reveals that the stems as well as the underside of the leaves are covered in tiny hairs. These hairs are called trichomes. A subset of these trichomes are what caused your discomfort. 

Anatomy of a stinging trichome

Anatomy of a stinging trichome

These trichomes have been honed by natural selection into a very effective defense. They are an elongated cell that sits atop of a multicellular pedestal. They are quite brittle and any contact with them causes their tips to break. They are also hollow and once they are broken, they essentially function like mini hypodermic needles. They penetrate the skin of any animal unlucky enough to brush up against them and inject an irritating fluid into the tissues of their "attacker." The fluid itself is quite interesting. Chemical analyses have revealed that it consists of a complex mixture of histamines, acetylcholine, serotonin, and even formic acid. Chemists are still working out the exact makeup of this chemical weapon and how much variation there is between different stinging species. 

As you might have deduced by this point, these stinging hairs are a defense mechanism. They protect the plant from herbivores. However, not all herbivores are deterred by this defense. It was found that invertebrates don't seem to have any issue navigating the stinging hairs. Instead, it is thought that the stinging nature of these plants evolved in response to large mammalian herbivores. This makes some sense as larger herbivores pose more of a threat to the entire plant than do invertebrates.

Stinging nettle ( Urtica dioica ) 

Stinging nettle (Urtica dioica

Even more interesting is the response of some nettles to varying levels of herbivory. It has been found that heavily damaged plants will regrow leaves and stems with higher densities of stinging hairs than those of plants that have experienced lower rates of herbivory. This too makes a lot of sense. Stinging hairs require resources to produce so plants that have not experienced high rates of herbivory do not bother allocating precious resources to their production.

Even more interesting is the fact that for stinging nettle (U. dioica), male and female plants tend to have differing densities of stinging hairs. Female plants produce more stinging hairs than males. It is thought that since females must invest more resources into producing seeds than males do into producing pollen, they must also invest in more protection for these valuable reproductive assets. 

These nettles are not alone in producing such stinging trichomes. Many other plant species have converged on this defensive strategy. If you have ever experienced this for yourself, you can really understand just how effective it can be. 

Wood nettle ( Laportea canadensis )

Wood nettle (Laportea canadensis)

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

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

The Termite-Eating Nepenthes

Plants and eusocial insects have some interesting ecological relationships with one another. A vast majority of these relationships are between a plant a members of the order Hymenoptera (ants, bees, and wasps), but what about those other eusocial insects, the termites?

Despite the social similarities they share with many ants, bees, and wasps, termites are actually distant relatives of the cockroaches. As many already know, termites also have a relationship with plants. Thanks to symbiotic bacteria residing in their gut, termites are able to make a living eating wood and building massive colonies, sometimes in undesirable locations like in the framework of your house. However, there is at least one species of plant out there that has evolved a different kind of relationship with termites.

Meet Nepenthes albomarginata. Native to Borneo, Malaysia, and Sumatra, this tropical carnivore seems to have a taste for termites. However, unlike flies or ants that are attracted to sweet nectar, termites have a different palate. Feeding on plant materials, termites don't necessarily seem like the kind of insect a plant would want to attract. N. albomarginata has seemingly found a way to attract tasty termites without becoming a meal itself.

As the specific epithet suggests, there is a white ring located around the margin of the pitchers' mouths. The ring is made up of a dense coat of hairs called trichomes. It was discovered that sometimes this white ring would disappear overnight. The pitchers without the white ring were also chock-full of partially digested termites. Just how the termites find these pitchers isn't quite certain. Researchers have not yet been able to isolate a scent compound.

Either way, the termites swarm the ring. While many termites make off with a free meal, plenty more of them slip and fall into the trap. It has been found that N. albomarginata obtains upwards of 50% of its nitrogen needs from termites in this way. What's more, all of this happens in a span of a single evening. Once the ring is picked clean, the pitchers are no longer attractive to the termites. They go their way and the plant has its meal. Because of the social structure of these peculiar insects, the loss of these individuals is never high enough to represent a serious selective pressure.

Further Reading: [1]