Lichens: A Burst of Color on a Dark Day

The lichens have awoken.

Lichens, moss, and mushrooms in the Palisades.
Photo by PBM. Used with permission.

 Late autumn, almost winter. The trees are bare and the sky is grey. A light mist falls. New York City seems devoid of life. Scaffolding drips and trains run late.

Grey day on the Hudson River.
Photo by PBM. Used with permission.

Across the Hudson River, a few miles away from the dead-grey of the city in the Palisades Interstate Park in New Jersey delicate greens and specks of mustard yellow coat rocks and trees in a quietly vibrant layer.

Lichens on a rock in the Palisades.
Photo by PBM. Used with Permission.

On a cheerful sunny day, the same lichen appear as a crusty dust on a rock, but in the dismal rain these creatures burst forth.

My rainy hike in the Palisades reminded me of my love for lichen and then I realized something rather embarrassing: I knew relatively nothing about my favorite organisms. The lichens I saw inspired me to learn more about them.

Lichens on a tree in the Palisades.
Photo by PBM. Used with permission.

Lichens are intricate beings, and before we can delve into their complexities, let’s start with the basics.

What Are Lichens?

Lichens are the product of a mutually-beneficial relationship between fungi (the mycobiont) and algae or cyanobacteria (the photobiont).

What does this mean?

Basically, a fungus and an algae or a cyanobacteria join forces for better living, and in the process, they form what we call a lichen.

Lichens on a gravestone at Cortland Rural Cemetery.
Photo by PBM. Used with permission.

Not all fungi and algae/cyanobacteria are well-suited for the lichen life. For fungi which thrive in a lichen relationship, the process of selecting a worthy photobiont is a ruthless one.

A fungi will try to form a partnership with any nearby algae/cyanobacteria. As the partnership is forming, the fungi try to kill off the algae/cyanobacteria. Any algae/cyanobacteria that survives the attempted slaughter is deemed a suitable partner by the fungi and the two will form a lichen together.

What do Lichens Look Like?

Internal Structure

In primitive lichens, the cells of the mycobionts and photobionts are thrown together in a miscellaneous mishmash, but more advanced lichens have distinct layers with specific functions.

Lichen diagram by blog author.

External Appearance

Lichens are strange hybrids which look like neither fungi nor algae/cyanobacteria. There are three main types of lichen: 

fruticose lichens, 

Fruticose lichen photo by Jason Hollinger. A creative commons image.
Source.

foliose lichens, 

Foliose lichen photo by Norbert Nagel. A creative commons image.
Source.

and crustose lichens.

Crustose lichen photo by Roger Griffith. A creative commons image.
Source.

Then, there are countless variations on these basic types.

The physical characteristics of lichens are influenced by lichen acids, byproducts of lichen metabolism. These acids often give a lichen its characteristic color.

Lichens in the sun look almost completely different than lichens on a rainy day. Why? Lichens shrivel up and hibernate when the air is dry to reduce water loss, they open up again to soak up water when the air is moist. Wet days are the best days to observe lichens in all their glory.

Pollution

Lichens absorb water indiscriminately without filtering it. This means they also absorb all the pollutants in the water. Often these poisonous pollutants are too much for the poor lichens to handle, which is why there aren’t many lichens in large cities. The lichens that survive intense pollution tend to be small since their already slow growth is stunted by pollution.

 

Lichen on a NYC street tree. Photo by blog author.

Lichen Reproduction

Some of the lichens I noticed on my hike were covered in dark dots. These dots are a common type of spore-producing growth called disc-shaped apothecia produced by the fungal partner of the lichen for sexual reproduction. Lichens produce spores all year round, and like lichens themselves, the spore production sites are mainly active when wet.

Lichens in the Palisades with spore-producing growths.
Photo by PBM. Used with permission.

The odd thing about lichens is since they are a combination of two organisms, only the fungal part of the lichen can reproduce sexually. The spores the fungi release may grow as a pure fungus, they may die, or they may find a photobiont partner and become lichens, possibly a different type of lichen then their parent.

Lichens also reproduce asexually when small pieces of the mother organism break off and are carried away animals, wind, or water to a new home. To speed along asexual reproduction, lichens produce isidia—small outgrowths which break off easily—and soredia—powdery granules of a few cells which can blow and float to a new location.

Lichen Longevity

Once a lichen does set up shop, it can live for thousands of years. Lichens grow incredibly slowly, so it is a good thing that they can live so long. One lichen in northern Sweden is thought to be over 9,000 years old!

Lichens on a gravestone at Cortland Rural Cemetery.
Photo by PBM. Used with permission.

Now that I’ve learned some lichen basics I hope to start identifying the species of lichens I see. If you know what species any of the lichens in the photos from this post are, please let me know in the comments!

Sources.

Invasive Species: Friends or Foes?

We’ve all heard horror stories about invasive species, but are introduced plants and animals really as bad as the media paints them?

News headlines on invasive species. Image by author. Sources for headlines.

The answer to this question depends on exactly which invasive species we are referring to.

The brown tree snake (Boiga irregularis) is an example of a truly harmful invasive species. 

Brown tree snake. Public domain image. Source.

The brown tree snake arrived on the small Pacific Island of Guam in the late 1940s or early 1950s as a stowaway on military cargo, and has since eaten most of Guam’s birds, lizards, and small mammals. Guam’s fauna never experienced predation by a large snake before, and they are suffering horribly under the reign of the invasive brown tree snake.

In addition to eating Guam’s native species, the brown tree snake causes frequent power outages. How can a snake cause power outages? By climbing pylons and shorting out the power circuits! Climbing pylons is a dangerous hobby as it kills the snake, inconveniences Guam’s citizens, and costs power companies too much money in repairs.

Brown tree snake on top of a fence post. Creative commons image. Source.

The brown tree snake also costs the US military stationed on Guam millions of dollars each year in safety measures to prevent this pesky predator from escaping to another island. So far, this money is well spent as the brown tree snake has not escaped Guam.

The only good thing about the brown tree snake is that it eats Guam’s rats.

But what about another invasive species, garlic mustard (Alliaria petiolata)? I remember when garlic mustard was one of the most hated plants in NYC. Garlic mustard grew in dense monocultures and drove out other plants. Every time I went on a walk I would see huge piles of garlic mustard the parks department ripped out of the soil in an attempt to eradicate this species. Now, I rarely see piles of dead garlic mustard. Instead, I see garlic mustard growing alongside other plants along the park paths. What caused this change?

Garlic mustard. Creative commons image. Source.

 Garlic mustard was deliberately introduced to the United States in 1868 on Long Island for its ability to control erosion and its medicinal properties. While garlic mustard may look innocent and charming with its little white flowers, this plant had serious issues with poisoning its neighbors. 

Sinigrin, the toxic chemical garlic mustard emits, kills nearby plants and mycorrhizal fungi. Mycorrhizal fungi are a type of fungi which colonize the roots of plants and help their plant partners absorb more water and nutrients. Without their mycorrhizal fungi, the lives of plants living near garlic mustard were jeopardized, which allowed garlic mustard to push these plants out. Soon garlic mustard was dominating the forest floor.

Now, garlic mustard has been living in the United States for a long time and it is learning to be a kinder neighbor. U.S. garlic mustard doesn’t emit as much sinigrin as it used to, and the plant can grow next to other species without killing them.

Garlic mustard flowers. Creative commons image. Source.

Why is this?

High levels of sinigrin release, seen when garlic mustard first was colonizing the states, is most beneficial for garlic mustard when the plant is mainly competing with other species. However, when garlic mustard is well established, it mainly competes with itself, which means low levels of sinigrin release are preferable. Low levels of sinigrin emission are currently so favorable for the U.S. garlic mustard population that the genetics of this population changed to make low levels sinigrin release widespread and innate.

Now that garlic mustard has had a chance to settle into its new home, it’s not acting particularly dangerous and invasive anymore, is it?

Purple loosestrife (Lythrum salicaria) and Canadian pondweed (Elodea canadensis) are two other introduced species, like garlic mustard, accused of forming monocultures and driving out native species. After forming dense stands during the first few years of introduction, purple loosestrife and Canadian pondweed populations both declined significantly, like garlic mustard did, and they now live peacefully next to native plants. Non-native species may need a few years to acclimate to their new homes. We may be accusing them of wrongdoing too quickly.

The purple haze in the background of this photo is purple loosestrife.
Photo by PBM. Used with permission.

Admittedly, garlic mustard, purple loosestrife, and Canadian pondweed are not the worst invasive species. These plants don’t hold a candle to the brown tree snake. Are the more wickedly painted invaders, like zebra mussels (Dreissena polymorpha) or tamarisk (Tamarix sp.), ever falsely accused?

News headline on invasive species. Image by author. Sources for headlines.

Yes, non-native species are often blamed for problems we humans caused simply because they are found near where the damage occurred. 

Illustration of a zebra mussel. A public domain image. Source.

Zebra mussels are incriminated for clogging pipes, covering every hard surface underwater, and eliminating endangered freshwater mollusks. The fact is, freshwater mollusks were on the decline long before zebra mussels ever arrived because of habitat degradation and pollution caused by humans. Zebra mussels do compete with these mollusks for food, but it is wrong to blame them for endangering the mollusks. 

Zebra mussels. Creative commons image. Source.

Zebra mussels do cover a large amount of hard surface area underwater, but while doing so, they filter polluted water. As a result of the zebra mussels’ filtering job, the water is clearer, which promotes the growth of aquatic plants. These plants provide cover for fish and invertebrates and help increase their populations. The fish then feed on the zebra mussels and help clear some of the hard surfaces they live on. Zebra mussels also serve as a major food source for waterfowl. 

Yes, zebra mussels may inconvenience water companies by clogging their pipes, but they aren’t the evil species the media makes them out to be.

Zebra mussels on water meter. Public domain image by NOAA. Source.

Tamarisk has a similar story. This small tree is blamed for being a water hog and for destroying native bird habitat. Actually, humans were the ones using all the water. Tamarisk use about the same amount of water as native plants. In addition, tamarisk provides habitat for native birds. This plant isn’t quite as bad as the headlines make it seem.

Tamarisk flowers. Creative commons image. Source.

The whole idea that introduced species must be harmful is a xenophobic attitude. Once a non-native species takes root, it is next to impossible to eradicate. Introduced species are not going away anytime soon either, as international trade and travel just make it easier for species to globe trot. It’s time to accept the innocent immigrants species and the benefits they can offer our ecosystem.

Sources.

The Brain on Coffee

I recently became a coffee drinker, and as I drank my coffee one morning, enjoying how much more awake and alert I felt with every sip, I began to wonder: why does coffee help me wake up and focus? What is caffeine doing to my brain?

Photo by History Underfoot, Used with permission.

Energized by my morning brew, I jumped right into research. Soon I learned many new words (neuroscience papers are superfluously verbose) and why coffee helps most of us feel more human on even the least promising morning.

A drawing of the brain.
A public domain image. Source.

The invigorating effect of coffee can be traced to the caffeine present in the beans. We all know that caffeine is a central nervous system stimulant. But how does it get into our nervous system in the first place? Caffeine can easily cross the blood brain barrier, a screening system which prevents most chemicals from entering the brain, to interact with our neurons. 

Stained neurons.
A creative commons image. Source.

What does that mean?

Before I explain exactly how caffeine interacts with our neurons let’s back up a little and briefly review how our brain works. Our brains are made of neurons which respond to stimulus by sending and receiving neurotransmitters. Neurotransmitters are chemical messages which either trigger neurons to release more neurotransmitters—or stop neurons from releasing more neurotransmitters. The complex patterns of releasing and withholding neurotransmitters are what allow us to move our muscles, to make decisions, and form memories.

A creative commons image. Source.

One end of a neuron—the end which receives neurotransmitters—is made of many branched extensions called dendrites. The dendrites are covered in receptors which interact neurotransmitters. Different types of receptors respond to different neurotransmitters. 

Now, let’s get back to the coffee. Caffeine mainly affects two types of receptors known as the A1 and A2A receptors. These receptors normally receive adenosine, a neurotransmitter which helps us sleep and relax. Caffeine blocks the A1 and A2A receptors and prevents them from receiving adenosine. With less adenosine interacting with our neurons we automatically feel more alert and awake. 

Drawing of neurons.
A public domain image. Source.

Caffeine,  in preventing adenosine from doing its job, has a few side effects like increasing anxiety. People who are prone to anxiety and panic attacks—and first-time coffee drinkers—are more likely to feel nervous after a cup.

Luckily for us regular coffee drinkers, most people’s brains habituate to caffeine very easily, and the caffeine jitters become less pronounced the more frequently one drinks coffee. After my first cup of coffee, I felt a little on edge, but now that I’ve been enjoying coffee daily for about two weeks, I don’t feel nervous after my morning mug.

Vintage coffee can.
A creative commons image. Source.

Caffeine promotes alertness, which is great during the day, but not as pleasant at bedtime. Even if one does manage to fall asleep after drinking coffee in the afternoon, caffeine reduces the amount of deep sleep per night and increases the amount of light sleep. The rapid eye movement (REM) sleep, or dreaming sleep, remains the same regardless of coffee consumption. First-time coffee drinkers are more likely to experience sleep disruption as increased caffeine tolerance reduces the drug’s influence on sleep.

Another thing I learned is exactly how quickly one becomes accustomed to caffeine depends largely on genetics. Some people’s livers metabolize coffee slowly, and since caffeine would remain in their bodies longer, it may have a stronger effect on them. In general, people of Asian and African descent tend to metabolize caffeine more slowly than those of European descent. 

Author's own image.

I have not gotten my DNA sequenced yet, but I’m guessing that my liver metabolizes caffeine fairly quickly because I quickly became habituated to caffeine and my ancestors were European.

Coffee has a few other side effects like excessive urine production, increase in systolic blood pressure, and dilation of the airways. I’ve read that drinking too much coffee can raise the blood pressure to dangerous levels in people with hypertension. (Always do your own research.)

Author's own image.

For people without hypertension or caffeine-induced anxiety, lifelong coffee drinking is suspected to delay the onset of cognitive decline, Parkinson’s disease, and diabetes. One recent study found that drinking coffee reduced the risk of brain tumors in Japanese populations. The benefits of coffee drinking tend to be more pronounced in women.

Unfortunately, these health benefits only apply to lifelong coffee drinkers. Starting to drink coffee as an older adult can possibly increase cognitive decline. People with Parkinson’s disease and diabetes who began to drink coffee after they were afflicted reported no change in their symptoms.

Even if coffee can’t change the symptoms of Parkinson’s or diabetes, it can improve one’s mood. Coffee paired with bread and blue light is one of the top mood improvers! I know next time I’m feeling down try this coffee, bread, and blue light strategy. (Where do I find blue lights?)

Coffee and bread, my breakfast this morning! Author's own image.

Happy coffee drinking to my fellow coffee lovers!

Inspired to learn more about coffee? Check out my post on my other blog, Totally Baroque, about 17th century men and women who couldn’t pass a day without the “drink which drove away drowsiness" here.

Sources.

Cemetery Series: The Cemetery Habitat

Cortland Rural Cemetery. Photo Credit to PBM.

Cemeteries are not just for the dead, but for the living. In fact, graveyards serve as safe havens for rare plants and animals. Tombstones in Britain are the only home of some species of endangered lichens. The Calvary Cemetery in north St. Louis, Missouri contains the last original prairie grasses in the area and is one of the few spaces where ground-nesting bees survive. The Weißensee Jewish Cemetery in Berlin is home to 608 species of wildlife including an arthropod (Agonum gracilipes) which was thought to be extinct. It’s not extinct! That little invertebrate has been living in a graveyard in the middle of Berlin.

The Trinity Church Cemetery. Photo by author.

Cemeteries are often the only fragments of natural habitat left in cities or areas dominated by agriculture. This makes graveyards vital to the survival of many species.

Author's own image.

The Trinity Church Cemetery. Photo by author.

Wildlife thrives in un-manicured cemeteries and in cemeteries with tombstones placed close together. At the Weißensee Jewish Cemetery mentioned above – the place that surprised scientists with the arthropod – most species are found in areas left to grow naturally. Closely-crammed tombstones provide hiding places for animals like foxes and coyotes. Tight spaces between grave monuments also makes it harder to remove sprouting plants, allowing them to grow and prosper.

Kings Chapel Burying Ground. Photo by Author.

Author's own image.

The flora and fauna in cemeteries provide data on many topics like conservation biology, species diversity, and climate change. Managers at some cemeteries embrace the wildlife found among the gravestones and host birdwatching programs or guided hikes through their land. One great example is the Cortland Rural Cemetery in Cortland, New York that offers a self-guided tour on the cemetery featuring the trees and lichen growing there.

Cortland Rural Cemetery. Photo Credit to PBM.

To learn more about cemeteries and science check out my previous posts: Cemetery Geology and Death and the Environment.

The Trinity Church Cemetery. Photo by author.

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Cemetery Series: Death and the Environment

Cortland Rural Cemetery. Photo credit to PBM.

I often think about the effect of my lifestyle and actions have on the environment, but one thing I do not often think about is my death and how it will impact the earth. Death is a natural process, and the earth easily takes it in stride, however, the way we humans take care of and dispose of cadavers can have a positive or negative impact on the earth.

The two most popular ways to dispose of human remains in the modern United States are traditional burial and cremation. Neither option is environmentally friendly. To understand why, let’s examine the processes involved in traditional burial and cremation.

Traditional Burial

If traditional burial, complete with an open casket funeral, is chosen, the deceased is first drained of all blood and pumped full of embalming fluid, which is mainly made of formaldehyde, to preserve the body for as long as possible. The cadaver is then made presentable for the funeral and placed in a casket made of varnished wood, metal, or plastic. Once the funeral is over, the body is laid to rest in a concrete vault in a nearby cemetery.

The procedure of traditional burial is largely centered on using chemicals to keep the body from decomposing for as long as possible. These chemicals, and products from the plastic used to make or coat coffins, leech into the soil where they can be absorbed by nearby plants and animals. The chemicals enter the groundwater, posing a threat to people who rely on well water for drinking. Over 750,000 gallons of formaldehyde enter the soil every year from embalmed corpses in the United States alone.

Formaldehyde does eventually decay, but it is not healthy to have 750,000 gallons of a known carcinogen released into the environment. Frequent exposure to formaldehyde during the embalming procedure puts funeral home employees at a higher risk of getting cancer. The effects of other chemicals in the coffins and burial vaults on the environment are largely unknown.

Cortland Rural Cemetery. Photo credit to PBM.

The idea of slowing down decomposition and preserving the body in an illusion of life promoted by traditional burial would make sense if the cadaver was being kept as a scientific specimen to be used for research. However, there is little point in preserving a body that will be left to rest permanently underground. Besides, without constant care it is impossible to stave off decomposition forever, even the most embalmed corpse will begin to decay after a while.

Slowing down decomposition is neither healthy nor safe. The World Health Organization (WHO) recommends disposing of cadavers in a way that will allow them to break down as quickly as possible, to reduce the risk of pathogens colonizing the corpse and leaking into the soil and groundwater.

Traditional coffins and vaults waste valuable resources like wood, concrete, and steel which could be put to more effective use. In the United States, 30 million board feet of wood, 1.6 million tons of concrete, and 90,000 tons of steel are used in traditional burials, never to be seen again.

Traditional burial is rarely required by law, expensive, wasteful, and pollutes the earth. I hope that as more options for body disposal become available traditional burial will become a thing of the past.

Cortland Rural Cemetery. Photo credit to PBM.

Cremation
Cremation is widely viewed as a “green” alternative to traditional burial, but is it actually earth-friendly? Not really.

Cremating a corpse correctly takes about one hour of intense heat, 1590-1797 degrees Fahrenheit (950 degrees Celsius). Maintaining this heat level takes a large amount of energy, often obtained from fossil fuels. During the combustion process, pollutants like carbon dioxide, carbon monoxide, soot, and trace metals such as highly toxic mercury from dental fillings are released into the atmosphere. Each body burned uses 28 gallons of fossil fuels, finite resources, and releases 540 pounds of carbon dioxide. This adds up, considering 912,000 cadavers are cremated each year in the United States. The ashes left after cremation are not harmful to the environment, but since they are devoid of any nutrients, they cannot be used to nourish future life.

There are ways to reduce the environmental impact of cremation, such as choosing crematoria with scrubbers or filters which help prevent pollutants from entering the atmosphere, wrapping the body in a natural shroud to make sure no excess chemicals are released as the shroud burns, removing dental fillings and medical implants before cremation to prevent release of trace chemicals, and using a biodegradable urn if the ashes will be buried. Some crematoria donate to the carbon fund, an organization which works to reduce pollution and conserve nature, but are there ways to avoid producing pollution in the first place? Fortunately—yes.

Cortland Rural Cemetery. Photo credit to PBM.

Green Burial

Green burial is less of a new earth-friendly fad and more of a return to burial practices before embalming and concrete vaults existed. During green burial, the body is simply buried in a biodegradable coffin or shroud without any invasive chemicals. After being laid to rest, the corpse naturally decomposes and nourishes the earth. Some traditional cemeteries have a section dedicated to green burial, but there are other burial grounds which only accept chemical free bodies and shrouds/coffins. A few green cemeteries allow headstones that lay flat to the ground, while other cemeteries serve as nature preserves and no grave markers are allowed at all. In those places, graves can be found by GPS.  Some states allow people to conduct funerals in their own homes and conduct green burials on their own property, but the laws on this practice vary considerably by locality.

Green burial is cheaper than traditional burial because no expensive supplies or chemicals are used. One downfall of green burial is that it requires lots of open space, which is generally not available to city dwellers.

Composting Cadavers
Someday city inhabitants may have their own form of green burial consisting of specialized buildings for composting cadavers. The fertile soil produced by these facilities could later be used as a fertilizer for flower gardens. Similar to green burial, body composting will not release greenhouse gases or chemicals. Currently, composting corpses is not available to humans, though farmers commonly compost their deceased livestock.

Promession and Alkaline Hydrolysis

Promession and alkaline hydrolysis are two methods of body disposal that are advertised as being good for the environment, though an in-depth analysis of their effects on the earth has yet to be conducted.

Promession is similar in concept to green burial or urban composting, except the body is reduced to small organic fragments by a treatment with liquid nitrogen before being buried. Like the body composting, promession is not yet available to customers.

Cortland Rural Cemetery. Photo credit to PBM.

Alkaline hydrolysis, however is available in a few facilities in the United States. Alkaline hydrolysis, also called green cremation, is a process where the body is reduced to a sterile fluid and some bone fragments, using an alkaline solution of water and potassium hydroxide. Instead of slowing down the decomposition process as embalming does, alkaline hydrolysis speeds it up. Alkaline hydrolysis uses one-eighth of the amount of energy that cremation does and it does not release trace metals or greenhouse gases into the environment. The leftover bone fragments can be buried or scattered, similar to the ashes left over after cremation.

Donate to Science

Donating one’s body to science may not be environmentally friendly in the same way green burial or body composting is, but cadaver donations help researchers in a variety of ways from teaching anatomy to students, studying how bodies decompose, to research projects like testing safe seatbelts and airbags for new car models.

Filling out an application before death and going to a medical evaluation session are necessary steps for those who wish to donate their bodies. Body donation is by far the cheapest option of body disposal, in fact it is completely free. Once a cadaver is successfully donated, the family of the deceased has no control over what research project their loved one will participate in. The family will later receive ashes of the deceased and a letter naming the project their loved one was involved in.

Body donation is not for everyone, but the donated corpses provide vital resources for scientists. It is difficult to judge the environmental impact of body donation because the corpses are used in a wide variety of ways. However the cadavers are being used to

expand our knowledge, which is valuable to our society.

Burial at Sea

Full-body burial at sea is a lesser-known option that helps the environment by saving land for other uses. If the body is wrapped in biodegradable material, as required by most providers of sea burial, then the body can decompose naturally, with little environmental impact.

Cremated remains can also be put to good use underwater by becoming building materials for reef reconstruction technology.

Cortland Rural Cemetery. Photo credit to PBM.

These are just a few of the popular and earth-friendly options for body disposal. There are many other more eccentric options such as cryonics, mummification, shooting one’s ashes into space, or transforming the carbon in one’s body into a diamond. Though interesting, these options do not benefit the earth and are highly expensive.

For more on cemeteries and the environment keep an eye out for my next blog post in the Cemetery Series: The Cemetery Habitat.

Note: Flowers

Cut flowers, popular at funerals, are often grown in overseas in countries with poorly enforced environmental policies or no environmental policies at all. A whole slew of chemicals are used to grow these flowers, including DDT, which is illegal in the United States. These chemicals harm workers in the flower industry and the wildlife around the greenhouse where they are grown. Employees in these greenhouses are not only exposed to dangerous chemicals, but start working at a young age for long hours and have very few rights. The transportation required to ship these flowers across the world uses resources like fuel and releases greenhouse gases. It is possible to still have nice flowers and avoid contributing to the environmental harm by buying organic or fair trade flowers.

Sources.