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The spiny sowthistle (Sonchus asper) is a flowering plant that grows up to three feet tall and sprouts small yellow flowers. A native of Europe, it’s an invasive weed here in the United States. If you found it in your garden, you’d pull it out or attack it with weedkiller.

Gerd A. Guenther of Düsseldorf, Germany, however, made the plant rather special when he put a section of the stem under a microscope and snapped this photo using a technique called darkfield illumination. The photo won second place in this year’s Nikon Small World Photomicrography Competition. (A gallery of winning images can be found at www.nikonsmallworld.com.)

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A living root bridge in the Khasi Hills, Meghalaya, India (courtesy of flickr user Seema K K)

In the United States, the lowly ficus sits quietly in the corners of our homes and offices, providing some much needed greenery and oxygen to our indoor spaces. But in the northeastern Indian state of Meghalaya, where Ficus elastica are large, native outdoor trees that live near water, the local people have been using the ficus’s roots as bridges for generations.

These aren’t trees that have fallen naturally over streams, though, which are commonly used as bridges in other places. Instead, the people train the trees’ roots to grow over the streams, guiding them over a period of 20 or so years into the shapes of paths and handrails until they have a bridge strong enough to carry many people at once. And as the tree grows, so does the bridge, gaining in strength over time, as the magazine Geographical noted earlier this year:

Once the roots have been trained across the stream bed, they anchor in the soil of the opposite bank, providing the foundations for a living bridge. Usually, several roots are threaded together for strength, while others provide handrails and supports for longer spans. Flat stones from the stream bed are used to fill gaps in the bridge floor and, in time, these are engulfed by woody growth and become part of the fabric of the bridge itself.

A root bridge takes around 20 years to become fully functional. Once complete, however, it will probably last for several hundred years and, unlike its non-living counterparts, will actually increase in strength with age.

Known in the Khasi language as jingkieng deingjri (‘bridge of the rubber tree’), the bridges may be anywhere from ten to 30 metres in span. Unlike most artificial structures, they are able to withstand the high level of soil erosion brought about by monsoon rains and, being living material rather than dead wood, are resistant to the ravages of termites.

There is even a double-decker bridge supposedly capable of handling the weight of 50 people at a time.

A trumpet flower caught as it unfolds (copyright Patsy Lieberman)

Summer is over–meteorologically speaking, at least–and the weather has finally cooled off here in Washington, D.C., but the flowers are still blooming prolifically. Patsy Lieberman (daughter of goSmithsonian and Around The Mall editor Beth Py-Lieberman) was talented and patient enough to capture this amazing image of a trumpet flower unfolding last week.

Have you taken your own amazing photo of the natural world? If so, you can enter it into Smithsonian’s 7th Annual Photo Contest (deadline: December 31, 1, 2009).

And if you’re here in Washington, you can check out the Smithsonian gardens (weekly tours are still available), the National Arboretum or, my favorite, the United States Botanic Garden to try to create your own photographic masterpiece or just enjoy the sight.

An orchid at the Smithsonian. Photo by Sarah Zielinski

Forget about birds and bees—if you want to learn about the varieties of sexual practices in the wild, study orchids. They’re the most rich and varied family of flowers by far, with about 24,000 species (another estimate is 30,000 species). And many of those species have evolved elaborate tricks to get hapless birds and bees and other pollinators to lovingly embrace their flowers.

Some orchid flowers look just like their pollinators and thereby lure the real thing. In a special issue on orchids in the Annals of Botany this month, an introduction points out that Carl Linnaeus appreciated one superb mimic:

Its flowers bear such a resemblance to flies, that an uneducated person who sees them might well believe that two or three flies were sitting on a stalk. Nature has made a better imitation than any art could ever perform.

(See for yourself here.)  Linnaeus didn’t figure out what the orchid was up to, but Darwin did. The National Museum of Natural History had a gorgeous exhibit of live orchids this spring called Orchids Through Darwin’s Eyes, which Sarah photographed.

Botanists recognized orchids’ visual mimicry first, but lately they’ve uncovered even more interesting scent-based mimicry. Basically, the orchids emit chemicals that smell, to a male insect, just like the sex pheromones emitted by the female of his species. In an interesting twist last year, researchers found that a bee-pollinated orchid produced chemicals that are similar but not identical to a female bee’s scent. It’s not that the orchid is a bad mimic, they researchers conclude, but that male bees are most attracted to a scent that’s not too familiar.

Aside from feeling used, do pollinators suffer from being tricked by orchids? Maybe so. As a paper in the American Naturalist last year pointed out:

While some sexually deceptive orchid species require only pollinator gripping or brief entrapment for effective pollination, other orchid species coerce their pollinators into energetic copulation. Although these copulations are often described as “pseudocopulations,” the vigorous response of pollinators suggests that true matings with ejaculation and costly sperm wastage may indeed occur.

Sure enough, they found that male wasps pollinating Australian tongue orchids do indeed ejaculate, which is a waste of time and energy for the wasps.

For the orchid, the relationship with pollinators is all about sex; but for the pollinators, sometimes it’s about food. A study that comes out in Current Biology later this month shows that a Chinese orchid mimics the scent of a honeybee’s distress signal—a scent that attracts honeybee-eating hornets. Wicked!

But orchids don’t always need pollinators. Sometimes they have sex with themselves. A study a few years ago showed that another Chinese orchid, if no wind or pollinators are around, will twist its pollinia into its own stigma:

Here we describe a new type of self-pollination mechanism in the tree-living orchid Holcoglossum amesianum, in which the bisexual flower turns its anther against gravity through 360° in order to insert pollen into its own stigma cavity — without the aid of any pollinating agent or medium.

How could I resist a book with the title Wicked Plants: The Weed That Killed Lincoln’s Mother & Other Botanical Atrocities? This small, elegant volume by Amy Stewart packs in a ton of information on plants that have been used to murder or to intoxicate, some that can inflict pain or cause hallucinations, and others that are “badly behaved” or are just illegal.

Wicked Plants, by Amy Stewart

The main entries for individual plants include common names, habitat and descriptions as well as stories from history. In the henbane entry, for example, Stewart mentions that the vegetable had been added to beer in the Middle Ages to enhance the intoxicating effects of the brew and that it was in part to eliminate it (and other “suspicious” ingredients) from beer that the Bavarian Purity Law was passed in 1516.

Interspersed are sections that group several plants (and in one, fungi) with similar effects, such as “Stop and Smell the Ragweed.” This entry particularly struck me, as we are deep into allergy season. There are several common garden trees and plants that could be the culprits in our suffering. The mulberry, for one, sheds billions of pollen grains. And it turns out that while Bermuda grass may be popular in the South, it is also one of the most allergenic of the grasses.

There are surprises throughout the book. Many familiar plants have hidden dark sides. My favorite flowers, tulips, produce an irritating sap—a reminder to wear my garden gloves when planting bulbs. And while there are the expected murder stories, more worrisome are the numbers of people who have died eating plants that they didn’t recognize or misidentified or simply didn’t think could be dangerous. Children and pets are, sadly, the most common victims. This is a reminder to not experiment with unknown vegetation, ever.

My one criticism is that the book lacks an index, which makes it difficult to find any specific plant, especially those that have entries only within groups. Overall, though, it is a fascinating read and a must-have for botanists and budding mystery writers.

Oh, and what was the weed that killed Lincoln’s mother? Well, that was white snakeroot (Eupatorium rugosum), which when eaten by cattle, poisons the milk and causes milk sickness. Symptoms include weakness, vomiting, tremors and delirium. Nancy Hanks Lincoln, her aunt and uncle and several other townspeople succumbed to the disease in 1818.

Irish Moss, Chondrus crispus (Credit: Andrea Ottesen, University of Maryland)

The National Science Foundation and the journal Science are now soliciting entries in their seventh International Science & Engineering Visualization Challenge (deadline for entries is September 15). There are five categories (photographs, illustrations, informational graphics, interactive media and non-interactive media), and the winners each year are truly wonderful. This photo of Irish moss (a type of seaweed) tied for first place in 2007.

The slimy, glistening mass of seaweed washed up on a sandy beach seems light-years distant from this feathery, dendritic image of Irish moss (Chondrus crispus) created by Andrea Ottesen, a botanist and molecular ecologist at the University of Maryland, College Park. “If you pull Chondrus out of the ocean, it’s folded on itself–really curled up,” she says. It wasn’t until after she had “pressed every one of those little ends down with sea stones” and left it to dry for 2 days that the seaweed’s beautiful, simple shape was revealed. Besides being one of the most common seaweed species on the Atlantic coast, says Ottesen, Irish moss and algae like it are sources of natural thickeners and stabilizers called carrageenans, which are widely used in processed foods as diverse as lunch meat and ice cream.

Purple loosestrife was brought to the U.S. in the 1800s as an ornamental plant, but it has taken over many wetlands. (Courtesy of flickr user poppy2323)

They have come to our country from all over the world, brought here in the bowels of ships, in the cargo holds of planes, even on the bottoms of our shoes. They’re called invasive species—plants, animals and microbes from foreign lands that in the absence of their natural competitors, predators or diseases not only establish themselves but sometimes even do better than the natives. The brown tree snake in Hawaii. Zebra mussels in the Great Lakes. Nutria in Louisiana. Leafy spurge throughout the West. Just to name a few.

Why are they a problem? They might crowd out or prey upon indigenous species, possibly leading to extinctions. They might take over a landscape—look at kudzu problem down South, for example, where the infestation can cover an entire house. They might even transmit diseases.

What can you do to help? The USGS is now encouraging people to volunteer with one of the agency’s invasive species networks to help record and submit data on these pests.

Looking for a good excuse to go outdoors this summer? Why not help the USGS track harmful, invasive species? By becoming a citizen scientist, you can help prevent invasive plants, animals, and diseases that are in ecosystems where they don’t belong and harming native species. During your summer hikes, you could be on the hunt for kudzu, giant hogweed, yellow star thistle, snakehead fish, feral cats and much more!

Why is this important? Well, government agencies can’t do anything about these species unless they know what is out there and where it is.

Organizations looking for volunteers include:

The Invasive Plant Atlas of New England
Invasive Plant Atlas of the Mid-South
The Cactus Moth Monitoring and Detection Network
CitSci.org
Purple Loosestrife Volunteers
Invaders of Texas Citizen Science Program
Maui County Report a Pest

Can anyone identify the orchids in these photos? I visited the orchid show at the Natural History Museum last week (Orchids through Darwin’s Eyes, which runs until April 26) intending to learn more about Darwin and his orchid research, as well as take a few photos for the blog. But I got distracted by all the lovely flowers—they’ve got 300 plants on display, including some that are really rare—and I forgot to keep track of what I was photographing. Oops.

Two of our Around the Mall bloggers were able to keep their wits about them, though (see Orchids Star in Darwin’s Garden and Orchids Show their Stuff on the Smithsonian Channel) and write something substantial about the exhibit.

Can’t make it to the exhibit before it closes? The museum has some lovely professional photos and background information on the exhibit’s Web site.