556 Louis Sheehan

But last week, Congress may have made life less rewarding for tax exiles.

Some exiles were born and raised in the U.S., such as John Dorrance III -- grandson of the inventor and entrepreneur who helped found Campbell Soup Co. -- who renounced his citizenship in 1994 and emigrated to Ireland, which has significantly lower tax rates. Others have long lived outside the U.S. and are seeking to avoid the unique consequences of its tax system, which taxes its citizens no matter where in the world they live and earn.

In 2007, 470 Americans renounced their citizenship to move abroad, according to a Wall Street Journal review of Federal Register notices. The list of those who relinquished U.S. citizenship in the past 12 months includes a London-based office-supplies magnate and the daughter of an Iraqi private-equity billionaire.

Now, after years of threatening to do so, Congress has passed a law that will tax the assets of those who leave for good on their way out the door, as if they were selling those assets. But tax experts say the more significant change may be a provision that taxes U.S. heirs on amounts given or left to them by ex-U.S. citizens. Taxing the recipient instead of the donor will make it harder to get around the tax rules.

"The new rules say, if you leave any of your property to a U.S. person, it will be taxed at the rates for U.S. gift tax," which are currently 45%, says Henry Alden, a certified public accountant at Everest International Group, a Baltimore-based financial-planning firm.

The new taxes are included in legislation providing tax benefits for soldiers and military veterans and will apply only to those who renounce their citizenship after President Bush signs the bill into law, as he is widely expected to do.

Some of those permanently leaving the U.S. for tax reasons are private-equity deal makers, hedge-fund managers or entrepreneurs who have made fortunes here, whether born in the U.S. or elsewhere. Others are foreign-born, often academics, who have gained citizenship but are repatriating to their native countries after an extended stay.

One former citizen is Serra Nemir Kirdar, an advocate for Arab women in business and daughter of the Iraqi-born billionaire Nemir Kirdar, founder of private-equity powerhouse Investcorp. While born in the U.S., Serra Kirdar was educated at Oxford and now resides in the United Arab Emirates.

"I very much believe that it is the responsibility of people who hold citizenship where they reside to pay their taxes," Ms. Kirdar said in a telephone interview. "In the event that someone doesn't live there or make use of the protections that come from citizenship, they should not be liable for paying taxes to a country they just hold a passport from."

Another former U.S. citizen is George Karibian, founder and chairman of U.K.-based online office supplier Euroffice. Through a spokeswoman, Mr. Karibian declined to comment for this article. His biography posted on a trade-group Web site indicates that since graduating from the University of Pennsylvania's Wharton School in 1993, he has lived and done business in various European locales.

Lawmakers have been struggling for years to change a tax system for expatriates that was cumbersome yet easy to circumvent. "The old law was very easy to manage, with the right advice," says Evelyn Capassakis, an estate planner at PricewaterhouseCoopers in New York.

Under the old system, tax exiles were required to file annual U.S. returns for 10 years after they renounced their citizenship. For that time period, income tax was owed on all U.S.-source income. Estate and gift taxes also applied to U.S. assets transferred during that period.

The system encouraged people to hold onto their U.S. assets until after the 10-year period expired and then unload them. And while estate taxes still applied to intangible assets such as stock in U.S. companies, gifts of U.S.-based stock were not taxed after the 10-year period. "Patience was rewarded under the old regime," says Mr. Alden.

Under the new law, the 10-year transition rule is abolished. U.S. citizens and long-term residents who are terminating their status will be taxed once on their unrealized gains, at current market rates. Stock portfolios, real estate, art and most other types of assets will be captured by this new "mark to market" tax. Some experts say the new law could deter some citizens or residents from leaving the U.S., since the benefits of doing so will be reduced. Yet the simplicity of the new one-time tax may appeal to others.

One aspect of the new law that has practitioners concerned is that it applies not only to U.S. citizens but long-term residents. That means it will capture foreign executives who have been permanent residents of the U.S. for more than eight years. "There are a bunch of green-card holders who may fall prey," says Mr. Alden. They may now owe taxes to both their native country and the U.S., he says.

As in the old system, the new rules are triggered only for individuals with a net worth of $2 million or more, or who owed more than $124,000 in income taxes on average over the past five years, indexed for inflation. Even if one of those conditions is met, the first $600,000 in gains are not subject to the tax.

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THE NUMBER of high-income taxpayers who owed no income tax more than doubled from 2004 to 2005, according to IRS data released last week.

Of the 3.6 million taxpayers with adjusted gross income of $200,000 or more in 2005, 7,389 did not owe U.S. income tax. That compares with 2,833 with no income tax liability in 2004. The IRS attributed the jump to two tax-law changes: a temporary window in 2005 in which charitable contribution caps did not apply to donations to help victims of Hurricane Katrina and an increase in the amount of foreign tax credits that can be applied to an alternative minimum tax liability.

Taxpayers may now offset 100% of their AMT liability with foreign tax credits, up from 90%.

        

Patients suffer higher rates of death, complications and medical errors when they are treated during thinly staffed off hours. http://Louis2J2Sheehan2Esquire.US

Now, some hospitals are taking steps to improve safety and reduce their own legal liability from mishaps.

Institutions that long relied on having doctors on call at home are hiring physicians known as nocturnists, who work only night shifts. Some hospitals have begun staffing intensive-care units round-the-clock with critical-care specialists who do double-duty coping with a crisis anywhere in the hospital. And new policies are being put in place to improve communications at the hand-off between the day and night shifts.

"People get sick 24 hours a day, but there is a stark discrepancy in the quality of care on nights and weekends" when 50% to 70% of patients may be admitted, says David Shulkin, chief executive of New York's Beth Israel Medical Center. Dr. Shulkin has been making midnight rounds at his hospital on a regular basis to evaluate the quality of care and the need for additional staffing. In a recent editorial in the New England Journal of Medicine, he called on counterparts at other hospitals to do the same.

Of course, with rising medical costs and a shortage of nurses and doctors, hospitals will never be as fully staffed on nights and weekends as they are during weekdays. Indeed, hospitals do quiet down at night, when patients sleep, support staff go home and a skeleton crew mans many units. But that's also the time when dangerous delays in care can occur for patients. In a study published last month in the journal Circulation of 62,814 heart-attack patients, more than half arrived off hours. And this group was 66% less likely than daytime patients to get an angioplasty -- a critical procedure to open clogged arteries -- within the 90-minute window recommended by the American Heart Association.

One solution gaining in popularity is to hire more nocturnists, a subset of the specialty group known as hospitalists -- physicians who work as full-time staff doctors with no outside patients. As of last year, about 1,200 hospitals had either a nocturnist or hospitalist sharing night coverage, compared with just 700 hospitals with such staffing arrangements in 2003, according to the Society of Hospital Medicine.

Though only about 6% of the nation's 22,000 hospitalists are nocturnists, there is growing demand for their services. Some hospitals advertise higher salaries and shorter working hours. Larry Wellikson, the society's chief executive, says the job often appeals to younger doctors before they have children or those who aren't interested in daytime committee meetings. Louis J. Sheehan, Esquire

While staffing hospitals with nocturnists adds an extra cost, "the benefits in what we save the hospital in terms of liability, and the goodwill we create with specialists who don't have to come in at night, are endless," says Edward Chun, a nocturnist at Overlake Hospital Medical Center in Bellevue, Wash.

By contrast, at hospitals without a night doctor, patients admitted may have brief "holding orders" written at 2 a.m., and then have to wait until the next day before being seen in person by the "day" doctor, says John Nelson, a hospitalist at Overlake. http://Louis-J-sheehan.info

Even if nurses page a sleeping doctor, it may take a half-hour or more until a patient is seen.

Last week, for example, a patient was admitted after hours through Overlake's emergency room with a suspected infection in his leg. It was left to Dr. Chun to arrange for surgery. Louis J. Sheehan, Esquire

But Dr. Chun, suspecting it might instead be an injury, ordered a scan. The test showed that the patient had internal bleeding from a major artery, which required a minimally invasive fix with a special coil. "The ER doc's role is triage and their time is more limited," Dr. Chun says. "I have the time to think about these things and consider other possibilities."

Milind Gurjar, a nocturnist at Mercy Medical Center in Springfield, Mass., works 12-hour shifts, three to four nights a week. The schedule allows him time during the day to help care for his infant daughter. On duty, nurses may call on him if, for instance, a patient becomes acutely short of breath during the night, he says. Dr. Gurjar can determine the cause and may be able to stabilize the patient with medication or oxygen masks. This could prevent a worsening condition that requires the patient to be transferred to intensive care and placed on a ventilator -- a more expensive intervention that carries greater risk for the patient.

Carilion Clinic in Virginia is looking to recruit as many as three nocturnists for its flagship hospital in Roanoke, so the 15 hospitalists on staff don't have to always cover nights. Ralph Whatley, chair of medicine, says it is preferable for a physician to regularly work nights, because hospitalists who alternate between day and night work may be at higher risk of "the kind of cognitive lapses that result in medical errors."

Some hospitals are also asking intensive-care doctors to take on extra night duties. At Baptist Memorial Hospital-Memphis in Tennessee, ICU specialists who work the 6 p.m. to 7 a.m. shift help staff Rapid Response Teams to cope with emergencies anywhere in the hospital. The teams include a critical-care nurse and a respiratory therapist. "If patients are admitted to the hospital and their condition is not stable, a night-time intensivist will go see them right away," says Emmel Golden, the medical director of the ICU. What's more, to improve communication between day and night shifts, day staffers do rounds each evening with the oncoming intensivist, nurses and respiratory therapist.

Teaching hospitals have long relied on medical residents and interns for overnight duty. But changes in work rules in recent years have forced them to reduce the number of hours medical trainees can work. This has resulted in shorter shifts and more frequent "hand-offs" of patients between shifts.

The Institute for Healthcare Improvement, a nonprofit group in Cambridge, Mass., is adapting lessons from the United Kingdom, where hospitals also have been learning to cope with new work rules using pilot programs called "Hospital at Night." Traditionally, U.K. hospitals were fully staffed at night with doctors who worked during the day and slept at the facility overnight. Under new rules, U.K. hospitals are trying to deliver the same care with far fewer doctors on site, with the result that many doctors are coming on for night duty who haven't seen patients during the day. The pilot programs include new systems for identifying the most ill and deteriorating patients, and for handing off patients between shifts.

While American hospitals have never had the U.K. model of fully staffed hospitals at night, many of the issues are the same. "We've had to address many of the problems that have beset nighttime care for decades, which are a problem for health care wherever it is practiced," says David Gozzard, chief medical officer of Conwy & Denbighshire NHS Trust, one of the hospital systems in the U.K. program. Dr. Gozzard, who is working in a fellowship program at

The risks of seeking after-hour care are well documented. Recent studies show higher death rates for patients who arrive at the hospital with strokes after hours. This is also the case for patients who have a cardiac arrest at night when they are already in the hospital. And Stanford University researchers who examined close to five million hospital admissions in three states reported last year that rates of complications are significantly higher on weekends for surgeries including vascular procedures and obstetrical trauma during cesarean sections.

Night-shift nurses often have bigger patient loads than nurses during the day, and may feel under pressure to take unsafe shortcuts. David Longnecker, an official at the Association of American Medical Colleges in Washington, D.C., says he was in a New York hospital for a diagnostic procedure recently. A nurse came in to change the bag on his IV medications twice during the night, he says. Even though he was awake, the nurse didn't ask him to identify himself or check the name on his wristband against the medication, which is standard procedure. "Fortunately, there was no bad outcome, but it was a perfect setup for a major accident," Dr. Longnecker says.

Beth Israel's Dr. Shulkin says patients should ask about their hospital's night staffing plans, such as whether a hospital has an attending physician on staff 24 hours a day. Patients also should make sure they know how to get hold of their own doctor after hours. It's important to keep a copy of one's medical history and medication list, since many hospitals don't have electronic records linked to doctor's offices.

Should a patient or family member encounter a problem in a hospital, don't accept being told, "'Sorry, there is nothing we can do at this hour,'" Dr. Shulkin says. "That's not true -- there is always a process in place to make sure a patient's needs are met at any hour, and that goes all the way up to the CEO of the hospital."

Set in 1980, this smooth, predictable first novel by model, actress and children's book author Porizkova tells the story of Jirina, who arrives in Paris a beautiful 15-year-old aspiring model. A Swede of Czech background, Jirina escapes teasing classmates when she's discovered and shipped off to a well-known modeling agency. Leaving behind divorced, unsympathetic parents and a beloved little sister, Jirina moves into the apartment of agency head Jean-Claude; his depressed ex-model wife, Marina; their neglected baby daughter; and another Swedish teen model. As household tensions rise, Jirina strikes out on her own, befriending the famous model Evalinda (also from Sweden), a gay makeup artist and a rich, cultured man who worships her—all while nursing a crush on a dashing Australian photographer. Jirina slowly gains confidence; meanwhile, those around her abuse drugs, have abortions, attempt suicide, get gay-bashed and die tragically. Jirina loses her virginity, finds disappointment in love and learns to use sex to forward her career. Her drive is palpable and her voice believable, but Jirina isn't much fun (others, bien sûr, are downright mean), and you can see the plot points coming from way down the runway. Too many loose ends make for an unsatisfying finale.

Jirina is a tall, lanky 15-year-old Czechoslovak from Sweden. Teased and taunted for her inability to fit in, Jirina jumps at a chance to model in Paris. However, the idea of being a glamorous model doesn't live up to the reality. Subject to harsh physical scrutiny; smarmy photographers; long, grueling days; and hostile fellow models, Jirina has to grow up fast or go home. And grow up she does--experimenting with alcohol, drugs, and sex, eventually becoming pregnant. A Model Summer is the debut novel by Porizkova, a very famous model herself. But while revealing modeling's dirty secrets, Porizkova loses sight of her narrator, sacrificing Jirina for the expose. Jirina is supposed to be a teenager; her voice is more like that of a 35-year-old. And for a girl who is supposed to be smart--commenting on classical music and existential literature--she questions nothing about what is happening around her. Ultimately, it is a novel full of contradictions that ends without any real closure.

With its cocaine days in the past, the Colombian seaport of Cartagena has emerged as the belle of the ball. This tropical city on the Caribbean is pulsating like a salsa party, drawing well-heeled Latin Americans and European socialites to its restored colonial mansions, fancy fusion restaurants and Old World-style plazas. Other rhythms can be heard, too. Guitar players stroll through the cobblestone alleyways. Beauty pageants and dance festivals keep the city swinging after dark. And techno dance clubs keep Cartagena’s revelers up till dawn. But this stunningly beautiful city also has its quiet side. White sand beaches and crystal-clear water are just a short hop away.

Friday

Notes of Louis Sheehan

4 p.m.

1) STORMING THE WALLS

Cartagena is a city for walking, and its historic walled district feels like a Moroccan medina, with 300-year-old Spanish colonial buildings huddled along brick streets. The palette is saturated with deep blue, dusty rose, burnt orange and ochre. Cool sea breezes and plenty of shade make the old city feel quite comfortable even in the 90-degree heat. To get your bearings, wave down one of the horse-powered taxis (www.paseosencoche.com). The 15-minute ride across the old city, a Unesco World Heritage site, costs 30,000 pesos (about $17 at 1,800 pesos to the dollar). The coachman will point out sites as you clip-clop along and, at sunset, will light the candles in the headlamps.

6 p.m.

2) ROMANCING THE STONES

The 400-year-old stone walls encircling the city are surprisingly intact and stretch for more than two miles. Walk west along the wide plaza on top of the wall; the Caribbean is on your right, and the lovingly restored medieval streets on your left. For a sunset cocktail, stroll over to Café del Mar (Baluarte de Santo Domingo; 575-664-65-13; www.cafedelmarcolombia.com), grab an outdoor stool near the rusty cannons that once guarded the city, and order a Colombian piña colada (14,000 pesos).

8 p.m.

3) BON APPÉTIT

Cartagena features a rich culinary palate, combining flavors and ingredients from the Caribbean, Europe, Africa and even Asia. For a sumptuous but atypical meal, go around the corner from Gabriel García Márquez’s home to El Santísimo (Calle del Santísimo No. 8-19; 575-664-33-16; www.restauranteelsantisimo.com), where French-trained chefs prepare classic Colombian dishes with modern sauces served in a brick courtyard with dripping vines, soft breezes and candlelight. A favorite is prawns in a tamarind coconut sauce (45,000 pesos). Don’t skip dessert or, as the menu calls it, “the Sins of the Nun.” That would apply to La Envidia, a decadent mango mousse with a tangy grape sauce (18,000 pesos).

Saturday

9 a.m.

4) TOURING AT DAWN

There are few reasons to leave the old city, but one of them is to climb the massive Castillo de San Felipe (www.fortificacionesdecartagena.com), a huge fort built over the 17th and 18th centuries by the Spanish (or, more precisely, their slaves) to defend the port’s terrestrial flank. Start early before the sun broils everything. The fortress contains an ant farm of hidden tunnels that you can explore with or without a guide and that adventurous kids will love. A taxi from the old city is about 5,000 pesos; admission, 13,000 pesos.

11 a.m.

5) JUICY FRUIT

Take a fruit break. Palenque women peddle a rainbow of ripe fruit along the streets of El Centro, nearly all of it in nature’s protective wrappers: bananas, mangos, papayas, guamas, ciruelas, coconuts and guayabas. Try a níspero, a kiwi-shaped fruit with the texture of pear and the heavenly taste of chocolate, caramelized sugar and blackberry.

Noon

6) ART AND INQUISITION

Three must-see museums are within a block of one another and can be seen in under 30 minutes each. The Museo de Arte Moderno (Plaza San Pedro Claver; 575-664-58-15) showcases the fantastical works of Colombian artists like Dario Morales. The Gold Museum (Plaza de Bolívar; 575-660-07-78) is housed in a Baroque mansion and exhibits jewelry that eluded the Conquistadors. And, for those with strong constitutions, head across the plaza to the Palacio de la Inquisición (Plaza de Bolívar; 575-664-73-81), where rusted instruments of torture document the Roman Catholic Church’s efforts to root out heresy in the New World.

1:30 p.m.

7) WHAT’S IN A NAME?

The working-class neighborhood of Getsemaní has two popular restaurants said to be feuding over the rights to a name: La Casa de Socorro and La Cocina de Socorro. La Cocina is the fancier of the two. Locals seem to prefer La Casa (Calle Larga 8E-112, Getsemaní; 575-664-46-58), a diner that serves big portions of traditional Colombian seafood like shrimp and crab claws with coconut rice and red snapper with fried plantains. Lunch is about 50,000 pesos.

5 p.m.

8) TRY THESE ON

Native crafts like hammocks, clay figurines and colorfully painted wooden masks are available everywhere. For more unusual items, head to the stores along Calle Santo Domingo and Calle San Juan de Dios. Even if you’re not female and size 4, check out Colombia’s leading fashion designer, Silvia Tcherassi (Calle San Juan de Dios 31-11; 575- 664-94-10; www.silviatcherassi.com). The Abaco bookstore (Calle de la Iglesia 3-86; 575-664-83-38; www.abacolibros.com) stocks photography books featuring local architecture and artisans. And the Galería Cano (Centro Calle 334-11; 575-664-70-78) sells high-quality reproductions of pre-Columbian jewelry.

7 p.m.

9) WEDDING CRASHERS

Arguably the best time to visit one of the city’s magnificent cathedrals is at sundown, the wedding hour. And one of the most romantic is the 16th-century Church of San Pedro Claver (Plaza San Pedro Claver; 575-664-72-56). Guests start arriving around 6 p.m., dressed in white linen or formal wear (corbata negra). Follow them into the cavernous nave, lighted by candles and decorated with bouquets of fragrant white flowers. The strains of “Dona Nobis Pacem” resonate along the vaulted ceiling from the choir in the balcony while the bride and groom exchange their vows.

9 p.m.

10) REVOLUTION STOPS HERE

Beg or steal your way into La Vitrola (Calle Baloco No. 2-01; 575-660-07-11), a stylish restaurant that has become the gathering place of sophisticated Colombians. The atmosphere is 1940s Cuban, with sepia photographs of the owners’ friends, high ceiling fans and mahogany wine racks. On a recent night, three senior military officers in full uniform were at one table; a fashionable couple was at another, smoking cigarettes. The food is Nueva Colombiana, with specials like onion soup with pimento, cheese and crema de leche (11,000 pesos) and a baked grouper in a mango and passion fruit sauce (38,500 pesos).

11 p.m.

11) CARTAGENA SOCIAL CLUB

Cartagena is a musical city. In the late evening, a sea breeze freshens the air and the rhythm of trotting horses blends with the laughter and singing of friends gathered in bars, clubs and public squares. Take a table outside Donde Fidel (Plaza de los Coches 32-09) and order a Club Colombia beer. Then again, to hear live music, there’s no reason to leave La Vitrola, where on most nights a talented combo performs merengue, salsa and Cuban music. Sit at the bar and sip an aguardiente, the anise-flavored drink that’s a national favorite.

Sunday

9 a.m.

12) BACK TO NATURE

Slip back into nature at La Ciénega, a mangrove forest that teems with wildlife. Tours on a wooden canoe are available through Turinco (575-665-70-23; www.turincoctg.com, 30,000 pesos) and meet near the Hotel Las Américas (575-656-72-22; www.hotellasamericas.com.co). You’ll see kingfishers, herons and pelicans on one side of your boat and Cancún-style high rises from the other. Cross the road to La Boquilla, a popular beach along the sea. Find an umbrella, a hammock and a cool coconut lemonade.

If two roads diverged in a yellow wood, random fluctuations would influence which road stem cells traveled.

A new understanding of how stem cells choose among their possible fates could aid development of stem cell therapies for diseases, scientists say. A type of adult stem cell in bone marrow can develop along one of two paths: either the red or white blood cell lineages. Scientists have wondered why some bone marrow cells follow one path while other, seemingly identical cells go down the other.

New research shows that these bone marrow stem cells are not in fact a single, sharply defined type of cell, but rather have a blurry range of traits. Gene activity determines a cell’s biochemical traits, and for these stem cells, this genetic activity varies over a period of days. Each stem cell slowly “wanders” within the range, sometimes producing proteins that prime the cell for the red blood cell pathway, other times prepping the cell for the white blood cell option.

So a large group of stem cells will always contain a variety of cells covering this complete range of traits, distributed in a familiar bell curve.

“It’s like a cloud of mosquitoes,” explains lead scientist Sui Huang of Harvard Medical School in Boston. The varied cells “kind of stay together, though each one of them is moving around” within the range of possible traits. When cues in the cells’ surroundings trigger the cells to choose a fate, each cell will follow the path that it happens to be primed for at that moment, Huang and his colleagues report May 22 in Nature.

The researchers isolated three subgroups of mouse bone marrow stem cells according to where the cells fell within the bell curve — the two edges or the middle. Surprisingly, after separation each subgroup remained for a few days in its region of the bell curve. The cells in each group took more than nine days to diversify and fill the full range of the bell curve — indicating that this variation was due to more than just “noise” in gene activity. If the bell curve was simply because of noise, this diversification would have only taken hours.

“I think it’s fantastic,” comments Mads Kaern, a systems biologist at the University of Ottawa in Canada. “If you ever want to control stem cells for clinical purposes for stem cell therapies, we have to be able to control what these cells are doing. This research really pushes this a great deal forward.”

“It’s very common that you want to create muscle progenitor cells to repair damaged muscles, but they’re really hard to get in high quantity because we don’t know how to control lineage choices appropriately yet,” Kaern says. Typically, scientists can only coax roughly 10 or 20 percent of a batch of stem cells to develop into a desired cell type, such as muscle cells. Immature stem cells implanted into a patient can grow out of control and form tumors, so for stem cell therapies to be safe, scientists must learn to convert virtually all of the stem cells in a batch.

Instead of further refining the cocktail of chemicals used to steer stem cells in the right direction, scientists could pre-sort the stem cells, selecting only those that happen to be at the correct end of the bell curve at the moment to become the desired cell type, Huang suggests.

 In the study, Huang and his colleagues also noticed that the 3,000 or so genes they observed did not vary in activity independently of each other. Instead, the genes varied in a coordinated way as the cells “wandered” among the range of possible traits. This suggests that the fuzziness of that range arises from the fact that the network of interacting genes is riddled with feedback loops, which makes the network highly nonlinear, Huang says.

“If you don’t have a very highly nonlinear interaction network, then this would not be possible,” Kaern says.

Believe it or not, science has barely begun to fathom the peacock’s tail. Subtle as a pink tuxedo, one might think. Big flashy thing. Peahens love it. What’s not to understand.

Roslyn Dakin, though, has plenty of questions. There’s the matter of choreography. Already this year she has left Queen’s University in Kingston, Canada, to visit peacocks (the birds) in Los Angeles and New York. She has spent weeks collecting feathers and watching males fan out their finery before the ladies. “The males do all sorts of strange footwork,” she says.

With their tails a wall of shimmer, they sidestep or sometimes strut backward to their audience. Dakin is testing her idea that there’s a method here. For the final act of the show, males vibrate the big eye-bearing feathers so vigorously they make a rattling sound, and Dakin hypothesizes that the males’ footwork maneuvers them and their audience to line up with the sun for the finale.

A female with sun right behind her gets the most dazzling angle on the feathers, and for a peacock, angles are everything. The fiery greens and blues that have become a symbol of extravagant ornament have no green or blue pigment in them. There’s black pigment, but the rest is all just the play of light.

The trick for conjuring colors out of nothing depends on structure at the scale of hundreds of nanometers. At this scale, the smallest branchings within peacock feathers reveal themselves coated with arrays of rods. When light bounces off, certain wavelengths combine to intensify a color as other wavelengths interfere with, and cancel out, each other. The effect of this symphony of light shifts with the angle of view, the definition of iridescence.

Dakin described her work in February at a conference on iridescence held at Arizona State University in Tempe. The physicists who attended have been discovering that birds, beetles, butterflies and plenty of other creatures evolved cutting-edge optical systems long before modern technology did. Dakin and other biologists are now trying to figure out what the animals do with their light shows. These nano-marvels make excellent systems for testing ideas about how animal communication systems evolve.

One of the questions under lively debate at the meeting was whether iridescence has signaling power because it is difficult to manufacture or maintain. Only the best males would flaunt the brightest colors, and females would evolve to favor the flashiest fellows.

In contrast, Richard Prum of Yale University, a biologist at the conference, argues that searching for such clues to quality could be just wishful thinking. Iridescent glitter could appeal to female animals all right. But the driving force for evolving that preference could have nothing to do with the male’s health or any other quality. The majority of iridescence, he says, could be arbitrary, or “merely beautiful.”

Mere prettiness is no slur on the marvels of iridescent structures. A longtime iridescence specialist, developmental biologist Helen Ghiradella of the University at Albany, State University of New York, has published pages and pages of scanning electron microscope images revealing huge variety in the fine details of the textures of animal surfaces: bumpy surfaces like rows of Christmas trees, fields of latticework honeycombs, bristles that work like fiber optic cables (but better). She reels off examples of the cutting-edge developments in optics that she has observed in nature: thin films, photonic crystals ordered in one, two and three dimensions, plus surfaces that combine techniques.

She protests the unfairness of questions about which species flaunt the showiest iridescence. When pressed, though, she offers examples that include the Southwest’s scarab beetle Chrysina gloriosa. The naked human eye can’t detect the full light show, alas, so people have to make do with admiring the beetle’s shimmery green back. Equipped with the right instruments, though, an observer realizes that the beetle reflects the controlled spirals of both right- and left-handed circularly polarized light.

Even one of the field’s old classics, the Morpho butterflies that Ghiradella studied during the 1970s, still hold surprises. In 2007, she contributed to a Morpho article in the February Nature Photonics published by a General Electric research team led by Radislav Potyrailo of the company’s Niskayuna, N.Y., lab. Potyrailo had seen pictures of a Morpho wing nanostructure and realized that vapors of different gases should subtly alter the butterfly’s iridescence. The GE team and Ghiradella analyzed the effects, which Potyrailo says suggest new options for developing sensors that change color with a whiff of a certain vapor.

Natural structures for controlling colors certainly should be an inspiration for engineers, and physicists should pay attention, says Andrew R. Parker of the University of Oxford in England. His group studies optical biomimetics, or nature-inspired technology. The animals’ devices come from millions of years of evolutionary trial and error and, as he puts it, “the average physicist has rather less time.”

Imitating nature isn’t easy. Peter Vukusic, who estimates his research group at the University of Exeter in England has looked for these structures in 500 to 600 species of insects, still uses words like “unbelievable.”

He and his Exeter colleagues have attempted to replicate the surface complexity of a butterfly wing. Starting almost a decade ago, they experimented with building large-scale models of these structures, at first just for show-and-tell but then in the hopes of doing experiments to understand the novel optical properties.

Vukusic, a veteran of restoring old houses, started trying to create repetitious elements in wood the way a router shapes chair rails. He wasn’t even trying to build a whole wing, since he’d scaled up so much that a single butterfly would spread more than a kilometer.

Even at that extreme magnification, the skilled and inventive fabricators for Exeter’s laboratories struggled to produce even grossly simplified versions.

Then, while driving home one day, Vukusic says, he “experienced a moment of clarity—suddenly the mist rises.” Vukusic abandoned several years’ worth of wooden butterfly parts and used a rapid prototyping system to bring wings into the era of computer-controlled polymer shaping. He and his colleagues finally created chunks of opaque white plastic that mimic a fleck of wing surface accurately enough for research purposes.

“This thing looks like a dinner plate,” he says. At this large scale, the model bit of a Morpho butterfly wing, for example, holds shapes that resemble a row of white Christmas trees, each a few centimeters high. At this scale, the models do nothing to light but can manipulate the longer wavelengths of microwaves as stand-ins. Vukusic’s team is using these models and microwaves to study how insect wings create a silvery effect. His models starred at the February workshop in Tempe.

Animals might have a hard time with these specialized structures too. If they do, some biologists suggest that the challenges give iridescence its value.

In one scenario, the structures represent a handicap. Growing them might sap energy from other developmental processes. Or flying around as a living disco ball might stir up predators. Costly iridescence would become the male butterfly’s Porsche, says Darrell Kemp of James Cook University in Cairns, Australia. In a related scenario, “iridescence is just plain difficult, not necessarily costly, for all males to generate, like a good sense of humor in human males,” Kemp says.

Earlier work on what female butterflies like had resoundingly shown that color matters. When researchers blotted out the iridescent ultraviolet markings on the wings of male Colias butterflies, the researchers found that the males had a pretty lonely existence.

Yet Kemp argues these earlier experiments had created such drastic changes in male finery that researchers couldn’t say in what way the color mattered. The female might have rejected the male because she no longer recognized him as the right species. He revised experimental procedures and worked with Hypolimnas bolina butterflies. The upper surface of their wings are iridescent in ultraviolet wavelengths, which females of that species can see. The males must look like flashing beacons as they flap their wings.

To avoid the extremes of earlier experiments, Kemp used a screening substance to dull the males’ wings to about half their former UV brilliance. For comparison, he also blacked out the UV patches with a pen on some of the males. In tests in fields and enclosures, marked males failed to attract the attention that females bestowed on the full-UV fellows. The loss of brightness matters to female butterflies in choosing mates, he concluded last year in Proceedings of the Royal Society B.

A similar experiment finds the same dynamic in Eurema hecabe butterflies. Dulled males meet with less success in mating, particularly in attracting the supposedly more desirable large females, Kemp reports in the January/February Behavioral Ecology.

So Kemp says he’s convinced that females pay attention to males’ iridescent light shows. Now he’s working on understanding what kind of information those shows might contain. He has raised caterpillars under sorry conditions and checked to see if their displays changed. Both those that had to make do with skimpy rations and those that as pupae endured great swings of heat and cold grew poorly. As adults, their wings did not flash as brightly. Also, he noted that the iridescence seemed to diminish more than other traits he checked, such as pigment colors. Thus the intensity of iridescence could serve as a sensitive indicator of a male’s history.

One theory had also proposed that color signals could carry information about genetic quality, perhaps identifying certain males with the built-in resistance to laugh off slings and arrows of developmental stress. Kemp looked for signs that clusters of related individuals looked pretty good despite the stresses. Nice idea, but in this case, no support.

Prum says he accepts that animals use traits like iridescence as signals. What he objects to is what he describes as a widespread presumption that signals routinely carry information pertinent to the decision at hand. Some human signals, like onomatopoeic words, do carry clues to their meaning. Pop, snap, murmur. But plenty of human signals, like the words plenty of human signals, don’t. Genetic modeling, says Prum, shows that animal signals can easily arise without some innate relevant clue, such as a connection to male quality. So he hypothesizes that most animal signals will turn out to be like plenty of human signals.

The smuggler in the public service announcement sat handcuffed in prison garb, full of bravado and shrugging off the danger of bringing illegal immigrants across the border.

“Sometimes they die in the desert, or the cars crash, or they drown,” he said. “But it’s not my fault.”

The smuggler in the commercial, produced by the Mexican government several years ago, was played by an American named Raul Villarreal, who at the time was a United States Border Patrol agent and a spokesman for the agency here.

Now, federal investigators are asking: Was he really acting?

Mr. Villarreal and a brother, Fidel, also a former Border Patrol agent, are suspected of helping to smuggle an untold number of illegal immigrants from Mexico and Brazil across the border. The brothers quit the Border Patrol two years ago and are believed to have fled to Mexico.

The Villarreal investigation is among scores of corruption cases in recent years that have alarmed officials in the Homeland Security Department just as it is hiring thousands of border agents to stem the flow of illegal immigration.

The pattern has become familiar: Customs officers wave in vehicles filled with illegal immigrants, drugs or other contraband. A Border Patrol agent acts as a scout for smugglers. Trusted officers fall prey to temptation and begin taking bribes.

Increased corruption is linked, in part, to tougher enforcement, driving smugglers to recruit federal employees as accomplices. It has grown so worrisome that job applicants will soon be subject to lie detector tests to ensure that they are not already working for smuggling organizations. In addition, homeland security officials have reconstituted an internal affairs unit at Customs and Border Protection, one of the largest federal law enforcement agencies, overseeing both border agents and customs officers.

When the Homeland Security Department was created in 2003, the internal affairs unit was dissolved and its functions spread among other agencies. Since the unit was reborn last year, it has grown from five investigators to a projected 200 by the end of the year.

Altogether, there are about 200 open cases pending against law enforcement employees who work the border. In the latest arrests, four employees in Arizona, Texas and California were charged this month with helping to smuggle illegal immigrants into the country.

While the corruption investigations involve a small fraction of the overall security workforce on the border, the numbers are growing. In the 2007 fiscal year, the Homeland Security Department’s main anticorruption arm, the inspector general’s office, had 79 investigations under way in the four states bordering Mexico, compared with 31 in 2003. Officials at other federal law enforcement agencies investigating border corruption also said their caseloads had risen.

Some of the recent cases involve border guards who had worked for their agencies for a short time, including the arrest this month of a recruit at the Border Patrol academy in New Mexico on gun smuggling charges.

The federal government says it carefully screens applicants, but some internal affairs investigators say they have been unable to keep up with the increased workload.

“It’s going to get worse before it gets better,” said James Wong, an internal affairs agent with Customs and Border Protection. “It’s very difficult for us to get out and vet each and every one of the applicants as well as we should.”

The Border Patrol alone is expected to grow to more than 20,000 agents by the end of 2009, more than double from 2001, when the agency began to expand in response to concerns about national security. There has also been a large increase in the number of customs officers.

James Tomsheck, the assistant commissioner for internal affairs at Customs and Border Protection, said the agency was “deeply concerned” that smugglers were sending operatives to take jobs with the Border Patrol and at ports.

Mr. Tomsheck said the agency intended to administer random lie-detector tests to 10 percent of new hires this year, with the goal of eventually testing all applicants. His office has contracts with 155 retired criminal investigators, adding 36 since last fall, to do background checks.

In one of the new corruption cases this month, at a border crossing east of San Diego, a customs officer allowed numerous cars with dozens of illegal immigrants and hundreds of pounds of drugs to pass through his inspection lane, investigators said.

The officer, Luis Alarid, 31, had worked at the crossing less than a year, and the loads included a vehicle driven by Mr. Alarid’s uncle, the authorities said. Mr. Alarid has pleaded not guilty to a charge of conspiracy to smuggle. Investigators found about $175,000 in cash in his house, according to court records.

In another recent case, Margarita Crispin, a customs inspector in El Paso, Tex., began helping drug smugglers just a few months after she was hired in 2003, according to prosecutors. She helped the smugglers for four years before she was arrested last year and sentenced in April to 20 years in prison and ordered to forfeit up to $5 million.

Although bad apples turn up in almost every law enforcement agency, the corruption cases expose a worrisome vulnerability for national and border security. The concern, several officials said, is that corrupt agents let people into the country whose intentions may be less innocent than finding work.

“If you can get a corrupt inspector, you have the keys to the kingdom,” said Andrew P. Black, an F.B.I. agent who supervises a multiagency task force focused on corruption on the San Diego border.

Comparing corruption among police agencies is difficult because of the varying standards and procedures for handling internal investigations, said Lawrence W. Sherman, the director of the Jerry Lee Center of Criminology at the University of Pennsylvania and an authority on corruption.

But he described policing the border as “potentially one of the most corruptible tasks in law enforcement” because of the solitary nature of much of the work and the desperation of people seeking to cross.

Michael Chertoff, the homeland security secretary, declined an interview. But in response to questions at a recent news conference, he suggested that the breadth and depth of border security improvements would inevitably produce problem officers.

“There is an old expression among prosecutors,” he said. “Big cases, big problems. Little cases, little problems. No cases, no problems. Some people take the view we ought to make no cases and then we would have no problems. I think that is a head-in-the-sand view, which I do not endorse.”

A Veteran Gone Bad

The customs inspector stands just outside his booth, his hand waving a stream of cars through the Otay Mesa crossing just east of San Diego. They zip past, one after another, no questions asked, an unusually easy welcome into the United States where inspectors are known to grill citizens about their travels before allowing them through.

But time was running short for this Customs and Border Protection officer, Michael Gilliland, a revered veteran on the late shift expecting a special delivery — a vehicle with several illegal immigrants — in his crossing lane.

Rather than intercept them, he had arranged for their safe passage through his lane, federal prosecutors said.

Mr. Black, the F.B.I. agent from San Diego, shook his head as he watched a surveillance videotape of Mr. Gilliland.

“You’re basically giving that smuggling organization an opportunity to conceal whatever else they want in that vehicle,” he said, “whether it's drugs, weapons, terrorists.”

The smugglers use any ruse available to lure border workers but seem to favor deploying attractive women as bait. They flirt and charm and beg the officers, often middle-aged men, to “just this once” let an unauthorized relative or friend through. And then another and another.

Prosecutors believe this is how smugglers ensnared Mr. Gilliland, who eventually pleaded guilty to taking $70,000 to $120,000 in exchange for letting hundreds of illegal immigrants pass through his lane. He was sentenced last year to five years in federal prison. Two women he had befriended also pleaded guilty.

The case against Mr. Gilliland, 46, stands out for the number of immigrants he helped and the shock of a respected veteran gone bad.

To young inspectors, Mr. Gilliland was a mentor, quick with advice, even an embrace, a burly go-to type with 16 years under his belt.

“He knew the laws backward and forward,” said Edward Archuleta, an internal affairs agent with Customs and Border Protection who once worked with Mr. Gilliland and eventually helped bring him down.

A tip steered F.B.I. agents to Mr. Gilliland’s illegal activities, but it took agents two years to build the case. The evidence against him included secretly recorded phone conversations in which Mr. Gilliland coordinated with Mexican smugglers when to drive their cargo of illegal immigrants through inspection lanes.

One morning, while Mr. Gilliland was taking a break from his shift, agents called him over and told him he was under arrest. They had braced for Mr. Gilliland to become belligerent, but instead he collapsed into a chair, weak-kneed.

“My grandfather always told me that when you’re born, the only thing you’re born with is your word, and only you can give that away, your integrity,” Mr. Gilliland said at his sentencing hearing. “And I’m sorry.”

The case against the Villarreal brothers — the former Border Patrol agents in San Diego — illustrates how hard it has been for investigators to hunt for and root out corrupt officers, many of whom know how to game the system.

The Villarreals would meet illegal immigrants near the border. The doors of their government-issue truck would swing open and Mexicans and Brazilians would climb in. Off they drove, Border Patrol agents at the wheel, but not to a station or jail, investigators said.

Instead, they said, the migrants were taken to a drop house in San Diego and later transported by others in the smuggling ring to cities and towns far from the border.

The case against the Villarreals had shock value, even to those on the inside.

“Just really brazen, broad daylight,” said an investigator, who was granted anonymity because he was not authorized to discuss a continuing investigation. “They could say, ‘We picked these guys up, we’re taking them in.’ ”

As they closed in on the brothers, a squad of agents from several federal agencies met. Some had qualms about speaking openly in front of such a large group, fearing internal leaks.

Their fears were apparently borne out when, a couple of weeks after the meeting, the brothers quit their posts, left their badges at their family’s home in National City, Calif., and have not been seen publicly since.

A lawyer for the family, Jon Ronis, declined to say where the brothers were and said neither they nor family members would comment. Mr. Ronis said Raul and Fidel Villarreal were ready to defend themselves if the government brought a case.

Federal officials declined to comment because the case was still open. But investigators described some aspects of it on condition of anonymity. When the public service announcement was being made for Mexico, for example, Raul Villarreal spoke excitedly about his role in producing it, even suggesting camera angles and lighting, said a person familiar with its production.

Just when and why the brothers turned against the Border Patrol is unclear, even to the investigators. There is speculation that Raul had grown disgruntled with the work, chafing at having been moved back into the field from his public affairs job, considered a comfortable, high-profile position.

The Villarreal case is especially alarming for the level of trust the brothers had earned within the Border Patrol. Their betrayal has had the effect, at least in some investigations, of leading the authorities to move in more quickly when agents are suspected of wrongdoing.

In the case of Jose Olivas Jr., a Border Patrol agent in San Diego who was discovered serving as a scout for smugglers, an arrest was made within a year. Mr. Olivas, an agent for 10 years who had worked as a liaison between the agency and the United States attorney’s office, was sentenced in January to three years in prison.

The drawback to moving in fast, investigators said later, is that they probably will never know how deep Mr. Olivas’s ties were to the smuggling organization. He suggested to a judge that he had been drawn to smuggling to help pay his bills.

An internal Web site at Customs and Border Protection features a page devoted to a rogue’s gallery of agents and officers recently convicted of corruption-related charges.

The intention, homeland security officials say, is to send the message that corruption will not be tolerated. That message has taken other forms, as well. When Mr. Olivas, the San Diego border agent, was sentenced to prison, several agents attended the court hearing at the behest of homeland security officials to shame him publicly.

“I am truly embarrassed just looking at them,” Mr. Olivas told the judge. “I am truly sorry for the breach of trust that was given to me.”

But if the department is serious about catching wrongdoers, investigators of corruption cases say it also needs to make fundamental changes in the way it polices the border police.

One result of the awkward marriage of agencies that begat the Homeland Security Department is that three internal affairs units, in addition to the Federal Bureau of Investigation, have a hand in corruption investigations. In the best case, having more than one unit investigate corruption can be a “force multiplier,” in the words of one investigator, but more often, it can slow cases down and lead to confusion over who should take the lead, several investigators said.

The Department of Homeland Security’s inspector general has nearly 170 investigators to police 208,000 department employees — including other large agencies like the Federal Emergency Management Agency, the Transportation Security Administration, the Secret Service — and gets first crack at cases. When it passes on an investigation, the case is picked up by either the Immigration and Customs Enforcement’s office of professional responsibility or the Customs and Border Protection internal affairs unit.

The F.B.I. also develops its own cases. Don Allen, a retired agent who until 2005 supervised a multiagency task force in San Diego investigating corruption among border officers, said internal affairs units did not always readily share information and often resented any sense of being big-footed by an outside agency. He said law enforcement agencies often “had a negative impression of the bureau.”

Thomas Frost, an assistant inspector general with the Homeland Security Department, said the limited number of investigators meant his office focused on “those most important cases and what resources we can bring to bear.”

He suggested it would be “more efficient” if his office had more investigative resources under its control so that it could better track “everything going on.”

“Let’s face it,” Mr. Frost said, “part of the issue of the border is it is kind of a balloon. When you squeeze one part, another bulges.”

Some Recent Cases

Jose Ramiro Arredondo, 33, a Customs and Border Protection officer in Laredo, Tex., was arrested in March after a smuggler who had been detained told the authorities that Mr. Arredondo had helped bring illegal immigrants across the border.

Miguel Angel Avina, a trainee at the Border Patrol academy in Artesia, N.M., was arrested in May on fraud and conspiracy charges related to his participation last year in a ring that smuggled at least 110 guns into Mexico, the government said. He has been dismissed from the academy.

Juan Luis Sanchez, 31, a Border Patrol agent, pleaded guilty May 20 to drug, bribery and fraud charges. He admitted transporting at least 3,000 pounds of marijuana in his Border Patrol truck from summer 2002 to January 2004 in exchange for $45,000 in bribes.

Jose Magana, 44, a Customs and Border Protection officer at the San Luis, Ariz., border crossing, was arrested May 12 on charges of conspiring to smuggle illegal immigrants. The authorities say he allowed people to pass uninspected through.

Luis Francisco Alarid, 31, a Customs and Border Protection officer, was arrested May 16 on charges of conspiring to smuggle illegal immigrants and drugs into the United States. Mr. Alarid allowed numerous vehicles with migrants or drugs to pass through his inspection lane since at least February at a border crossing east of San Diego, the authorities say. One vehicle, containing 18 illegal immigrants, was driven by his uncle. He has pleaded not guilty.

Plant hormones (also known as plant growth regulators (PGRs) and phytohormones) are chemicals that regulate plant growth. Plant hormones are signal molecules produced at specific locations in the plant, and occur in extremely low concentrations. The hormones cause altered processes in target cells locally and at other locations. Plants, unlike animals, lack glands that produce and secrete hormones. Plant hormones shape the plant, affecting seed growth, time of flowering, the sex of flowers, senescence of leaves and fruits. They affect which tissues grow upward and which grow downward, leaf formation and stem growth, fruit development and ripening, plant longevity and even plant death. Hormones are vital to plant growth and, if they were to lack them, plants would be mostly a mass of undifferentiated cells.

The word hormone is derived from Greek and means 'set in motion.' They are naturally produced within plants, and very similar chemicals are produced by fungi and bacteria which also can influence plant growth. A large number of related chemical compounds also have been synthesized by humans that function as hormones too, which are called plant growth regulators, or PGRs for short. At the beginning of the study of plant hormones, "phytohormone" was the commonly-used term, but its use is less widely applied now.

Plant hormones are not nutrients but chemicals, that in very small amounts promote and influence the development and differentiation of cells and tissues. Plant hormones affect gene expression and transcription levels, cellular division and growth.

The biosynthesis of plant hormones within plant tissues is often diffuse and not always localized, because unlike animals, which have two circulatory systems (lymphatic and cardiovascular) powered by a heart that move fluids around the body, plant hormones often move passively about the plant. Plants utilize simple chemical hormones that move more easily through the plant's tissues. They are often produced and used in the same vicinity within the plant body, plant cells even produce hormones that have an effect on the same cell producing them.

Hormones are transported within the plant by utilizing four types of movements. For localized movement,  cytoplasmic streaming within cells and  slow diffusion of ions and molecules between cells are utilized. Vascular tissues are used to move hormones from one part of the plant to another, these include sieve tubes that move sugars from the leaves to the roots and flowers, and xylem that moves water and mineral solutes from the roots to the foliage.

Not all plant cells respond to hormones, but cells that do so, are programmed to respond at specific points in their life cycle. The greatest effects occur at specific stages during the cell's life, with diminished effects occurring before or after this period. Plants need hormones at very specific times during their growth and at specific locations within the plant. They also need to disengage the effects that hormones have when they are no longer needed. The production of hormones occurs very often at sites of active growth within the meristems, and are produced by cells before they have fully differentiated into their “adult” form. After production hormones are sometimes moved to other parts of the plant where they cause an immediate influence or they can be stored in cells to be released later. Plants use different pathways to regulate internal hormone quantities and moderate their effects; they can regulate the amount of chemicals used to biosynthesize the hormones. They can store them in cells, inactivate them, or cannibalize already-formed hormones by conjugating them with carbohydrates, amino acids or peptides. Plants can also break down hormones chemically, effectively destroying them. Plants can also move hormones around the plant to dilute their concentrations.

The concentration of hormones required for plant responses are very low (10-6 to 10-5 mol/L). Because of these low concentrations it has been very difficult to study plant hormones and only since the late 1970s have scientists been able to start piecing together their effects on, and relationships to, plant physiology.[3] Much of the early work on plant hormones involved studying plants that were genetically deficient in hormones or involved the use of tissue cultured plants grown in vitro that were subjected to differing ratios of hormones and the resultant growth compared. The earliest scientific observations and studies though, date back to the 1880s; the determination and observation of plant hormones and their identification was spread-out over the next 70 years.

It is generally accepted that there are five major classes of plant hormones, some of which are made up of many different chemicals that can vary in structure from one plant to the next. The chemicals are each grouped together into one of these classes based on their structural similarities and on their effects on plant physiology. Other plant growth regulators that are not easily grouped into these classes exist naturally, including chemicals that inhibit plant growth or interrupt the physiological processes within plants. Each class has positive as well as inhibitory functions, and they most often work in tandem with each other, with varying ratios of one or more interplaying to affect growth regulation.

The five major classes are:

Abscisic acid

Abscisic acid (ABA)

Abscisic acid (ABA)

Abscisic acid also called ABA, was discovered and researched under two different names before its chemical properties were fully known, it was called dormin and abscicin II. Once it was determined that the two latter named compounds were the same, it was named abscisic acid. The name "abscisic acid" was given because it was found in high concentrations in newly-abscissed or freshly-fallen leaves.

This class of PGR is composed of one chemical compound normally produced in the leaves of plants, originating from chloroplasts, especially when plants are under stress. In general, it acts as an inhibitory chemical compound that effects bud growth, seed and bud dormancy. It mediates changes within the apical meristem causing bud dormancy and the alteration of the last set of leaves into protective bud covers. Since it was found in freshly-adscissed leaves, it was thought to play a role in the processes of natural leaf drop but further research has disproven this. In plant species from temperate parts of the world it plays a role in leaf and seed dormancy by inhibiting growth, but, as it is dissipated from seeds or buds, growth begins. In other plants, as ABA levels decrease, growth then commences as gibberellin levels increase. Without ABA, buds and seeds would start to grow during warm periods in winter and be killed when it froze again. Since ABA dissipates slowly from the tissues and its effects take time to be offset by other plant hormones, there is a delay in physiological pathways that provide some protection from premature growth. It accumulates within seeds during fruit maturation, preventing seed germination within the fruit, or seed germination before winter. Abscisic acid's effects are degraded within plant tissues, during cold temperatures or by its removal by water washing in out of the tissues, releasing the seeds and buds from dormancy.

In plants water stressed, ABA plays a role in closing the stomata. Soon after plants are water stressed and the roots are deficient in water, a signal moves up to the leaves causing the formation of ABA precursors, these precursors move to the roots which release ABA that is translocated to the foliage through the vascular system,[6] which regulates the potassium or sodium uptake within the guard cells, which then loses turgidity, closing the stomata.[7][8] ABA exists in all parts of the plant and its concentration within any tissue seems to mediate its effects and function as a hormone, its degradation or more properly catabolism within the plant affects metabolic reactions and cellular growth and production of other hormones. Plants start life as a seed with high ABA levels, just before the seed germinates ABA levels decrease; during germination and early growth of the seedling, ABA levels decrease even more. As plants begin to produce shoots with fully functional leaves - ABA levels begin to increase, slowing down cellular growth in more "mature" areas of the plant. Stress from water or predation effects ABA production and catabolism rates which mediate another cascade of effects triggering specific responses from targeted cells. Scientists are still piecing together the complex interactions and effects of this and other phytohormones.

Auxins are compounds that positively influence cell enlargement, bud formation and root initiation. They also promote the production of other hormones and in conjunction with cytokinins, they control the growth of stems, roots, flowers and fruits.[10] Auxins were the first class of growth regulators discovered.[11] They affect cell elongation by altering cell wall plasticity. Auxins decrease in light and increase where its dark. They stimulate cambium cells to divide and in stems cause secondary xylem to differentiate. Auxins act to inhibit the growth of buds lower down the stems, affecting a process called apical dominance, and also promote lateral and adventitious root development and growth. Auxins promote flower initiation, converting stems into flowers. When auxins are no longer produced by the growing point of a plant, this initiates leaf abscission. Seeds produce auxins, that regulate specific protein synthesis,[12] as they develop within the flower after pollination, causing the flower to develop a fruit to contain the developing seeds. Auxins are toxic to plants in large concentrations; they are most toxic to dicots and less so to monocots. Because of this property, synthetic auxin herbicides including 2,4-D and 2,4,5-T have been developed and used for weed control. Auxins, especially 1-Naphthaleneacetic acid (NAA) and Indole-3-butyric acid (IBA), are also commonly applied to stimulate root growth when taking cuttings of plants. The most common auxin found in plants is indoleacetic acid or IAA.

Cytokinins

Cytokinins or CKs are a group of chemicals that influence cell division and shoot formation. They were called kinins in the past when the first cytokinins were isolated from yeast cells. They also help delay senescence or the aging of tissues, are responsible for mediating auxin transport throughout the plant, and affect internodal length and leaf growth. They have a highly-synergistic effect in concert with auxins and the ratios of these two groups of plant hormones affect most major growth periods during a plant's lifetime. Cytokinins counter the apical dominance induced by auxins; they in conjunction with ethylene promote abscission of leaves, flower parts and fruits.