1. Manchineel

Would you believe that there's a tree so poisonous that you don't actually have to touch it to be harmed? It's called the manchineel tree (Hippomane mancinella), found throughout the Florida Everglades, Central America and the Caribbean.

Inhaling sawdust or smoke from the 30-foot (9-meter) tall tree may result in a variety of uncomfortable side effects, including coughing, laryngitis and bronchitis. Some reports suggest that simply standing beneath the tree during a rainstorm and being splashed by runoff may result in rashes and itching. Your car isn't even safe from this toxic tree: Park under its low branches, and dripping sap can seriously damage the paint.

Direct contact with the manchineel tree is far more hazardous. Its milky sap can squirt from the tree when twigs are snapped off, painfully irritating the skin and eyes. Ingestion of the deceptively sweet, crabapple-like fruits is known to blister the mouth and cause the throat to swell shut, then inflict severe gastrointestinal problems. These harmful effects result from the toxin hippomane A and B, which are present in every part of the tree.

The manchineel tree sometimes grows near the beach, giving it another of its common names, "beach apple." Hapless tourists vacationing on the warm coasts of Central America and the Caribbean often encounter its poisonous boughs with unfortunate consequences. So if you're heading to that region's beach resorts, make sure to avoid the manchineel tree or else your dream vacation could turn into a nightmare.

2. Jimsonweed

With pointy leaves and spiky fruit, jimsonweed (Datura stramonium) definitely looks the part of a poisonous plant. Its toothed foliage emits an unpleasant odor and branches from reddish-purple stalks, which grow to a height of 3 to 4 feet (0.9 to 1.2 meters). The plant's fruit is particularly wicked-looking. The green spheres, measuring about 2 inches (5 centimeters) across, are covered with long, sharp spines. Even the nectar and petals of its beautiful white or lavender trumpet-shaped flowers are dangerous. They, like the rest of the plant, are tainted with the toxins atropine and scopolamine.

European settlers in the New World quickly discovered the potency of jimsonweed, which grows throughout Canada, the United States and the Caribbean. The plant was plentiful at Jamestown, where some colonists made the mistake of having it for dinner as early as 1607. They would have experienced horrific symptoms, including dilated pupils, racing heartbeat, hallucination, delirium, aggressive behavior and possibly coma or seizures.

The plant has been linked to darker arts, like witchcraft and voodoo, because of its delirium-inducing and hallucinogenic properties. For most people, though, jimsonweed is a dangerously poisonous plant that's best avoided completely.

3. Aconite

Aconite (Aconitum napellus) is commonly referred to as monkshood because the top of the flower resembles the monastic head covering. But there's nothing holy about this plant. A perennial, it stands 2 to 6 feet (0.6 to 1.8 meters) tall and produces blue, white or flesh-colored bunches of flowers at the tops of its stalks. Every part of the aconite plant is laced with the toxin aconitine, making it dangerous to consume or even touch.

Poisonings from aconite are rare but typically occur when gardeners mistake its white carrot-like root for horseradish or some other edible herb. Consuming the plant causes burning in the mouth followed by increased salivation, vomiting, diarrhea, a tingling sensation in the skin, blood pressure and heart irregularities, coma and sometimes death. Just touching aconite can cause tingling, numbness, and in severe cases, heart problems.

People have used aconite in the past to intentionally harm people or animals. Nazi scientists used the plant's toxin to poison bullets, while shepherds in ancient Greece laced bait and arrows with aconite to kill wolves, that preyed on their stock. From this latter use came another common name, "wolfsbane." Fans of the Harry Potter series will recognize this as the plant Professor Snape brews to help Remus Lupin turn into a werewolf.

4. White Snakeroot

Most people know that John Wilkes Booth assassinated Abraham Lincoln, but did you know that a plant killed the president's mother? The culprit: white snakeroot (Ageratina altissima), a shade-loving weed native to the forests of the eastern and southern United States. This shrubby plant grows to a height of 18 to 60 inches (46 to 152 centimeters) and boasts leaves that are serrated around the edges. Its flowers, which emerge from the ends of the branches in late summer, are small and grow in white clusters. Don't let these beautiful blooms fool you, though; the plant contains high levels of tremetol, a powerful toxin.

White snakeroot causes "milk sickness," a condition that afflicts people who consume milk or meat from a cow that has grazed on the highly poisonous plant. (Snakeroot is also poisonous to the cow.) Those affected can experience a variety of symptoms, including bad breath, loss of appetite, listlessness, weakness, vague pains, muscle stiffness, vomiting, abdominal discomfort, severe constipation, coma and possibly death. Milk sickness was common until the 1920s when farmers widely recognized white snakeroot as the cause, eradicating the weed from their pastures and fencing them to prevent cows from wandering into the woods to graze. Unfortunately, this discovery came much too late for Lincoln's mother, Nancy, who fought milk sickness for two weeks before passing away on Oct. 5, 1818.

5. English Yew 

Given the English yew's highly toxic nature, it's fitting that the tree is commonly found growing in church graveyards across Great Britain. Some scholars believe that this tradition started when early Christians incorporated the trees — which already had spiritual value to the pagans — into their new religion. Today they stand not as symbols of death, but of the immortality of the soul.

The English yew (Taxus baccata) is an evergreen tree with needlelike leaves and red arils, or fleshy seed-coverings. It grows to a height of 60 to 70 feet (18.3 to 21.3 meters) and is found throughout Great Britain, but is also cultivated in the southern United States. Every part of the tree is toxic due to taxine alkaloids, except for the aril flesh. Consumption of the leaves, and to a lesser extent the seeds, can lead to increasingly serious symptoms, including dizziness, dry mouth, dilation of the pupils, weakness, irregular heart rhythm and possibly death.

Despite its harmful qualities, English yew has been used for a variety of productive purposes. Its wood was valued across Europe for bow-making as early as the Neolithic period, which lasted from approximately 7000 B.C.E to 2500 B.C.E Later, the Anglo-Saxons explored the tree's medicinal qualities, including yew berries in a 10th-century formula for the treatment of "water-elf disease" (probably measles or chicken pox). More recently, researchers have studied the English yew for its potent antitumor qualities. Today, yew extract is used to formulate the drug paclitaxol, or Taxol, which slows the growth of ovarian, breast and lung cancers.

6. Castor Bear

The castor bean plant, or Ricinus communis, is widely cultivated for its castor oil and is also used as an ornamental plant. Neither of these uses would clue you into the fact that this plant has deadly contents: ricin.

Castor oil is a mild-tasting vegetable oil that is used in many food additives, flavorings and in candy production. It's also available to the consumer as a laxative and to induce labor (though no scientific evidence shows it's successful in inducing labor). Castor oil comes from the plant's seeds, which are 40 to 60 percent oil.

The castor bean plant probably originated in Africa, but is now found throughout the world. This large, shrubby plant is popularly used in gardens because of its hardy nature. It grows well in barren areas and doesn't require special care. It's fast-growing and can reach 36 feet (11 meters) in a season. The flowers of the plant are yellowish green, and the centers of the flowers are red. The leaves are large with toothed edges.

Ricin is present in low levels throughout the plant, but it's largely concentrated in the seed coating. Seed poisonings are rare and usually involve children and pets, but they can be deadly. As few as three seeds, which are green with brown markings, could kill a child who swallows them.

Symptoms of castor bean poisoning include nausea, abdominal cramps, vomiting, internal bleeding, and kidney and circulation failure. Many people suffer from an allergic reaction to the dust from the seeds and may experience coughing, muscle aches and difficulty breathing. Exposure to the dust is most common in areas where the beans are processed for commercial use. In ancient times, the castor bean was used in ointments, and allegedly, Cleopatra applied the oil to the whites of her eyes to brighten them.

7. Deadly Nightshade

Deadly nightshade, or Atropa belladonna, contains poisonous atropine and scopolamine in its stems, leaves, berries and roots.

Deadly nightshade is a perennial plant that grows between 2 and 4 feet (0.6 to 1.2 meters) tall. You'll recognize it by its dull, dark green leaves and bell-shaped purple, scented flowers, which bloom from mid-summer through early fall.

Deadly nightshade berries are green when they form and turn to a shiny black as they ripen. They're sweet and juicy, which makes them tempting to children. The plant requires rich, moist soil to thrive, and it grows wild in some areas of the world, but in the United States is limited to cultivation. Not all animals are affected by deadly nightshade. While it's deadly to humans and some animals, horses, rabbits and sheep can eat the leaves without harm, and birds feed on the berries.

The poisons contained in deadly nightshade affect the nervous system. Taken in sufficient doses, the deadly poison paralyzes nerve endings in the involuntary muscles of the body, such as the blood vessels, heart and gastrointestinal muscles.

Symptoms of deadly nightshade poisoning include dilated pupils, sensitivity to light, blurred vision, headaches, confusion and convulsions. As few as two ingested berries can kill a child, and 10 to 20 berries would kill an adult. Even handling the plant can cause irritation.

8. Rosary Pea

The rosary pea, or Abrus precatorius, has very pretty seeds. Two-thirds of the seed is red, and the top third is black. These decorative seeds are often used to make jewelry, and that jewelry is imported to other countries. In fact, these seeds are especially popular for rosary prayer beads.

But rosary pea seeds contain the poison abrin. The seeds are only dangerous when the coating is broken — swallowed whole, the rosary pea doesn't present any danger. But if the seed is scratched or damaged, it's deadly. The rosary pea poses greater danger to the jewelry maker than to the wearer. There are many reported cases of death when jewelry makers prick a finger while handling the rosary pea.

Rosary pea plant is an aggressive grower and can take over an area if not kept in check. One rosary pea vine can grow and climb more than 20 feet (6 meters) in a single season. The plant, which is native to Indonesia, has spread across the world, in tropic and sub-tropic climates. It's even located in several states in the United States, including Alabama, Arkansas, Florida, Georgia and Hawaii. The plant has long leaves with off-shooting leaflets and red flowers.

Abrin, the poison found in the rosary pea seed, is more deadly than ricin. Less than 3 micrograms of abrin in the body is enough to kill, which is less than the amount of poison in one pea. In the human body, abrin bonds to cell membranes and prevents protein synthesis, one of the most important duties of the cell. Symptoms of rosary pea inhalation poisoning are: difficult breathing, fever, nausea and fluid in the lungs. If ingested — and the seed coating is broken — rosary pea seeds cause severe nausea and vomiting, which eventually leads to dehydration, and ends with the kidneys, liver and spleen shutting down. Death usually follows within three to four days.

9. Water Hemlock

The water hemlock, which is native to North America, is considered by many to be the most deadly plant on the continent. The wildflower, which grows to 6 feet (1.8 meters), thrives along stream banks, in marshy areas, and in low-lying, damp meadows.

For those unlucky enough to taste the water hemlock, the onset of illness is rapid. The cicutoxin contained in the plant causes violent and painful convulsions, nausea, vomiting, cramps and muscle tremors. Those who survive the poisoning experience long-term health conditions, such as amnesia. No amount of water hemlock root is considered safe to ingest.

10. Oleander

The oleander, or Nerium oleander, is considered by many to be the most poisonous plant in the world. All parts of the beautiful oleander contain poison — several types of poison. Two of the most potent are oleandrin and neriine, known for their powerful effect on the heart. An oleander's poison is so strong, in fact, that it can poison a person who simply eats the honey made by bees that have digested oleander nectar.

The oleander is an attractive plant, and despite its deadly reputation is often planted for decorative purposes. Although native to the Far East and the Mediterranean areas, oleander has been introduced in the United States, where it grows easily. It's tolerant of poor quality soil and dry weather. The plant grows as a dense shrub, and is typically 6 to 18 feet (1.8 to 5.4 meters) tall at maturity. It has thick, dark green leaves, and the flowers, which grow in clusters, can be yellow, red, pink or white.

Even in barren areas, the oleander produces lovely flowers and fragrance. Animals instinctively avoid the plant, and it grows rapidly, so it's often used for highway barriers and other areas that require screening from noise and pollution. Its rapid growth also makes it a popular choice around new construction zones, as it prevents erosion.

Unlike some toxic plants, the oleander is poisonous to most animals as well as humans. A single ingested oleander leaf can kill a child. Ingestion of oleander results in diarrhea, vomiting, intense stomach pain, drowsiness, dizziness, an irregular heartbeat, and often, death. If the victim survives the initial 24 hours after ingestion, his or her odds of surviving increase dramatically. For successful treatment, the patient is induced to vomit, his or her stomach may be pumped, or he or she may be fed activated charcoal to absorb as much of the poison as possible.