Poisonous plants have evolved toxic substances that function to defend them against herbivores and thereby better adapt them for survival.
After evolving adaptations that facilitated colonization of terrestrial habitats, plants were confronted with a different type of problem.
This was the problem of herbivory, or the inclination of many different types of organisms, from bacteria to insects to four-legged herbivores, to eat plants. Pressures from herbivory drove many different types of plants, from many different families, to evolve defenses.
Some of these defenses included changes in form, such as the evolution of thorns, spikes, or thicker, tougher leaves. Other plants evolved to produce chemical compounds that make them taste bad, interrupt the growth and life cycles of the herbivores, make the herbivores sick, or kill them outright.
One of the most interesting aspects of plants, especially prevalent in the angiosperms (flowering plants), is their evolution of substances called secondary metabolites, sometimes referred to as phytochemicals.
Once considered waste products, these substances include an array of chemical compounds: alkaloids, quinones, essential oils, terpenoids, glycosides (including cyanogenic, cardioactive, anthraquinone, coumarin, and saponin glycosides), flavonoids, raphides (also called oxalates, which contain needle-like crystals of calcium oxalate), resins, and phytotoxins (highly toxic protein molecules). The presence of many of these compounds can characterize whole families, or even genera, of flowering plants.
Effects on Humans
The phytochemicals listed above have a wide range of effects. In humans, some of these compounds will cause mild to severe skin irritation, or contact dermatitis; others cause mild to severe gastric distress. Some cause hallucinations or psychoactive symptoms. The ingestion of many other types of phytochemicals proves fatal.
Interestingly, many of these phytochemicals also have important medical uses. The effects of the phytochemicals are dependent on dosage: At low doses, some phytochemicals are therapeutic; at higher doses, some can kill.
Alkaloids are nitrogenous, bitter-tasting compounds of plant origin. More than three thousand alkaloids have been identified from about four thousand plant species. Their greatest effects are mainly on the nervous system, producing either physiological or psychological results.
Plant families producing alkaloids include the Apocynaceae, Berberidaceae, Fabaceae, Papaveraceae, Ranunculaceae, Rubiaceae, and Solanaceae. Some well-known alkaloids include caffeine, cocaine, ephedrine, morphine, nicotine, and quinine.
Glycosides are compounds that combine a sugar, usually glucose, with an active component. While there are many types of glycosides, some of the most important groups of potentially poisonous glycosides include the cyanogenic, cardioactive, anthraquinone, coumarin, and saponin glycosides.
Cyanogenic glycosides are found in many members of the Rosaceae and are found in the seeds, pits, and bark of almonds, apples, apricots, cherries, peaches, pears, and plums. When cyanogenic glycosides break down, they release a compound called hydrogen cyanide.
Two other types of glycosides, cardioactive glycosides and saponins, feature a steroid molecule as part of their chemical structure.
Digitalis, a cardioactive glycoside, in the right amounts can strengthen and slow the heart rate, helping patients who suffer from congestive heart failure. Other cardioactive glycosides from plants such as milkweed and oleander are highly toxic.
Saponins can cause severe irritation of the digestive system and hemolytic anemia. Anthraquinone glycosides exhibit purgative activities. Plants containing anthraquinone glycosides include rhubarb (Rheum species) and senna (Cassia senna).
Many common household plants are poisonous to both humans and animals. One family of popular household plants that can cause problems is the Araceae, the philodendron family, including plants such as philodendron and dieffenbachia.
All members of this family, including these plants, contain needle like crystals of calcium oxalate that, when ingested, cause painful burning and swelling of the lips, tongue, mouth, and throat.
This burning and swelling can last for several days, making talking and even breathing difficult. Dieffenbachia is often referred to by the common name of dumb cane, because eating it makes people unable to talk for a few days.
Many landscape plants are also poisonous. For example, the yew (genus Taxus), commonly planted as a landscape plant, is deadly poisonous.
Children who eat the bright red aril, which contains the seed, are poisoned by the potent alkaloid taxine. Yews are poisonous to livestock as well, causing death to horses and cattle. Death results from cardiac or respiratory failure.
Other poisonous landscape and garden plants include oleander, rhododendrons, azaleas, hyacinths, lily of the valley, daffodils, tulips, and star-of-Bethlehem. Many legumes are also toxic, including rosary pea, lupines, and wisteria.
Castor bean plant, a member of the family Euphorbiaceae, produces seeds that are so toxic that one seed will kill a child and three seeds are fatal to adults. The toxin produced by the seeds is called ricin, which many scientists consider to be the most potent natural toxin known.
Toxic plant and animal products have been used for thousands of years in hunting, executions, and warfare. Usually the poisonous extracts were smeared on arrows or spears.
The earliest reliable written evidence for these uses comes from the Rigveda fromancient India. Arrowpoisons come in many different varieties, and most rain-forest hunters have their own secret blend. SouthAmerican arrow poisons are generically called curare.
There are more than seventy different plant species used in making arrow poisons. Two of themain arrow poison plants are woody vines from the Amazon: Strychnos toxifera and Chondodendron tomentosum. Some types of curare have provenmedically useful.
They are used as muscle relaxants in surgery, which lessens the amount of general anesthetic needed. A plant called Strychnos nux-vomica from Asia yields the poison strychnine, a stimulant of the central nervous system.
In ancient times, toxic plant products were also commonly used in executions. Many people were expert, professional poisoners in the ancient world.
They could select a poison that would take days or evenmonths to take effect, thus ensuring, for example, that an unfaithful spouse or lover would not suspect the reason for his or her lingering illness. On occasions when a more rapid result was required, a strong dose or more powerful poison could be prescribed.
Toxicodendron radicans, commonly known as poison ivy, is well known for causing contact dermatitis. Poison ivy is a member of the Anacardiaceae, or cashew family, and is a widespread weed in the United States and southern Canada. It grows in a variety of habitats: wetlands, disturbed areas, and the edges of forests.
It has many forms, appearing as either a shrub or a woody vine which will grow up trees, houses, fences, and fence posts. It has alternate leaves with three leaflets, forming the basis of the old saying “Leaves of three, let it be.”
After poison ivy flowers, it develops clusters of white or yellowish-white berries. Related species are poison oak, western poison oak, and poison sumac, which some scientists consider to be different types of poison ivy.
Roughly half the world’s population is allergic to poison ivy. Very sensitive people develop a severe skin rash; about 10 percent of the people who are allergic require medical attention after exposure.
The chemical compound causing the allergic reaction is called urushiol, a resin found in all parts of the plant. Urushiol is so potent that in some individuals, just one drop produces a reaction.
Inhaling smoke from burning poison ivy can result in eye and lung damage. For some people, mere contact with the smoke from burning poison ivy can trigger a reaction. Urushiol lasts forever; in herbaria, dried plants one hundred years old have given unlucky botanists contact dermatitis.