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Poisonous and Allergy Plants

Chapter Summary

1. Poisonous and allergy plants are known to adversely affect the health of humans and other animals. As foraging societies identified plants according to use, knowledge of poisonous plants accumulated. Poisonous plants have been employed as arrow poisons, as “succession powders,” and as a form of capital punishment. Glycosides and alkaloids are usually the toxic compounds in most poisonous plants.

2. Many plants in the natural environment and those in the garden or house are poisonous. Some plant toxins, such as curare, have been used for beneficial purposes in medicine. Most cases of plant poisoning in the home involve the ingestion of aroids by young children.

3. White snakeroot, locoweed, Indian hellebore, and St. John’s wort are plants that have been implicated in the poisoning deaths of livestock.

4. Plants can cause mechanical injury by puncturing the skin via thorns, barbs, or spines. Stinging hairs not only penetrate the skin but also inject histamine-like irritants. Other plants injure by releasing irritating resins or latexes that damage the skin and other organs.

5. Plant toxins have become an important source in the search for natural insecticides. Pyrethrum and rotenone are two examples of plant-derived insecticides; the neem tree of India is the origin of an oil that has been shown to be effective against insect pests, bacteria, viruses, and fungi.

6. Allelopathy is a type of chemical warfare waged by plants that affects the growth and development of competing plants. The effects of allelopathy on the spacing of vegetation are especially pronounced in desert and chaparral plant communities.

7. Allergies are hypersensitivity diseases caused by the immune system’s response to harmless substances such as ragweed pollen or mold spores. Hay fever and asthma are the most common forms of respiratory allergies, affecting 20% to 25% of the human population. Hay fever is characterized by sneezing, runny nose, nasal congestion, and watery eyes; the symptoms of asthma include wheezing, coughing, and choking.

8. Pollen from ragweed plants is the most important cause of allergic rhinitis in North America. Ragweeds are pioneer species that are well adapted to invading disturbed soils. They occur abundantly through the eastern two-thirds of the continent, and in most parts of North America. Ragweed pollen occurs in the atmosphere from August through October. Approximately a million tons of pollen are produced each year in the United States.

9. In addition to ragweed, pollen from hundreds of seed plants is known to trigger respiratory allergies. Like ragweed, hay fever plants produce abundant lightweight, airborne pollen. Pollen allergies are usually categorized into tree, grass, and weed allergies, correlating the symptoms with the season of pollen production. Tree pollen is considered the leading cause of spring hay fever, grasses the leading cause of summer hay fever, and weeds the leading cause of fall hay fever.

10. The most common cause of contact dermatitis is poison ivy, a widespread climbing shrub in North America. The actual allergen is a resin, urushiol, present in all parts of the plant. The resin can be picked up by touching the plant directly or by touching clothing, animals, or even gardening tools that were in contact with the plant.

11. Food allergies can produce gastrointestinal symptoms, dermatitis, hives, or respiratory symptoms. A wide variety of plant foods are known to cause reactions in hypersensitive individuals.