Plants & Human Affairs - Introduction
Cherries.wmf (7140 bytes) Plants & Human Affairs (BIOL106)  -  Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;

Poisonous & Harmful Plants

I. Why are plants poisonous or otherwise harmful?
    Plants are poisonous, or have other nasty features, primarily because they can't move around to avoid predators (herbivores). Plants have developed an array of weapons (like thorns, hairs) and a lethal chemical arsenal (secondary metabolites) to ward off herbivores while remaining rooted firmly. Thus, plants are "intentionally trying to hurt us".  In addition, plants also unintentionally cause great human suffering (i.e., hayfever) when they shed their pollen, which is a also direct consequence of their non-motile lifestyle. 

II. Why don't poisonous plants kill themselves?
    Did you ever wonder why toxic secondary metabolites don't kill the plant that makes them? Primarily because after the chemical is produced, it is "locked away". In other words, the poisonous chemicals are stored in the vacuole in the cells. The vacuole is a large water-filled sac teeming with a variety of materials including poisonous chemicals and other waste products. As long as the chemicals are tucked away in the vacuole the remainder of the cell is safe. 

III. Allelopathy – plant vs. plant
    Plants are also "poisonous" to one another.  Allelopathy is essentially chemical warfare between plants. Good gardeners know that you shouldn't plant a walnut tree near your garden because a chemical called juglone leaches from the roots and leaves. Juglone will inhibit the growth of many common garden plants especially tomatoes.

IV.  Accidental Ingestion of Poisonous Plants (not on exam)
One common way in which people come into contact with poisonous plants is to accidentally eat them.  Our ancestors quickly learned by trial-and-error which plants could be eaten and which to avoid.  However, even today many people are poisoned by plants because they:

  1. didn't bother to confirm the identity of the plants they were eating; 
  2. mistakenly identified a plant a plant they presumed to be edible; 
  3. relied on erroneous folktales for information about edibility (i.e., assumed a plant is edible because it was nibbled by an animal.  Some of our most poisonous mushrooms are eaten with impunity by a variety of insects and other animals;
  4. ate the wrong part of an otherwise edible plant (i.e., potato leaves are poisonous); 
  5. ate the plant during the wrong season (i.e., rhubarb stalks are rich in calcium oxalate taken identify); 
  6. ate a poisonous plant after received erroneous about its edibility (i.e., many report that milkweed is edible but you must be very careful because it is rich in cardiac glycosides and is toxic throughout most of its life; 
  7. the plant is poisonous to some but not others (i.e., Broad or fava beans are very toxic to certain individuals who have a single gene difference, often of Mediterranean descent); 
  8. didn't know better (i.e., children are probably the source of the greatest number of calls to poison control centers); and
  9. ate too much of an edible plant that contains low levels of toxins (i.e., apple seeds contain HCN which in high doses can be lethal).

Some plants that have caused accidental human poisonings include:

A. Henbane/Mandrake (Mandragora officinale)

  • used in witches brews
  • contain alkaloids, rather toxic.

B.  Jimson Weed (Datura stramonium; Solanaceae – Tomato Family)

  • named after Jamestown where some British sailors died from ingesting it.
  • deadly toxic, but hallucinogenic in extremely low doses – a good example of the fine line that exists between medicinal/poisonous and toxic plants.

C.  Strychnine (Strychnos nuv-vomica)

  • nerve toxin
  • used in past medicinally, but rather toxic and not used much anymore
  • rodent poison

D.  Milkweed (Asclepias)

  • can be eaten by monarch larvae, poison stored, and makes them toxic to birds. Viceroy looks similar but doesn't eat them to take advantage of protective coloration. Toxic to humans.
  • active ingredient is cardiac glycosides (steroids)

E.  Rosary Pea (Abrus precatorius; Fabaceae – Bean Family)

  • made necklaces but children ate; now illegal to do so

F.  Lily of the Valley (Convallaria majalis; Liliaceae – Lily Family)

  • extremely poisonous, all parts

G. Spurge (Euphorbia; Euphorbiaceae – Spurge Family)

  • plants with a milky sap (and other distinctive features)
  • milky sap can cause skin burns
  • many are also toxic
  • Poinsettia is a relative that has bad reputation but probably has minimal toxicity.

H.  Dumbcane (Dieffenbachia sp.; Araceae – Arum Family)

  • contains calcium oxalate crystals
  • contained in sac-like vase, crystals needle-like and shot out of opening
  • irritate the lining of the mouth and esophagus

I. Jack-in-the-pulpit (Arisaema triphyllum; Araceae – Arum Family)

  • another calcium oxalate producer

J.  White snakeroot (Eupatorium rugosum; Asteraceae – Sunflower Family)

  • milk sickness
  • tremetol

K.  Oleander (Nerium oleander; Apocynaceae – Dogbane Family)

  • very toxic, cardiac glycosides
  • widely planted ornamental shrub

V. Turning the Tables on Poisonous Plants
    Humans have taken advantage of poisonous plants in several ways:  (a) Medicines -  sub-lethal doses of some plants that would otherwise be poisonous can be used medicinally;
  (b) Hunting - to prepare poisons to put on the tip of arrows; and (c) Murder and other nefarious purposes.  We will discuss plants used in hunting and murder briefly:

A.  Plants and Hunting.
    The main use of plants in hunting is for producing a poison for the tips of arrow.  Curare is a arrow poison, mixture of many plants. Cause a rapid paralysis.

  • Main active ingredients include Strychnos toxifera (Loganiaceae) and Chondrodendicus tomentosa (Menispermaceae).
  • Blocks nerve impulse at muscle/nerve junction
  • muscle relaxant used in surgery

B.  Plants and Murder
    Some poisonous plants have been put to rather sinister uses.

1.  Hemlock (Conium maculatum; Apiacee – Carrot Family)

  • used to kill Socrates
  • kills by paralysis beginning with lower limbs and eventually paralyzes diaphragm leading to respiratory failure.
  • note – this is not the hemlock tree, which is a conifer

2.  Castor Bean (Ricinus communis; Euphorbiaceae - Spurge Family)

  • the seeds produce ricin a very toxic protein

VI. Plants causing mechanical injury

    1. Cactus - the thorns are bad, but the tiny hairs at the base of the thorns are even worse (called glochids).
    2. Nettles - stinging hairs, filled with histamine or other chemical; injected into skin; can cause severe reaction.

VII. Insecticides

    1. Pyrethrins - found in Chrysanthemum sp. (Asteraceae – Sunflower family). Nerve toxin to insects extracted from dried flowers
    2. Rotenone - isolated from roots of trees in bean family. Kills fish, also active against insects. Good garden pesticide, biodegradable.
    3. Neem (Azadirachta indica) - tree from India, has show great promise in insect control as well as human medicine.

VIII. Allergy Causing Plants
    Antibody/antigens - antibodies are proteins in the blood that bind to and destroy antigens. There is a unique antibody for every antigen. Proteins on the surfaces of cells are common antigens. An antibody is "Y"-shaped, made up of two heavy chains and two light ones. There is a conserved region - found in all antibodies - and a variable region that is different for each antibody and leads to the specificity of antibodies.

    Stem cells (bone marrow) blood cells B-lymphocytes antibodies (Ig series) five different kinds (including IgE which is involved in allergenic responses) attached to mast cells (line respiratory system, intensine, skin) and basophil cells (blood) after first exposure when bind antigen on subsequent exposures cause release of histamine symptoms

    Hay-fever - wind pollinated species such as ragweed, pines, oaks, grasses

    Contact dermatitis - urushiol produced by poison ivy, and relatives. 


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Last updated:  01/07/2005 / � Copyright  by SG Saupe / URL: