USA Science and Engineering Festival: The Blog

From the Manchineel to the Yew, Trees are a Source for Good or Evil

joe_schwarcz (2)Guest Blog by Festival Nifty Fifty Speaker Joe Schwarcz PhD

According to Talmudic tradition, a wise rabbi once proclaimed that if a person planting a tree were told that the Messiah had arrived, he should finish planting before going to greet him. That of course was long before there was any awareness of the important role trees play in generating oxygen and soaking up carbon dioxide. Neither was there any knowledge about trees preventing soil erosion, absorbing air pollutants, preventing water runoff or controlling climate by moderating the effects of the sun, rain and wind. Nevertheless, the importance of trees was clear. They produced olives, pomegranates, nuts, dates and figs while providing wood for cooking and timber for building.

As time went on trees would prove to be a source of important commodities such as paper, rubber, maple syrup, cork, turpentine, amber, tannins, fragrances (eucalyptus, sandalwood), flavourings (cola, cinnamon, bay leaves, nutmeg) and medicines (camphor, quinine, paclitaxel, salicin). Little wonder that environmentalists who appreciate the value of trees are referred to as “tree-huggers.” But they do need to be careful about which trees they hug, with the manchineel tree being a particularly poor choice. In this case expressing affection can prove to be a blistering experience. Manchineel trees, native to the Caribbean, Central America and Florida, produce a variety of toxins, phorbol being an example, that are not only irritating to the skin but can produce severe illness when ingested. Biting into the tree’s fruit, which resembles an apple, can cause dramatic swelling of the airway and severe gastroenteritis.

Legend has it that some Caribbean tribes would tie prisoners to the trunk of the manchineel tree as a form of torture, subjecting the unfortunate captives to terrible blistering as rain washed the tree’s milky sap over their bodies. It seems that concentrated manchineel sap can even kill! Juan Ponce de Leon, the Spanish explorer who led the first European expedition to Florida, can attest to the lethal effects of the manchineel’s poison. The explorer and his men were attacked by natives as they sought to colonize the land he named after its floral landscape, searching for gold, not for the fountain of youth, as is commonly believed. During the scuffle, Ponce de Leon was struck in the thigh with an arrow that was apparently poisoned with the sap of the manchineel tree. He managed a retreat to Cuba where he died of his wounds.

Some African tribesmen tip their arrows or spears with an extract of the bark of the Acokanthera schimperi tree. Ouabain, the toxic ingredient is a “sodium/potassium ion pump inhibitor,” capable of stopping the heart in its tracks. Even that of an elephant! But the African crested rat is immune to the toxin and actually uses it to its advantage. The rat chews on the bark of the tree and then licks the fur on its back. Any predator that looks on this foot-long rat as a tasty morsel ends up making a fatal error when it grabs its prospective meal. Why the rat is unaffected by ouabain is a mystery, the solution of which may have implications for the treatment of heart disease in humans.

Perhaps the best known tree poison is strychnine, found in the seeds of the species Strychnos nux-vomica, already familiar to the ancient Egyptians. Legend has it that Cleopatra used her servants as guinea pigs, forcing them to swallow the poisonous seeds when she was contemplating her own suicide. Upon witnessing their agonizing death, she decided that the bite of an “asp,” likely an Egyptian cobra, was a better way to go. Some historians however suggest that Cleopatra actually made her exit with a mixture of hemlock, wolfsbane and opium.

While Cleopatra did not end up using a tree poison, Boudica, the warrior queen of the ancient Britonnic Iceni tribe did choose to ingest the poisonous leaves of the yew tree, preferring death to being taken prisoner by the Romans after losing a battle. The English Yew, or Taxus baccata, contains taxane, an alkaloid that can cause cardiovascular collapse by interfering with calcium and sodium channels in heart muscle cells. There’s no way to confirm the story of Boudica’s suicide, but a case report in a 2011 issue of Clinical Medicine does describe a seventeen year-old girl’s attempted suicide with yew leaves. Apparently she changed her mind after swallowing the toxic leaves and sought help in a hospital’s emergency room. After being admitted her heart began to beat very erratically, necessitating defibrillation and the administration of the antiarrhythmic drug, amioderone. After recovery, she admitted being taken by the story of Boudica, encountered in her ancient British history class.

A young man who was trying to carry out a mystical ritual to experience the afterlife through the use of yew leaves was not so lucky. He had read about some ancient practice of consuming yew seeds when Venus and Jupiter came into alignment, allowing the indulger to experience the afterlife and return. We’ll never know what he experienced because he did not return. The coroner’s verdict was cardiac arrest caused by ingesting yew seeds.

While the yew tree’s toxins can take a life, they can also save one. Back in the 1960s, after screening thousands of plants, a compound derived from the bark of the Pacific Yew was found to be effective in the treatment of cancer. However, there was a problem. The isolation of “paclitaxel,” required large amounts of bark and stripping the bark killed the tree. Chemists quickly went to work to find a synthetic method to produce the compound but were stymied until they discovered a precursor in the leaves of the English Yew that could be converted into paclitaxel. Since the collection of fresh clippings did not damage the tree, a supply of paclitaxel for the treatment of cancers of the lung, ovaries, breast, head and neck became available.

So, are the chemicals found in the leaves of the yew species good or evil? Neither. It’s a question of how they are used. Chemicals don’t make moral decisions, but people do. Maybe a bite of the forbidden fruit picked from the tree of knowledge of good and evil in the Garden of Eden wasn’t such a bad idea.

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