Living with poisonous plants
Plants cannot run, fight or hide, so they make themselves inedible with poisons, otherwise they would get completely eaten up and become extinct. Animals that live on plants have some degree of resistance to plant poisons, but humans mostly lack such resistance. Salad plants have been selected and grown for their lack of poisons but such plants cannot survive on their own without human protection.
Some plant poisons act by contact. The most notorious are those of the poison ivy family which cause irritation in contact with bare skin. People learn to avoid touching these plants after one experience.
The most common plant poisons are tannins. Tannins react with proteins and are used to convert the perishable protein in animal skins into tough durable leather. Tannin will similarly react with the skin of the mouth and throat. Plants that contain tannins in large amounts are impossible to eat. Just try raw bananas or raw persimmons! When fruits ripen, their tannin content is deactivated to allow animals to eat the fruits and disperse the seeds. In dilute amounts, tannin is what gives taste to beer, wine and tea.
The next most common poison is calcium oxalate in the form of sharp microscopic crystals in the plant cells. When these cells are broken in the mouth, the crystals embed themselves in skin of the mouth and throat and cause swelling. The aptly named ‘dumb cane’ and other members of its family such as the popular ‘money plant’, as well as the ‘ZZ plant’ Zamiocalcus zamiifolia (marketed as a Chinese good luck plant), contain such crystals. Kids that put a leaf containing oxalate crystals in their mouths will learn never to put unknown plants into their mouths ever again, but it may be hours before the pain and swelling passes.
Then there are the plants that contain cyanide compounds, especially those in the tapioca (cassava) family that includes the ‘sweet leaf’ (sayur manis or Sabah vegetable). These poisons are totally inactivated by cooking. There was a case in Taiwan of death from eating raw ‘sweet leaf’, that caused a panic and a sharp drop in consumption of this popular vegetable when reported in the press. The person who consumed the raw leaves had done so deliberately in the belief that it would help her lose weight.
Finally there are the plants that contain alkaloids and other chemicals that can cause convulsions, breathing problems, nervous disorders, hallucinations or kidney failure. These plants are used to kill e.g. to tip arrows and darts for hunting and warfare, for execution (e.g. the famous case of Socrates who was executed with a drink of hemlock juice), for murder (when disguised as food) or suicide. Also to kill pain and give pleasure (e.g. opium). Many of these plants are used as medicines in small dosages.
Cases of accidental fatal poisoning by eating poisonous plants are extremely rare. The one case that everybody quotes is the deadly nightshade in Europe that has juicy delicious-looking but poisonous fruits. Because tomatoes resemble the deadly nightshade and belong to the same family, it took a very long time before Europeans would accept tomatoes as food after their introduction from the Americas.
In brief, fatal poisoning by accidental consumption of poisonous plants is almost impossible because poisonous plants are so unpleasant to eat and the human mouth is so sensitive to unpalatable substances. All the worry about poisonous plants is unnecessary.
Posted by Dr Francis Ng at 6:31 AM
