Understanding Toxic Chemicals that Naturally Occur in Plants
There are many commonly grown plants that are quite poisonous. Some, such as Nerium oleander are quite well known as being poisonous, while others, such as Narcissus pseudonarcissus (daffodil), many people may not realise that they too, have poisonous parts. Some are poisonous to only some people, possibly causing a skin rash or some other allergic reaction in that person. In some plants, it is only a certain part of them which is poisonous, for instance rhubarb stem can be eaten, but the leaves are quite toxic (obviously it is best to know what a plant is, and what parts of it can be used, before taking it internally). This is just meant as a caution ‑ it is only a small group of plants, that even following ingestion of a small amount of material, results in serious poisoning.
Plants may be considered to be poisonous, but the actual toxicity of the plant can depend on locality, age of plant and various environmental factors.
Sorbus aucuparia (rowan tree) produces small scarlet fruit, which are often made into jams and jellies. In past times they were used for various remedies such as for an anti‑diarrheic and they were also used as a source of vitamin C. But on the other hand, the fresh fruits are considered to be poisonous (eating them only produces mild symptoms).
Tanacetum vulgare (tansy) can be used in cooking as a flavoring, or made into a tea which reputedly has medicinal properties. The younger leaves are always used, but if taken in too large a quantity, the tea can supposedly act as an irritant narcotic and therefore be quite toxic.
Many plant poisons are extremely active substances, in suitable doses they are important medicines, though in ancient times they were often used for committing murder! Some plants contain poisons that are not found in other plants. The most important plant substances that act as poisons are as follows...
Essential oils. They are often used as scents and as constituents of spices and medicinal herbs. Such oils are well taken up by the skin and mucous membranes and after being absorbed they act as general cell poisons. For example pulegone, the oil from Mentha pulegium (pennyroyal), can be fatal when the essential oil of the plant is misused as an abortifacient.
Alkaloids are basic substances, widely distributed in the plant kingdom. There are numerous types of alkaloids, such as Pyridine/Piperidine and steroidal alkaloids.
Amino acids. While amino acids are essential for synthesizing body proteins, there are a large number of non‑protein amino acids derived from plants, and some of these have toxic properties. Some of these are responsible for poisoning grazing animals, such as indospicine, which is present in Indigophera spp.
Cyanogenic glycosides. These are one of the most toxic substances to man. It occurs in the seed of some Rosaceae genera and in linseed (in the normal use of linseed as a laxative it is not toxic).
Cardioactive glycosides are present in a large group of plants, including Digitalis sp. (foxgloves) and Euonymus europaeus (spindlebush). These plants usually taste extremely bitter and often cause vomiting; poisoning is therefore rare.
Furanocoumarins can cause injury and blistering to the skin. Plant acids can be non‑toxic and add to the flavour of the fruit etc., though others such as oxalic acid can be toxic, disrupting the blood clotting mechanism.
Saponins have long been known as fish poisons. Some saponins irritate the skin and the mucous membranes, and are known to be the toxic constituents of several poisonous plants including Hedera sp. (Ivy).
As can be seen from this list, there are many substances responsible for the toxic effects of plants and caution should be taken at all times.
How to Avoid Toxic Plants
You need to learn how to identify plants better -this is the first step!
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