Why Are Plants Poisonous

Producing toxic chemical substances is often beneficial to plants, making them less palatable and providing them with protection against plant-eating animals or insects. Milkweeds, for example, produce several types of

A baneberry bush, a perennial herb of the buttercup family (genus Actaea) with poisonous berries.

toxins that render them generally distasteful to foraging animals. A mere taste of the bitter leaves will turn away most would-be browsers, unless they are extremely hungry.

Many toxic compounds are secondary metabolites, which are produced as by-products of a plant's primary physiological processes. In some cases scientists do not yet understand why a particular type of plant or mushroom produces such poisons. Even within a single species, some individuals may have high concentrations of toxic compounds while others have minimal amounts. Over thousands of years, in the process of domesticating plants, we have learned to select and propagate less-toxic strains, and by these means, humans have been able to convert poisonous species into major foods. The common potato (Solanum tuberosum) is a good example; its wild relatives in the South American Andes are bitter and toxic due to intense concentrations of harmful alkaloids. Indigenous horticulturalists over many generations developed sweet and edible varieties of potato and learned how to process them to minimize these toxins. The Spanish introduced potatoes to the rest of Europe some time in the late 1500s, and, after a period of doubt and suspicion, they were adopted as a staple in many countries. Still, the domesticated potato produces harmful alkaloids in its leaves, fruits, and sprouts, and even in its tubers if they are left exposed to light and start to turn green. Many relatives of the potato, including belladonna (Atropa belladonna), black nightshade (Solanum nigrum), henbane (Hyoscyamus niger), and tobacco (Nicotiana spp.), contain these alkaloids and are thus quite poisonous to humans and animals.

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