A plant or mushroom is considered poisonous or toxic if the whole organism, or any part of it, contains potentially harmful substances in high enough concentrations to cause illness or irritation if touched or swallowed. From the waxen-leaved dieffenbachia in your living room to the delicate foxglove blooming in your garden to the shoots sprouting from a forgotten potato in your refrigerator, poisonous plants are a common part of our lives. Since it is neither desirable nor practical to eliminate poisonous plants from our surroundings, we need instead to educate ourselves about their potential dangers. At the same time we need to understand that, like all plants, poisonous species have important ecological roles and many of them are also useful to us as medicines or for other purposes.

Some plants and mushrooms are extremely toxic and can quickly cause coma or death if consumed. Others, though...