In many cases, the poisonous substances (PLANT TOXINS) are merely by-products or wastes from the essential functions of the plant. Often, toxic compounds- or at least stuff that is unpleasant to taste or smell- have evolved in plants to discourage or repel vegetarian insects or grazing animals. Plant toxins don't bother the plant that produces them, and often, they don't bother many other living things- but it only takes one species, you see...
Ironically, many toxic compounds from plants, such as MORPHINE FROM POPPIES or DIGITALIS FROM FOXGLOVES, have great medicinal value, even though overdoses can kill.
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