What harm could come to you when eating a close relative of carrots, parsley, and celery? As it turns out, quite a bit. A certain relative of the carrot can kill you, and if it doesn’t, it still permanently darkens your skin.
Bishop’s weed started out in Asia and east Africa, but has made its way around the world because it’s both hardy and decorative. It grows long, light green leaves, and long, light green stems which end in pretty, lacy clusters of small white flowers. It’s a relative of carrots, parsley, dill, fennel, and chervil, and you might be okay with plunking it, or its seeds, in a stew and having them for dinner. The next morning, though, you probably won’t feel as well.
The plant contains compounds called psoralens, which are meant to help it deal with harsh sunlight and react with UV light. They keep this trait when they’re transplanted from bishop's weed to the human body. This is an unnerving instance of real life going fantastical. If you ran across a reference in a novel about how eating a certain plant can help the hero gain the traits of the plant, you might roll your eyes, but it's hard to roll your eyes at reality. The process of gaining these traits is far more unpleasant than it would be in most novels. The plant is phototoxic, and unleashes its unpleasantness in the presence of sunlight. Often this means terrible rashes and blistering when the eater is exposed to sunlight for any length of time. Enough exposure to light and it becomes carcinogenic. But it does have one interesting effect — when a person who has eaten bishop's weed exposes their skin to sunlight, it darkens dramatically. Those who take it often, or in high enough doses, find that their skin turns permanently darker.
This was a good prospect for some, despite the chance of terrible rashes. Bishop's weed has been used since ancient times to treat vitiligo – the localized whitening of patches of skin. Psoralens is even used in drugs today, although the doses are measured and often the chemical is manufactured. Mix a careful dose of the psoralens with a careful exposure to UV light, and unusually lightened skin darkens. A darker skin, in this case, doesn’t always make its bearer any less sensitive to sunlight. Patients often need to wear sunblock and limit their exposure to the sun in order to avoid easy blistering and burning.