The opium poppy is Papaver somniferum, and it is the most consequential medicinal plant in human history.
The latex bled from the scored unripe seed capsule contains a family of isoquinoline alkaloids, principally morphine at around ten to sixteen percent of dried opium, with codeine, thebaine, papaverine and noscapine. Morphine was isolated by Friedrich Serturner in the early nineteenth century and named for Morpheus; it was the first alkaloid ever isolated from a plant, and it founded the discipline of alkaloid chemistry. It remains a reference standard for analgesia. Heroin is diacetylmorphine, synthesised from morphine and marketed by Bayer from 1898 as a supposedly non-addictive cough suppressant, which it was not.
The seeds are a different matter and are ordinary food: poppy seed contains no meaningful alkaloid content of its own, though it carries surface contamination sufficient to produce positive opiate results on drug tests, which is a documented and litigated phenomenon.
Culpeper gives the poppy to the Moon, on its narcotic and sleep-inducing action, the Moon governing sleep and dreams in his scheme. This is one of the very few attributions in the Complete Herbal where the reasoning starts from an effect that is real and reproducible rather than from an appearance. He describes syrup of poppy heads for sleep, cough and pain, and warns of its cold and stupefying force. Every element of that description is pharmacologically accurate. It is the best-founded entry in the book, and it is worth noting that it is best-founded precisely where the plant's action is strong enough to be unmistakable without any theory at all.