I didn’t use the frame narrative for the Petamerone, because its racist, and not just “Well, in their time..” racist, but full on speaking-in-pidgin caricature racist. In the story the main character, our heroine, is a princess who cannot laugh of smile. In the list of things that cannot make her grin it includes the “sardonic herb” and I thought “Hey, what’s this?” assuming it was a recreational stimulant.
It turns out that the Sardonic Herb is from Sardinia and it causes a sardonic grin: a sort of facial rictus similar to that caused by strychnine poisoning. Its modern name is probably hemlock water-dropwort (Oenanthe crocata). Its a neurotoxin used in pre-Roman Sardina as part of a euthanasia ritual for the elderly.
The story of the Flea doesn’t have a lot of additional material from the modern version. The flea becomes gigantic because it is fed on the blood of the king. The giant that floods the area with soapy water when he washes his hands is actually spitting the water, so presumably its a sort of sticky mucus.
Cenetrolla’s story has one major change. Under the guidance of her tutor, who will eventually become her evil stepmother, the heroine kills her previous stepmother. She gets her to lean into a wooden chest and slams the lid on the back of her head. Why we don’t care about this is absolutely unclear to me. You’ll note the fairies again come from Sardinia.
The merchant story has been cut down by about a third, which explains why so many parts of it seem to go nowhere. When the dragon’s head is knocked off, it rubs a herb on its wounds and reattaches the head. After the hero marries the princess, he becomes enthralled by a woman he can see from his window. She has a magical power that she can bind men with her hair. Why she would want to do this is not explained, but when the man nips over the roads to try his luck he vanishes.
The main character has a twin brother, who goes looking for his sibling after seeing magical signs he’s in trouble. He finds the man with the treasure in the well, and the faerie the brother saved from ravishment, and they each mistake him for his sibling. This lets him know he’s on the right track, and eventually he finds his brother’s wife. He pretends to be his brother, to prevent people knowing he is missing. She thinks he’s been off with the woman across the road, particularly when the brother refuses to sleep with her except under “divided sheets”.
The twin follows the lead, finds his brother enchanted, and uses two hairs from his magical dog to break the binding. Then they slay the lady with the magical hair. The twin explains what was going on, and when he gets to the bit where he slept in the same bed as his brother’s wife, the brother slashes his head off “like a cucumber” with his Spanish sword. The princess sees this through her flirty neighbour’s window, and rushes across, to tell her husband he’s an idiot. He recalls the dragon’s herb, rubs it on his brother’s neck, and reattaches his head. The brothers reconcile surprisingly quickly.
Games From Folktales 