Gillyweed

• Type: Magical Plant
• Owners: Primarily found in the private stores of potioneers and Herbology experts. Known users include Harry Potter. It was part of Severus Snape's private ingredient stores at Hogwarts.
• Origin: A magical plant native to the Mediterranean Sea.

Gillyweed is a magical plant that, when consumed, allows a witch or wizard to breathe underwater. In its raw form, it is described by Harry Potter as a “slimy, grayish-green bundle of what looked like rats' tails.” It is noted to be unpleasant to eat, having a rubbery texture.

The primary property of Gillyweed is to induce a temporary, partial Transfiguration in the consumer. Upon ingestion, the user develops functional gills in their neck, allowing them to process oxygen directly from water. Simultaneously, webbing grows between their fingers and toes, which aids in swimming with greatly enhanced speed and agility. The effects are not instantaneous but develop over a few moments. The duration of the magic is approximately one hour, after which the physical changes revert, and the user's normal human physiology is restored. To use it, one must simply chew and swallow the plant. It is effective in both freshwater and saltwater environments, as demonstrated by Harry's use of it in the Black Lake.

According to the book Magical Water-Plants of the Mediterranean, Gillyweed was first discovered by the wizard Beaumont Marjoribanks. However, supplementary materials offer differing accounts of who first perfected its use. The witch Elladora Ketteridge is credited with discovering its properties after she nearly suffocated from eating it and only survived by sticking her head in a bucket of water (Pottermore). A Chocolate Frog Card also names Sacharissa Tugwood as the discoverer (video game). In the 1994-1995 school year, Gillyweed was a crucial element in the Triwizard Tournament. Severus Snape accused Harry Potter of stealing Gillyweed and other ingredients from his private stores to brew a potion for the Second Task. In reality, the Gillyweed was stolen by Dobby the house-elf. Dobby acted on information overheard from Barty Crouch Jr., who was disguised as Alastor "Mad-Eye" Moody, as part of a complex plan to aid Harry. Crouch Jr.'s initial strategy was for Neville Longbottom, an avid Herbologist, to suggest Gillyweed to Harry after being given the relevant book. When Neville proved too shy to offer the advice, Crouch Jr. pivoted, ensuring Dobby would give the plant to Harry just moments before the task began.

Gillyweed's most significant role is as the plot device that enables Harry Potter to survive underwater for an hour during the Second Task of the Triwizard Tournament in Harry Potter and the Goblet of Fire. It allowed him to navigate the Black Lake, evade Grindylows, and ultimately rescue both Ron Weasley and Gabrielle Delacour from the merpeople's village. The subplot surrounding the acquisition of Gillyweed serves to highlight several key character traits and plot points:

• Severus Snape's Prejudice: It reinforces his unwavering suspicion of Harry and his willingness to believe the worst of him.
• Dobby's Loyalty: It provides a critical moment for Dobby to demonstrate his fierce devotion to protecting Harry, acting in what he believed to be Harry's best interest.
• Neville Longbottom's Knowledge: It showcases Neville's expertise in Herbology while also emphasizing his lack of self-confidence at the time.
• Barty Crouch Jr.'s Cunning: It reveals the intricate and manipulative nature of his plan to guide Harry through the tournament and deliver him to Lord Voldemort.

• In the film adaptation of Harry Potter and the Goblet of Fire, Harry's transformation after eating Gillyweed is portrayed as more dramatic and painful. He writhes as large, fin-like webbing erupts from his hands and feet, and the gills tear open on the sides of his neck. (film)
• In the Harry Potter and the Goblet of Fire video game, the player must acquire Gillyweed from the Hogwarts greenhouses before beginning the Second Task level. (video game)
• According to Wizarding World, Herbologists have long sought a method to make the effects of Gillyweed permanent, but so far have been unsuccessful. (Pottermore)
• The name “Gillyweed” is a straightforward combination of “gill,” the respiratory organ of most aquatic animals, and “weed,” a common term for a wild or uncultivated plant.