Arsenius Jigger was already 101 years old. Though he had never written anything as famous as Dumbledore's article on the twelve uses of dragon blood, he was an immensely prolific scholar, and all he had achieved today had been accumulated over the years.
He thought very little of Snape's title of "youngest Slytherin Head of House." Since Slughorn had not properly taught Snape, Jigger intended to make up for it. Thus, the youngest Potions master was sent running around London like an apprentice to buy Potions ingredients for Jigger.
Most plant lice fed primarily on plant sap, but for some unknown reason, the ones that attacked rowan trees began sucking blood. Humans and animals bitten by them became stiff and could even lose the ability to move for a time.
It was not a deadly illness, but it was extremely irritating. Some of the gatherers in the Forbidden Forest had already been bitten. Fortunately, they had not encountered any Acromantulas while paralyzed.
Rowan was one of the few wand woods capable of resisting plant lice, because it was naturally more defensive than other woods and exceptionally powerful for casting protective charms. Moreover, rowan wood did not become associated with the Dark Arts.
It was like the Unicorn among trees. If a Unicorn had not died, its blood could be used to make Felix Felicis. If it had died, then its blood became the most powerful curse.
An old superstition about wand woods said, "Rowan chatters, chestnut loafs, ash is stubborn, hazel is coy."
That did not mean rowan wood could truly speak, or chestnut wood was truly lazy.
In any case, Pomona had never heard a plant speak, apart from those that naturally made sounds—and they did not understand the meaning of what they said.
"Have you bought everything?" Pomona