Whisperweed
ひそひ草 / ひそひそう [hisohisou] in Japanese. It is a mash-up of the words ひそひそ [hisohiso] or 'whisper' and 草 [sou] or 'grass'.
Final Fantasy IV
Obtain: from Gilbart in Troia
Use: triggers after the first fight with the Dark Elf
Final Fantasy V
Obtain: from Xezat in Barrier Tower
Use: used to communicate between party and Xezat in Barrier Tower
Grimoire of the Rift

Buy: - (sell: 40 gil)
Use: Druid Mace
Treasure: It's the Thought (once)
Type: Flora, Rank: —
Description: Strange tubes in the stalk of this plant can carry the faintest noise across long distances. Increasingly rare of late.
Category: Item