A plain seed is the result of a used Reviver Seed in the Pokémon Mystery Dungeon series. When a Pokémon faints the Reviver Seed can be used. When the Reviver Seed revives the pokemon, the Reviver Seed turns into a plain seed. Plain seeds are pretty useless, as they only restore 5 belly and are worth one poke in resale.
Stub!
Please help the Pokémon Wiki by adding more info.