Beautiful white flower petals from the beautiful white flower.
The flower petals are found in front of the Elderly Tree in the garden on the 5th floor after you have killed the white flower. You need them to be able to go into the dark maze. Since the white flower's petals glow when water touches them (as said in The Talkative Plants book), a bottle full of water and the petals become a light for you in the dark maze.
To obtain them you need to kill the white flower. How to kill her you need to talk to the red grasses. They will tell you how, but remember, the red grasses are always lying. Once you have killed the white flower she will drop a few flower petals on the ground that Viola can pick up.
Once you have the flower petals you put them in the bottle you found in the poisonous room and fill it with water from the water chamber. Now you have a glowing bottle you can use to enter the dark maze.
In Extra mode, the flower petals don't exist. There is no dark maze to enter so you don't need the flower petals. The petals were simply burned away by the poison you poured on the white flower.