In the maze room, in the path where you build a key, the leave is in the bottom left of the screen. Also in the maze room where there is a glass case and numbers (the left most path?) the leaf is under the cushion.
In the number puzzle room the leaf is right there when the door opens after solving the puzzle. After that on the gate, where you use colors and symbols on them (blue yellow purple red) to enter the symbols on both sides of the gate, the leaf is somewhere on the left half of the screen I think...if I recall correctly.