First I've heard of it.
As a small boy I don't remember the poppy having a leaf on it.
The poppy was decided upon as a symbol for the fallen of The Great War (the war to end all wars) as it was the hardiest flower to recover quickly after and during the conflict.
During the following years the act of rememberance was performed on the 11th of November, but then the Second Great War broke out and the act of rememberance was moved to the nearest Sunday to respect their memory.
Personally I agree with David Niven in the film "The Dawn Patrol" (1938) when he said "the poppy is for the dead and dying of all wars, and all future wars."