As a child, I remember that we wore little paper poppies on our lapels on Memorial Day and this is why. After World War I the poppies grew in profusion over the battlefields and cemeteries in Flanders, thus the poppies became a symbol of remembrance.
Below is one of those poems that Grams had to memorize as an elementary school student in the 1960's. I don't think they make kids memorize and recite any more and it's kind of a shame. I can't recite it all any more, but it's message is so powerful that it has stayed with me all these years.
In Flanders Fields by John McCrae
In Flanders fields the poppies blow
Between the crosses, row on row
That mark our place; and in the sky
The larks, still bravely singing, fly
Scarce heard amid the guns below.
We are the Dead. Short days ago
We lived, felt dawn, saw sunset glow,
Loved and were loved, and now we lie
In Flanders fields.
Take up our quarrel with the foe;
To you from failing hands we throw
The torch; be yours to hold it high.
If ye break faith with us who die
We shall not sleep, though poppies grow
In Flanders fields.