In Britain the poppy, but in France the bluet (cornflower). The flowers are used as symbols of remembering the dead of the First World War, and raising much needed funds for the support of ex-servicemen and their families.
In the UK there is huge pressure placed on people to wear a poppy, and the date on which they appear is getting earlier. When we arrived in the UK 12 years ago, poppies came on sale the week of Remembrance Day. Five years ago they arrived on the 1st of November, and this year they arrived on the 26th of October. Anyone appearing on TV in the UK not wearing a poppy gets accused of dishonouring the dead, and there are letters to the newspapers (I would guess from people who can't be bothered actually going to a Remembrance Day ceremony) in incandescent outrage.
In France, little stickers with cornflowers on them are sold at the start of Remembrance Day ceremonies, and in Preuilly sur Claise half the town seems to attend.
The difference between the two countries is greater than simply that of language.
A Floral Spring Treat - The Chateau of Cheverny is surrounded by an extensive park and in the autumn they plant 60 000 tulip bulbs, ready to put on a stupendous show in the spring...