"It would be a mistake to imagine that the poppies in Great War writings get there just because they are actually there in the French and Belgian fields.
"Flanders fields are actually as dramatically profuse in bright blue cornflowers as in scarlet poppies. But blue cornflowers have no connection with English pastoral elegiac tradition, and won't do.
...Poppies had accumulated a ripe tradition of symbolism in English writing, where they had been a staple since Chaucer.
"The same principle [of literary selection––as opposed to documentary or photography] determines that of all the birds visible and audible in France, only larks and nightingales shall be selected to be remembered and 'used.'
"One notices and remembers what one has been 'coded'––usually by literature or its popular equivalent––to notice and remember.
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"(Sometimes it is really hard to shake off the conviction that this war has been written by someone.)"
--The Great War and Modern Memory, by Paul Fussell, 1975, Oxford University Press, about literature and the First World War
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Googled "chaucer and flowers". Found this.