As well as the dandelion clocks, grass blade whistles and buttercup "test" mentioned by Metchosin, my friends and I used to make snapdragons have "conversations" by squeezing them so that their "jaws" moved up and down. We also used to put any bell-shaped flowers upturned over our fingers to play:
"Two little dickie birds sitting on a wall,
One named Peter, one named Paul.
Fly away, Peter; fly away, Paul.
Come back, Peter; come back, Paul."

(on "fly away..." you put your hands behind your back and bring them out again with different fingers visible, so that it looks as though the birds have gone, then on "come back", your hands go behind your back again and return with the fingers which have the "birds" on them).

It is sad to think that some of these games are dying out. (Though I was glad to hear children at the school where I am currently working saying "Zero the hero, first the worst, second the best, third the one with the hairy chest, etc. etc." when queuing up. - Not flower/plant-related, I know. Sorry for thread drift).

- jeanie