meadow, field and preposition

Hello,

I´ve learned I´ve to say: I sit on the chair/on the floor/on the grass (English Grammar in Use), but what about:

I sit on a meadow?

I sit in a field???

or: There grow beautiful flowers on/in a meadow.???

Therer grow potatoes in/on the field.???

Is there any difference between British and American English in this case?

And what about the singular and plural concerning field.

I have this sentence: My mother was/worked in the fields. Could I also use singular here?