I can't find any Solanum with this leaf shape though Hetty.. there are some with white flowers though.
I found a herbarium specimen of Sicyos angulatus, enlarge the photo then use a magnifier. You can see some spines on the underneath side of a leaf, they probably have random spines..
http://florida.plantatlas.usf....
Solanum capsicoides is in Florida, and has white flowers. I found one photo with leaves which are similar to yours although they look more hairy, but other photos show them to be different!
http://en.wikipedia.org/wiki/F...
http://www.google.co.uk/search...
Reading on wiki, they say it's included in Solanum aculeatissimum, there we might have a match!
http://www.ukrop.info/html/top...
If it is that, it's an introduced plant..
http://plants.usda.gov/java/pr...
It has a very spiny stem..
http://www.zimbabweflora.co.zw...
http://two-ton.blogspot.com/20...
Those in Brazil have a different shaped leaf..
http://sites.google.com/site/f...
I went through the list of those in Florida on the USDA plants site, the only other one coming close is Solanum viarum but it didn't quite look right.
