There are 26 species of Desmodium in Florida. Wunderlin (1998) has a key
http://www.regionalconservatio...
The information on that site is for south Florida, but there are still 26 species of Desmodium in Florida.
23 species are listed on the Florida plant atlas, that should give you a clue as to which species you can expect to find in your location. Having looked at some they are close enough to leave room for doubt and as we know, plants are often found in wider locations than given.
http://florida.plantatlas.usf....
Elimination can be a good way to do it, that requires searching all the species listed which might give clues. By doing that you can often find keys which may or may not help.
I searched Desmodium cuspidatum because the stem shot appears to show bracts which 'cusp' the stem although it's not clear, missouriplants has some clues. If it doesn't fit, eliminate. Flowers would be a great help!
http://www.missouriplants.com/...
By using the 'back' button you will find more species to check out on the missouriplants site, don't forget to use the 'next' button.
http://www.missouriplants.com/...
You can often find a book which is referenced on Google books, at least parts of it which can be useful. I found the Guide to Vascular Plants of Central Florida by Wunderlin which might be of use as many plants will be common to other part of Florida.
http://books.google.co.uk/book...
I found a link to a book with keys for the south -eastern United States, page 634 mentions Desmodium in the links at the side but the keys don't appear to have it! The keys might be useful for other plants though ..
http://eol.org/pages/639561/li...
http://eol.org/pages/639561/li...
http://www.biodiversitylibrary...
I found a key for Desmodium of North Carolina! They appear to include the same species ..
http://www.cals.ncsu.edu/plant...
