Growing in the butterfly garden at Merritt Island National Wildlife Refuge; photo was taken March 1, 2014.
There are two different plants growing in this area and I'm not sure which foliage the flower goes with. From a distance I thought the flower was Buttonbush (Cephalanthus occidentalis) but if the leaves with the serrated margins go with the white flower it's not that one, and the bloom doesn't look quite right for Buttonbush either. Any suggestions?

Melanthera nivea ?? for the white flower.

If it was a fall pic, I'd think Spermacoce. Are there some that bloom in spring?

Greene: Thank you, I do believe you nailed it! Snow Squarestem (Melanthera nivea) is a Florida native; it's found all over the state and is a definite match to both leaves and bloom of the plant with the white flower.

Tiffany: False Buttonweed (Spermacoce verticillata) is not native here but is found in many areas of Florida; it reportedly blooms year round but the leaves don't match for the plant in question with the white flower.

I hope to visit the NWR at a time when that other plant is in bloom to try and get an ID on it.

LOL, and I see I've already uploaded photo's to the database. This plant is growing here in our Botanical Gardens and I'd forgotten I'd taken pictures of it there. Snow Squarestem (Melanthera nivea)

That's funny!! It really does catch your eye, doesn't it?

There are over 200 hundred Spermacoce species, I've only seen the one you mentioned. Sorry I wasn't clear that I wasn't suggesting any particular species, just a possible genus. I couldn't tell which leaves were attached to the flower either. No idea what the other species look like, when they bloom, or where they grow.

I found a link (Hawthorn Hill) that gives Snow Squarestem (Melanthera nivea) the common name 'Salt and Pepper".