Ask Mr. Smarty Plants

The USDA Plants Database reports seven species of Merremia that occur in North America.  There are four native species and three introduced ones that have been naturalized.  Merremia quinquefolia, with white flowers, is native to Florida, the West Indies and Central and South America. Merremia dissecta (Alamo vine) occurs in Texas, the southeastern states, and Pennsylvania.  Its flowers are white with dark red centers.  It does have five-fingered leaves like M. quinquefolia, but the leaves don't have smooth edges.  There is, however, a variety, Merremia dissecta var. edentata, with leaves with smooth edges.  Merremia cissoides is native to Florida and also has white flowers with a dark red center. Merremia umbellata is native to Florida and has yellow flowers.  Merremia aegyptia, a South African native also has white flowers.  Merremia tuberosa is yellow and a native of Central and South America. Merremia gemella is an Asian native and is also yellow.  You can see photos of Merremia gemella here with photos of still more species of Merremia, most of which have not been naturalized in the US.  Of these seven, your plant could possibly be a Merremia dissecta var. edentata with a flower color mutation.

However, I think the best bet for your Merremia-like flower is another member of the Family Convolvulaceae (Morning Glory Family)—Ipomoea cairica (Sweet mile a minute vine). Here are more photos and information from the University of Queensland, Australia.  Its origins are probably Africa and/or Asia, but it is naturalized in the US as well as other countries (e.g., Australia).  Here is the distribution map for the US.  It has pink or lavender flowers with a deeper pink throat and its leaves are similarly shaped to those of M. quinquefolia. 

If this is not the plant that you have seen and you have photos, please visit our Plant Identification page to find links to plant identification forums that accept photos for identification.