Common Greenbrier
Smilax Rotundifolia

The Common Greenbrier is a lily vine that was introduced to the United States. It has green leaves and spikes that are blackened at the tip. Flowering in April-June, the greenbrier thrives in moist/wet soil. It is a climbing plant, and can grow extremely long. Its leaves can grow up to 6 inches, and its flowers reach about 3/4 inches.

Sources:
http://www.npwrc.usgs.gov/resource/1999/soutflor/species/9/smilrotu.htm