The Collaborative International Dictionary of English v.0.48:

Sundrops \Sun"drops`\, n. [Sun + drop.] (Bot.)
   Any one of the several species of Kneiffia, esp. Kneiffia
   fruticosa (syn. Oenothera fruticosa), of the
   Evening-primrose family, having flowers that open by
   daylight.
   [Webster 1913 Suppl.]