Perezia nana.

Botanical name: 

Perezia (Acourtia) nana, Gray (natural size). - 1. Corolla. 2. Stamens. 3. Akene (magnified). 4. Floret (natural size).

This image is from the article "On the Presence of Pipitzahoic Acid in the Perezias ..." in the American Journal of Pharmacy, 1884.

Perezia nana.