View photos of the medicinal plant Tribulus terrestris (Puncturevine), profiled in Medicinal Plants of the American Southwest.

A passage from Medicine Plants of the American Southwest:

Puncturevine is known to alter/augment circulating androgens, particularly DHEA (dehydroepiandrosterone) and testosterone within the body. Although the mechanism of action is still unclear, its effects are noticeable: libido is increased, and in men, erectile response is heightened, as is the quantity and quality of sperm. Nocturnal emissions or spermatorrhea lessens under its use. Many men using the plant (it combines well with Ginseng) often notice a related sense of increased physical strength and will – a good tonic for older men and the metrosexual alike. Women too may notice an increase in libido.