This herb is a traditional Chinese remedy used as an aphrodisiac and tonic for reducing blood pressure and treating male impotence, premature ejaculation, rheumatism, and weakness and/or pain in the lower back and knees. Research indicates that this herb dilates the capillaries, thereby increasing blood flow to the extremities (and sexual organs), while simultaneously reducing blood pressure. This herb is reported to prevent impotence caused by the stress hormone cortisol, elevate sperm count, and increase testosterone levels. Animal studies show that this herb significantly increases the frequency of copulation (sexual intercourse). Likewise, a study involving patients with chronic renal (kidney) failure receiving regular hemodialysis found that this herb provided a sexual potentiation effect and improved quality of life. What is this herb?