Achillea millefolium

Yarrow (Achillea millefoium) rules the blood. Look at the slight pinkish tinge on these flowers from our garden - sometimes you'll find one that is blood-red amongst its more typical white cousins.
The name Achillea comes from the legend that Achilles, hero of the Trojan war, used it to heal the wounds of his soldiers in battle. Indeed, it is probably the best vulnerary we know, serving to instantly stop bleeding and to disinfect even a horribly deep wound. It is always a good idea to carry Yarrow if you are out hiking or far from home.
It has a particular power to curb inflammation and pain in the urinary system, and is used for infections of the prostate or the urinary tract to this end (and also because of its antiseptic and diuretic actions).
As a remedy for fever, it combines especially well with Elder flowers in the form of a hot infusion, take often, until the fever subsides.
Traditionally, Yarrow has a famous history as a plant with the power to unlock the psychic mind. It was eaten or placed under the pillow, at first bloom, for prophetic dreams. The flower stalks are dried and used to cast the I-Ching, the Chinese oracle called The Book of Changes. Perhaps this power is related to its ability to loosen, or unblock, internal stagnations (it is especially good for stagnant blood from old wounds, often colored purple under the skin) in the whole body/mind: as we know, our psychic powers are merely forgotten or repressed.