The root of the mandrake, or mandragora, often divided in two and looking vaguely like a human shape.

Go and catch a falling star,
Get with child a mandrake root,
Tell me where all past years are,
Or who cleft the devil's foot,
Teach me to hear mermaids' singing,
Or to keep off envy's stinging,
And find
What wind
Serves to advance an honest mind.
- John Donne

In ancient times, those parts of the roots resembling man were cut out of the root, and had various magical effects ascribed to them. It was believed that they produced fecundity in women (this belief is even mentioned in the Bible in Genesis 30 14-16). For this reason, one of Venus' names is Mandragoritis.

It was also thought that mandrakes could not be uprooted without fatal consequences to the person involved, so a cord would be attached to a root and then tied around a dog's neck. When chased, the dog would then proceed to draw out the mandrake - and supposedly perish as a consequence of this.

In some areas people believed that a small dose of mandrake would make a person very vain and enamoured of his own beauty; whereas a larger dose would make him an idiot.

As for the scream which mandrakes were supposed to let out when uprooted, it was said that a mandrake root was a living being, some kind of semi-human, "engendered under the earth of the seed of some dead person put to death for murder".

Worsworth dictionary of Phrase and Fable
Thomas Newton: Herball to the Bible

Thanks due to dutchess for helping me out with the song.