The Doctrine of Signatures states that each plant is marked with a sign to indicate its place in grand scheme of life. Although the actual Doctrine of Signatures is usually credited to a German named Jakob Böhme (1575-1624), author of Signatura Rerum (Signature of all things), many civilisations and cultures have had similar beliefs. Although in Böhme's time this grand scheme of life was God's great plan, in more recent times people have subsituted whatever higher being or power they choose to believe in.
Even in Europe the idea was by no means new, (the early 16th century alchemist, herbalist and physician Paracelsus had similar beliefs for example, teaching that "equal cures equal"), but Böhme is the one who popularised the idea. As a result the doctrine was endorsed by people such as Culpepper. It is however worth noting that Mr Böhme was not a herbalist or a botanist, but a shoemaker (probably a good one if he had the capital to have a book produced). His book was based on his hallucinations and visions.
Pretty flower, miraculous cure or deadly poison?
Plants were then (and still are today) a valuable source of medicinal compounds. The difficulty is of course working out which plant will kill you dead, which one will cure your warts and which one will help with your insomnia.
It follows from the doctrine that, in order to facilitate the task of herbalists and healers, our ever benevolent creator will have placed clues as to which plants will help with which ailments.
For example, skullcap, a traditional headache remedy, has seeds that look like miniature skulls. Because Hepatica leaves turns liver coloured in winter, and apparently look vaguely like chicken livers if you look at them really hard they must be good for the liver. And just about anything red will help with blood disorders.
Malarkey?
Although this superstition is alive and kicking in certain circles, it is obviously not one upheld by modern medicine. It is true that there are plants which have traditionally been used as remedies (and have subsequently been shown to contain interesting substances), that agree with the doctrine. It is equally true that there are countless examples of plants that have no effect whatsoever, despite parts of them looking like various organs or presenting "signs" that they will help with certain disorders.
Whether a certain feature of a plant looks like a certain body part or suggests it might cure a certain ailment is rather open to interpretation and as it is often the case with such things, it is easy to see signs and clues if you want to see them hard enough. The doctrine of signatures had something of a cult following, herbalists deluded themselves into finding ways in which such a plant obviously bore signs that it would cure a certain disease. There were also quacks using the doctrine as "proof" that their cures would be effective.
All your bodily fluids are belong to the Moon
The doctrine of signatures is also mixed in with a variety of beliefs such as astrology. Culpepper assigned both plants and diseases to various planets, according to the "personalities" of these planets. To treat a disease caused by a certain planet, you would need a plant from an opposing planet. If it looked like an appropriate organ then all the better. Others also assigned body parts to planets, for example Mars is said to be in charge of the gallbladder, muscles, reproductive organs and adrenal glands.
If you believe in a benevolent creator, the doctrine of signatures is not an unattractive idea, that somehow your creator is trying to give you pointers. Even without believing in a god, the idea that there is some of logic or higher purpose behind the design of plants may be what enables the doctrine of signatures to still be popular in the right circles.
Sources:
http://www.paghat.com/doctrine.html
http://www.ubcbotanicalgarden.org/education/doctrine.php
http://www.mounet.com/~jdye/doctrine.html
http://www.hole.gs.rl.no/engl/garden/medisinplanterE.htm
http://www.users.globalnet.co.uk/~rxv/sebpc/signature.htm
http://www.b-and-t-world-seeds.com/signs.htm