Wikipedia has this to say about the Doctrine of Signatures.
Jude's Herbal Home Remedies (Jude C. Williams, MH) has a section that talks about plants with signatures. I have included some of it below for your benefit"The doctrine of signatures is a philosophy shared by herbalists from the time of Dioscurides and Galen. This doctrine states that herbs that resemble various parts of the body can be used to treat ailments of that part of the body. Although the doctrine of signatures was formalized in early modern times, the theme of natural objects' shapes having significance is a very old one and is not confined to Western thought. Examples include the plants liverwort; snakeroot, an antidote for snake venom; lungwort; bloodroot; toothwort; and wormwood, to expel intestinal parasites. The occasional resemblance of mandrake root to a human body has led to its being ascribed great significance (and supernatural powers) since ancient times and in many places. The 17th-century botanist and herbalist William Coles (1626–1662), author of The Art of Simpling and Adam in Eden, stated that walnuts were good for curing head ailments because in his opinion, "they Have the perfect Signatures of the Head". Regarding Hypericum, he wrote, "The little holes whereof the leaves of Saint Johns wort are full, doe resemble all the pores of the skin and therefore it is profitable for all hurts and wounds that can happen thereunto." Nicholas Culpeper's often-reprinted Complete Herbal takes the doctrine of signatures as common knowledge, and its influence can still be detected in modern herbal lore.
 Some -wort plants and their signatures
- Lousewort, Pedicularis - thought to be useful in repelling lice
- Spleenwort, Asplenium - thought to be useful in treating the spleen
- Liverwort, Marchantiophyta - thought to be useful in treating the liver
- Toothwort, Dentaria - thought to be useful in treating tooth ailments
- Hedge woundwort, thought to have antiseptic qualities
- Lungwort - thought to be useful in treating pulmonary infections "
Herbs with Yellow Flowers-generally are thought to be good for the liver, gallbladder and urinary system as they tend to be diuretic and flush the body of toxins and infections.
Herbs with Red Flowers-generally are thought to be blood purifiers, alterative and astringent in nature. Some are also thought to have antibacterial qualities.
Herbs with Blue or Purple Flowers-generally are thought to be sedative or relaxing in nature. They have some calming effects on the body and some are also considered to be blood purifiers as well.
Herbs that grow in areas with alot of gravel-generally are thought to be good for illnesses where gallstones or kidney stones are a problem. It is believed that herbs of this nature help in removing accumulations from the alimentary and bronchial systems.
Herbs found growing in mucky, swampy or wet ground-generally believed to be good for conditions with excess mucus.
Herbs that grow near fast moving water-generally are thought to be diuretic in nature and help to flush the body of waste.
Herbs with a soft texture to them-generally are thought to be good for swollen or inflamed areas.
Herbs with thorns or that are prickly-generally thought to help with sharp pains in the body and are said to be tonic in nature for all organs.
Herbs that cling to themselves-generally are said to help the body remove excess hardened mucus in the inner systems.
Herbs that are also vines-generally are said to be good for the blood and nervous systems.
Herbs that have thin, thread-like roots and stems-generally are said to be good for skin issues.
Fissures in the bark of certain trees-generally these are said to help with skin disorders.
Herbs that resemble or are named for body parts-generally are said to be good for those parts of the body they resemble.
Herbs whose roots resemble the human torso-these are said to be aphrodisiac in nature and help with sterility.
Herbs that resemble the human head-generally are believed to help with headaches, nervous disorders, etc.
Herbs that attract bees-generally these are believed to be good for insect bites or stings.
Strong smelling herbs-generally are said to be good at disinfecting or antiseptic in nature.
There are many sites online that can tell you plenty about the doctrine of signatures. Even wikipedia talks about it although they take the mass view and state it as being purely superstitious and has no scientific proof to it. However, I would like to say that is not an entirely true statement on their part as there is much evidence showing how plants work for us now.
For instance, a tomato has four chambers and is red. The heart has four chambers and is red. All of the research shows tomatoes are loaded with lycopine and are good for the heart and blood. Sweet potatoes look like the pancreas and actually balance the glycemic index so as such are good for people with diabetes. A walnut looks like a little brain, a left and right hemisphere, upper cerebrums and lower cerebellums, even the wrinkles on the nuts are just like the neo-cortex. We now know walnuts help develop more than three dozen neuro-transmitters for brain function. So there are many many foods that follow this doctrine and people are beginning to see more and more how effective they can be when used appropriately.
It is a shame that allopathic medicine does not teach much about nutrition or alternative medicine. I believe if more doctors integrated their practices to include such things they would have far more success with their patients.
I will be posting more things from henceforth about individual herbs and/or foods that can be used for a variety of things. I will also post recipes that might come in handy for those of you experimenting at home with some of these things in order to benefit yourself or those you love in some capacity.