For much of period medical knowledge was based on hersay and conjecture. The reasons for this are many but one large factor was Church regulations regarding examination of the body. In particular the banning of human dissections lead to a number of false assumptions. When doctors were finally allowed to perform dissections they were limited to working on criminals. It becam obvious that the church teaching regarding the body were wrong but the this was rebuffed by saying that the bodies dissected were those of crimnals and thus abnormal.
Some cultures believed that plants have souls. In 4th century BC, Aristotle believed that plants had a psyche.
In medieval Europe, the Doctrine of Signatures stated there was a connection between how a plant looked (God's signature) and how it could be used medicinally, such as the Lungwort (which looks like a lung) which was/is used to treat respiratory illnesses.
For a sore throat
For healing wounds
To soothe your mouth/gums
Interesting texts: Chevallier, A. The Encyclopedia of Medicinal Plants, DK, London. 1996.
See also Herbs.