Read it through once

A firm belief existed in the Middle Ages that every plant was a good remedy for something. There is a real basis in fact for this superstition, because every plant in the world has, so far as it can do so, to protect itself. The attacks of all sorts of grazing animals, from the mouse to the elephant, as well as the infinitely more dangerous and destructive insects, bacteria, and fungi, have to be provided for. By far the commonest form of protection is to develop within the plant strong medicinal or strongly smelling substances. These are far better as protective agents than the thorns and spines characteristic of deserts and half-deserts. We have already glanced at the turpentines and resins of Coniferous forests and at the odorous gums, frankincense, and myrrh of the Acacia scrub.