Culpeper gives St John's Wort to the Sun, and the case is built from three visible features of the plant. All three are genuine observations; the inference from them is the doctrine of signatures, which is a false theory.
The first is the flowering time. It blooms around midsummer, and it takes its name from St John's Day, 24 June, the feast set at the summer solstice. Gathering it at midsummer and hanging it at doors and windows is attested English and continental folk practice.
The second is the perforated leaf. Held against the light, the leaves show scattered translucent dots, which is what perforatum records. They are not holes; they are glands filled with volatile oil, and the light passes through because the oil is clear. Reading them as pinpricks of sunlight is the signature argument.
The third is the red oil. Crushed flowers stain the fingers, and flowers steeped in oil turn it deep red over some weeks. This is real and it is the one that connects to the pharmacology: the red compound is hypericin, a genuinely active naphthodianthrone and the marker constituent by which commercial extracts are standardised. The plant does bleed red, and the red substance does something.
The older name Fuga Daemonum, the flight of demons, records the protective use. It is worth observing that a plant used against melancholy and demonic oppression in the seventeenth century is used against depression in the twenty-first, and that the categories are not the same category even where the plant is the same plant.