Hypericaceae is a plant family containing ten genera (The plant list, 2010), although some classification systems consider it a subfamily of Clusiaceae (Heywood et al, 2007). One of the genera, Hypericum (or St John’s-wort) is widespread with 370 species, especially in the northern hemisphere (Heywood et al, 2007).
The name Hypericum is derived from the Greek ‘Hyper’ – above and ‘Eikon’ – picture, which refers to the traditional hanging of the plants above religious pictures, to ward off evil, when they were harvested on St John the baptist’s day (June 24th – midsummers day)- hence the common name (Mabey, 1996).
UK Hypericums include perennial herbs or small shrubs. The species Hypericum perforatum, or common St John’s wort, can be found growing on campus in grasslands and at the edges of wooded areas in late spring and summer. 
It has a creeping habit with erect, branching stems, reaches up to one metre in height and can be identified by its small (just over 1cm), simple, bright green, oblong leaves, in opposite pairs, with no petioles; 
small translucent glands on the leaves, which can be seen when held to the light; flowers up to 2.5cm across, with five yellow petals bearing small black glands and five sepals, as well as an ovary borne above these (hypogynous); many stamen, in 3 bundles; and stems with a small ridge on each side. 
St John’s wort is commonly used in herbalism and medicine for its antibacterial and anti-inflamatory properties, as well as being an anti-depressant. (Mabey, 1996)
Outside its native range, e.g. in the USA, Hypericum perforatum can become an invasive and toxic weed out-competing native plants and poisoning livestock.
References:
Heywood, V.H., Brummit, R.K., Culham, A. and Sedberg, O. (2007). Flowering Plant Families of the World. Firefly books, Ontario
Le Grice, D. and Jury, S.L. (2011). Flora of the Whiteknights park. Unpublished
Mabey, R. (1996). Flora Britannica. Sinclair-Stevenson, Great Britain
Stace, C. A. (2010). New Flora of the British Isles, 3rd edition. Cambridge University Press, Cambridge
The Plant List (2010). Version 1. URL: http://www.theplantlist.org/
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