Hypericum is a genus of flowering plants in the family Hypericaceae (formerly considered a subfamily of Clusiaceae). The genus has a nearly worldwide distribution, missing only from tropical lowlands, deserts and polar regions. Many Hypericum species are regarded as invasive species and noxious weeds. All members of the genus may be referred to as St. John's wort, and some are known as goatweed.
Hypericum is unusual for a genus of its size because a worldwide taxonomic monograph was produced for it by Norman Robson (working at the Natural History Museum, London). Robson recognizes 36 sections within Hypericum.
Description
left|thumb|Hypericum hidcoteense
thumb|left|Ripe berries of Hypericum androsaemum
Hypericum species are quite variable in habit, occurring as trees, shrubs, annuals, and perennials. Trees in the sense of single stemmed woody plants are rare, as most woody species have multiple stems arising from a single base. Shrubs have erect or spreading stems but never root from nodes that touch the ground. However, perennial herbs tend to root from these horizontal nodes, especially those that occur in wet habitats. Annual herbs tend to have taproots with a developed system of secondary hair roots. Many species of Hypericum are completely glabrous, others have simple uniseriate hairs, and some species have long, fine hairs.
thumb|Glandular dots on a leaf of [[Hypericum perforatum]]
Two types of glands form the characteristic punctiform (dots) patterns of Hypericum, "dark glands" and "pale glands". Dark glands consist of clusters of cells with a distinct black to reddish color. Their hue is indicative of a presence of naphthodianthrone, either hypericin or pseudohypericin, or both. These glands occur in about two-thirds of Hypericum sections and are usually restricted to certain organs. When these glands are crushed, the naphthodianthrones give a red stain. Paracelsus called the red secretions "Johannes-blut" in the 16th century, linking the plant to the martyr St. John and giving rise to the English and German common names of "St. John's wort". The pale glands, forming the pellucid dots, are each a schizogenous intracellular space lined with flattened cells that secrete oils and phloroglucinol derivates, including hyperforin. The distribution of these hypericin glands dissuades generalist herbivores from feeding on the plants. When generalist insects feed on Hypericum perforatum, 30-100% more naphthodianthrones are produced, repelling the insects. i.e. the Greek name for Hypericum crispum and Hypericum revolutum, itself possibly meaning "above pictures", for its use over shrines to repel evil spirits, though some have translated it as "above the heath".
Sections
Hypericum is broken up into 36 sections, each with its own subsections and species. They include:
- Adenosepalum
- Adenotrias
- Androsaemum
- Arthrophyllum
- Ascyreia
- Brathys
- Bupleuroides
- Campylopus
- Concinna
- Coridium
- Crossophyllum
- Drosocarpium
- Elodeoida
- Graveolentia
- Heterophylla
- Hirtella
- Humifusoideum
- Hypericum
- Inodora
- Monanthama
- Myriandra
- Oligostema
- Olympia
- Origanifolia
- Psorophytum
- Roscyna
- Sampsonia
- Santomasia
- Taeniocarpium
- Takasagoya
- Triadenoides
- Trigynobrathys
- Tripentas
- Umbraculoides
- Webbia
Ecology
H. perforatum is an invasive species and noxious weed in farmland and gardens in the humid and sub-humid temperate zones of several continents. Hypericum species are the only known food plants of the caterpillar of the treble-bar, a species of moth. Other Lepidoptera species whose larvae sometimes feed on Hypericum include the common emerald, the engrailed (recorded on imperforate St. John's-wort, H. maculatum), the grey pug and the setaceous Hebrew character. A leaf beetle, Paria sellata, feeds on the foliage of Hypericum adpressum, while ant species Formica montana and F. subsericea decorate their nests with its bright yellow petals. H. perforatum is the most commonly used species – especially in Europe – as an herbal substitute for prescription drugs to treat depression, and is also sold as a dietary supplement.
There is evidence that combining St. John’s wort with prescription antidepressants may cause adverse effects, such as a life-threatening increase of serotonin, the brain chemical targeted by some drugs used for depression.
Ornamental plants
thumb|right|Hypericum olympicum in Botanic garden LiberecSome species are used as ornamental plants as many have large, showy flowers. Species found in cultivation include:
- H. aegypticum
- H. androsaemum
- H. balearicum
- H. bellum
- H. calycinum
- H. elodes
- H. forrestii
- H. kalmianum
- H. kouytchense
- H. olympicum
- H. perforatum
Numerous hybrids and cultivars have been developed for use in horticulture. The following have gained the Royal Horticultural Society's Award of Garden Merit:
- H. × moserianum (H. calycinum × H. patulum)
- 'Hidcote'
- 'Rowallane'
Most species of Hypericum are prone to thrips, scale, anthracnose, rust, and leaf spots. They are also eaten or infected by aphids, white flies, and spodoptera littoralis.
Fossil record
The oldest fossil species is †Hypericum antiquum from the Eocene of Siberia. Many fossil seeds of †Hypericum holyi have been described from middle Miocene strata of the Fasterholt area near Silkeborg in Central Jutland, Denmark.