Common St. John's-wort
St. John the Baptist's Day, celebrated on the 24th June, gives Hypericum perforatum its common name. The bright yellow flowers of this herbaceous perennial are prominent in grasslands, roadsides and field margins and waste places across the species' native Eurasian distribution around the time of this Christian midsummer festival.
Besides flower colour, Hypericum perforatum is easily identified in the field because of the black glands on its petals and sepals, the numerous stamens arranged in clumps, the red latex in its buds and the leaves covered in translucent dots. It is these glandular dots that give the plant its scientific species name.
In alternative medicine, St. John's-wort has become popular as an antidepressant and antibiotic, although clinical evidence regarding efficacy in vivo is contradictory and controversial. Two compounds, hyperforin and hypericin, appear to be biologically active and are likely to have evolved to deter herbivores. Livestock exposed to large amounts of St. John's-wort show photosensitive skin lesions which can become severe. Such responses have economic effects on productivity in areas where St Johns-wort has become invasive. For example, in regions outside of its native range such as North America and Australia.
St. John's-wort has numerous characteristics that one might expect of a successful invasive species. It can reproduce sexually through seed and vegetatively via rhizomes (underground stems). Furthermore, the seeds are small, easily dispersed by wind and can survive for decades in the seed bank. As a species of disturbed habitats, St. Johns-wort is well-adapted to habitats created by human activities.
Across its native range, Hypericum perforatum shows complex patterns of variation associated with morphology, chromosome numbers and breeding behaviour. Natural populations of St. John's-wort reproduce sexually and through apomixis (production of seed without the involvement of a male parent). The latter mode of reproduction is particularly common in tetraploid populations, that is where individual plants have four complete sets of chromosomes. The patterns created mean Hypericum perforatum has been divided into numerous subspecies; sometimes these have been given specific rank. However, genetic analyses show there are two groups of distinct populations, which may interbreed, throughout the range of Hypericum perforatum.
The closest relative of Hypericum perforatum is the European and western Asian Hypericum maculatum, as species of damp grassland in Europe and western Asia. Where the two species' ranges meet each other they cross producing a partially fertile hybrid that is capable of backcrossing to either of its parents.
Crompton CW et al. 1988. The biology of Canadian weeds. 83. Hypericum perforatum L. Canadian Journal of Plant Sciences 68: 149-162.
Koch MA et al. 2013. Evolution of cryptic gene pools in Hypericum perforatum: the influence of reproductive system and gene flow. Annals of Botany 111: 1083-1094.