Hypericum

Hypericum is a large genus of flowering plants within the family Hypericaceae.

These plants are found almost everywhere on Earth. They grow in most regions except for deserts, polar areas, and tropical lowlands. Many people know them by the common name St. John's wort. Some species are also called goatweed. Because they grow so quickly in some areas, many species are classified as invasive or noxious weeds.

To understand how these plants work, we must look at their unique glands. These glands create distinct dot patterns on the leaves and stems. There are two main types: dark glands and pale glands.

Dark glands consist of cell clusters that appear black or reddish. They contain chemicals called naphthodianthrones, such as hypericin or pseudohypericin. When these glands are crushed, they release a red stain. Pale glands are small spaces that secrete oils and substances like hyperforin. These glands serve as a defense mechanism. They discourage generalist herbivores from eating the plant. In fact, when insects feed on Hypericum perforatum, the plant can produce 30% to 100% more naphthodianthrones to repel them.

The physical structure of Hypericum is very diverse. Plants in this genus can grow as trees, shrubs, annuals, or perennials.

While trees are rare, most woody species grow with multiple stems from a single base. Perennial herbs often root from horizontal nodes, especially in wet areas. The stems themselves are a key way to identify species. Some stems are smooth, while others have ridges or "wings." For example, Hypericum perforatum has two lines, whereas Hypericum maculatum has four. Most leaves are arranged in an opposite and decussate pattern. This means the leaves grow in pairs, and each pair is turned at a right angle to the one below it.

Flower parts also vary significantly across the genus. Most petals are yellow, ranging from pale lemon to deep orange.

However, exceptions exist, such as the white or pink petals of Hypericum albiflorum. The flowers contain between five and 200 stamens, which are grouped into four or five bundles called fascicles. Some specialized species use a mechanism called lodicules to spread their petals for pollination. The fruit of the plant is usually a capsule that opens at the top. These capsules can be dry or fleshy when they mature. Inside, the seeds are small and range from yellowish brown to dark purplish brown.

History shows how humans have interacted with these plants for centuries. In the 16th century, the physician Paracelsus called the red secretions "Johannes-blut." He linked the red color to the martyr St. John, which helped create the name St. John's wort.

Modern science has also mapped this genus extensively. Norman Robson, working at the Natural History Museum in London, produced a worldwide taxonomic monograph. He identified 36 different sections within the genus to help scientists organize the more than 490 known species.

The significance of Hypericum extends into medicine and ecology. St. John's wort is frequently used as an herbal supplement to treat mild-to-moderate depression.

One study found it had similar efficacy to certain prescription drugs. However, it can cause dangerous interactions. It can increase serotonin levels to life-threatening amounts if mixed with other antidepressants. It can also reduce the effectiveness of hormonal contraceptives. In the wild, the plants support many animals. Certain beetles and moths rely on them for food. Interestingly, some ant species use the bright yellow petals to decorate their nests.

Finally, the fossil record provides a glimpse into the ancient past of Hypericum. The oldest known fossil species is †Hypericum antiquum. This plant lived during the Eocene period in what is now Siberia.

Other fossil seeds, such as †Hypericum septestum, have been found from the early Miocene. This long history shows how well the genus has adapted to different environments across the globe.