St. John's Wort or Klamath Weed
St. John's Wort (Hypericum perforatum) grows throughout North America. It is an erect perennial herb that grows up to 3 ft (1 m) tall with woody lower stems. The branches are opposite and sterile. Usually both stems and branches are two-edged or winged. The leaves are opposite, sessile, linear-oblong, 3/ 4 inch (2 cm) long, and spotted with tiny dots which are translucent when held against the light. The flowering part of the plant has a cyme arrangement with numerous flowers 1/2 to 3/ 4 inches (1 to 2 cm) in diameter with five bright yellow petals, five green sepals, many stamens in clusters of three to five and an ovary with three widely spreading styles. The petals have fimbriate margins and may have black glandular dots on the margins. These dots contain the toxin, hypericin.
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Hypericin is a photodynamic pigment that remains chemically intact through digestion and is readily absorbed into the blood. It has no effect on the liver or other organs unless it is exposed to ultraviolet rays. This occurs especially in nonpigmented skin, causing primary photosensitization. Its presence in the glandular dots on the leaves suggests that all Hypericum spp. plants with similar glands are potentially capable of causing primary photosensitization. The young plants are as toxic as the mature plants and more palatable to livestock; they are thus more likely to cause poisoning in grazing animals. However, the toxin is not destroyed by drying and, therefore, hay containing it may also cause poisoning.
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