A member of the honeysuckle family, the twinflower is the only species in its genus.  There are three subspecies: Linnaea borealis borealis, Linnaea borealis americana, and Linnaea borealis longiflora.  The first is found in northern Europe, the second in eastern North America, and the last in Asia and western North America.  It prefers cool forests and bogs, where it grows in moist, acidic soils.  In the UK, where it is now considered “nationally scarce,”  it is an indicator species of ancient woodlands.

Twinflowers get their name from the two bell-like flowers that hang from a shared stem.  They are white to pink, with darker pink or red stripes, and have a scent that is sweet but not strong.  Flowers are pollinated by syrphid wasps and native bees, producing little dry fruits that hold but a single seed.  The plant itself is a low-lying perennial creeper, with small, evergreen leaves and thin hairy stems.  It spreads by runners (after the plant is about five years old) and via seeds carried on the feathers and fur of animals.  It is vulnerable to fire, drought, and soil disturbance, putting it at risk from forestry and overgrazing.  Once common throughout the United States, it is now threatened, endangered, or “extirpated” (in the words of the USDA) in many states. 

It has been used in teas by native peoples (what hasn’t, one sometimes wonders) and you can buy twinflower essence on the internet that is supposed to “calm the sense of being overwhelmed from sensory overload.”  The charming quality of the twin bells has led a number of writers and poets to include it in their work, where it frequently evokes courting couples and young love.  It was also adopted by Carl von Linné (the father of taxonomic system we still use today); in an oft-quoted passage he explained that it “was named by the celebrated Gronovius and is a plant of Lapland, lowly, insignificant and disregarded, flowering but for a brief space - from Linnaeus who resembles it.” 

Sources:

EOL
Wikipedia
USDA
Connecticut Botanical Society
ARKive
Nearctica
Wildflower Center
Natural History of the North Woods
Britannica

Tree Frog Farms (source for twinflower essence)

Poetry and Literature:

Excerpt and analysis of Lorine Niedecker’s poem, “Wintergreen Ridge”  (The rest of her poetry is worth seeking out, by the way.)
Little Rivers: A Book of Essays in Profitable Idleness by Henry Van Dyke (1880s-1890s) (mentioned in “Sign of the Balsam Bow, Part III: The Island Pool”)