White Clover (Trifolium repens) is a familiar weedy herbaceous plant with sprawling stems that is native or naturalized across most of the temperate regions of the world. The white or pinkish flowers are produced in heads consisting of dozens of flowers. It is a perennial and an insect-pollinated, obligate outcrosser; plants also spread vegetatively by stolons (Olsen et al. 2008). Like other plants in the Fabaceae (legume family), White Clover harbors microbes in its root system that are capable of fixing nitrogen, i.e., converting atmospheric nitrogen into a form that is usable by plants. White Clover is one of the three-leafleted clover species that occasionally produces a leaf with four (or more) leaflets, i.e., a "four-leafed clover". The genetics underlying this phenomenon has been at least partly worked out (see Tashiro et al. 2010). Images of four-leafed clovers can be seen above and examples of multifoliolated clover leaves can be seen here, along with this intriguing quotation from Masters (1869): "[Trifolium repens] was gathered at night-time during the full moon by sorceresses, who mixed it with vervain and other ingredients, while young girls in search of a token of perfect happiness made quest of the plant by day."
White Clover exhibits a geographic polymorphism for cyanogenesis (the release of cyanide following tissue damage, a phenomenon seen in a very large and diverse number of plant species). Both cyanogenic and acyanogenic plants occur in natural populations, with acyanogenic plants predominating in colder climates for reasons that are not yet clear (Olsen and Ungerer 2008; Olsen et al. 2008 and references therein). This polymorphism has been studied since early in the 20th century, and represents one of the most thoroughly studied examples of an adaptive polymorphism in plants. Cyanogenic plants are generally found to be strongly favored in the presence of generalist herbivores, which avoid eating them. However, a number of costs appear to be associated with cyanogenesis, such as reduced drought tolerance, resulting in trade-offs that may favor cyanogenetic or acyanogenetic plants depending on local conditions. (Olsen et al. 2008 and references therein)