With some 280 species, Penstemon is the biggest genus of flowering plants endemic to North America. Penstemons are herbaceous perennials or small shrubs found from sea level to alpine zones, in habitats ranging from desert through arid scrub to moist forest. Despite the genus being widespread across the continent, many of its species have narrow ranges.
First described in the mid-eighteenth century, the number of known Penstemon species rose rapidly during the 1800s and 1900s as North America was explored by botanists. In Europe, by the mid-1800s, some horticultural plant breeders had become fascinated with the colour and form of penstemon flowers. As mutants and the products of interspecific hybridisations were selected, the numbers of cultivars rose into their hundreds. Penstemons have remained popular garden plants, albeit that the numbers of cultivars grown today are a fraction of those raised by Victorian gardeners.
Penstemons are known for their ‘showy’ flowers but to separate Penstemon species, subtle technical characters are necessary, including the size, shape, arrangement, hairiness and splitting of the stamens. The genus gets its name from having five stamens. Only four of the stamens are fertile and produce pollen; the fifth stamen, which produces no pollen (technically called a staminoid), is frequently the most prominent of the quintet.
Flower colour is a good predictor of pollinator in the genus; red-flowered species are likely to be hummingbird pollinated, purple-blue-flowered species are likely to be bee pollinated. The difference in flower colour is due to the loss of an enzyme that converts precursors for red pigments into blue pigments, i.e., purple-blue-flowered penstemons have the enzyme, red-flowered species lack the enzyme.
The genus Penstemon has proved useful for investigating the floral morphology, the evolutionary consequences of pollinator behaviour and the biology of plant reproduction. For example, two facts are revealed when the distribution of presumed bird- and bee-pollinated species is mapped across the Penstemon evolutionary tree. Firstly, there are many fewer Penstemon species with hummingbird-pollinated flowers than with bee-pollinated flowers. Secondly, hummingbird-pollinated flowers evolved from bee-pollinated flowers on at least 15 separate occasions; shifts did not happen in the opposite direction. This apparent anomaly may be explained by hummingbird-pollinated species having evolved recently, therefore there has been insufficient time for hummingbird-pollinated species to diversify. Alternatively, investigations based on modelling data, suggest another explanation; rates of species diversification in lineages leading to hummingbird-pollinated penstemons are lower than in those leading to bee-pollinated species.
Cardona J et al. 2020. Pollinator divergence and pollination isolation between hybrids with different floral color and morphology in two sympatric Penstemon species. Nature Scientific Reports 10: 8126.
Wessinger CA et al. 2019. Adaptation to hummingbird pollination is associated with reduced diversification in Penstemon. Evolution Letters 3-5: 521-533.
Wilson PP et al. 2007. Constrained lability in floral evolution: counting convergent origins of hummingbird pollination in Penstemon and Keckiella. New Phytologist 176: 883-890.