Flowers of the genus Phaseolus, the New World’s wild bean

Flowers in the Fabaceae, or legume family, are highly diverse in appearance, from the tiny florets in a mesquite tree’s catkin to the radiant stamens of the Baja fairy duster’s flower. There are (at present) six subfamilies in the legume family, and each produces distinctly different flowers. Even within a subfamily, the flowers can look very different to an untrained eye.

The Sonoran Desert Florilegium includes several of the legume illustrations prepared for the upcoming book, Legumes of Arizona: An Illustrated Flora and Reference, and those plants all fall within two Fabaceae subfamilies, the Papilionoideae (the butterfly flower subfamily with some 14,000 species) and the Caesalpinioideae (the peacock flower subfamily with roughly 4,400 species). So far this year, we’ve looked at Margaret Pope’s illustration of brazilwood, Haematoxylon brasiletto, and Susan Ashton’s Mexican palo verde, Parkinsonia aculeata, both from the subfamily Caesalpinioideae. Last month we looked at John Gerdes’s drawing of Clitoria mariana, the butterfly pea, from the Papilionoideae subfamily. Another group of plants from the Papilionoideae is the genus Phaseolus, the New World’s wild bean.

Phaseolus grayanus, or Gray’s bean, is a beautiful Sonoran Desert native that can be found at higher elevations to 8,500 feet. It has a trailing habit, striking trifoliate leaves with silvery markings along the veins, and, from July to September, deep pink flowers. The fruit is a curved seed pod whose shape is aptly described by the name of the genus that is derived from the Greek phaselos, or “little boat”. The plant was named in honor of Asa Gray (1810–1888), professor of botany at Harvard and America’s preeminent botanist in the 19th century.

The illustration of Phaseolus grayanus shown here is by Tucson artist Chris Bondante. At top right is a drawing of the flower that is the typical form for the Papilionoideae: a five petal structure with a large banner petal at the top; two wing petals on the sides; and a central keel, two petals that are partially joined to enclose and protect the stigma and anthers. Below that drawing is a vertical section of the flower that shows the stamens (male) and pistil (female) within the keel. Nine of the stamens are fused into a tube with a tenth “free” stamen standing apart from the other nine. This arrangement of the male structures is referred to as diadelphous and is common in the Fabaceae.

Below Chris’s illustration of Phaseolus grayanus is an illustration of Phaseolus acutifolius var. tenuifolius, the tepary bean, by Susan Ashton. Below Susan's illustration is a comparison of the flowers of each of the two plants. While the flowers appear very similar, there is variation in the reproductive structures and the keel. The tepary bean has a more tightly coiled keel, and within the keel, the pistil is also coiled and has a brushed tip, a pollination strategy that allows the pistil to extend from the keel and brush pollen from the anthers onto the pollinator.

An illustration of a different tepary bean, Phaseolus acutifolius var. latifolius, by Wendy Hodgson, can be seen here in the online exhibit Botanical Art of the Sonoran Desert: Past and Present.

Chris Bondante’s process for illustrating Phaseolus grayanus can be seen here in an presentation on how to create a botanical illustration. Included in this presentation are several photographs of the plant that provide the color information missing from the pen and ink drawing. — Cindy Hartwell

An this article appeared in the June 2018 issue of the Desert Breeze, the monthly newsletter of the Tucson Cactus and Succulent Society.