Flowers of the legume subfamily caesalpinioideae,
the peacock flowers

To continue last month’s survey of flowers of the legume family, here are drawings of two plants in the Fabaceae subfamily Caesalpinioideae, the peacock flowers. Last month we looked at flowers of the subfamily Papilionoideae, specifically two species of Phaseolus. The flowers in this group have a unique arrangement of five petals, each with a specific function: the banner petal is generally larger than the others and serves to attract pollinators; the two wing petals on each side serve as a landing platform for pollinators; and two partially joined keel petals at the center enclose the male and female reproductive structures (for a diagram, see Desert Breeze June 2018). Not only do the flowers of the Caesalpinioideae look very different from the Papilionoideae’s pea-like flowers, there is also much variability of appearance of the flowers within the Caesalpinioideae.

The drawings here are of two of our area’s common trees, the foothill palo verde, Parkinsonia microphylla, and the catclaw acacia, Senegalia greggii (formerly, Acacia greggii). The palo verde flower has a simple five-petal arrangement, with all the petals roughly the same size except for a slightly larger uppermost petal that is white, yellowing with age. Flowers of catclaw acacia are actually florets clustered together on a dense catkin-like inflorescence. The floret’s petals are fused and resemble a small cup.

The male reproductive structures of these two species are very different from those of the papilionoids. Stamens of the Phaseolus flowers are fused into a tube, while stamens of the caesalpiniods are distinct and not connected to each other. Foothill palo verde flowers have 10 distinct, or separate, stamens. The catclaw acacia flower has numerous distinct stamens that extend well beyond the petals, much like stamens of the fairy duster flower. (Both the Senegalia and Eriophylla genera were formerly in the legume subfamily Mimosoideae.) With these mimosa-type flowers, the stamens are very prominent and sometimes quite showy while the petals and sepals are mostly inconspicuous.

Illustrations of plants in the Caesalpinioideae that have been included previously in the Desert Breeze are Chris Bondante’s Senna lindheimeriana (Nov. 2016), Susan Ashton’s Parkinsonia microphylla (Jan. 2017), Margaret Pope’s Haematoxylon brasiletto (Feb. 2018), and Susan Ashton’s Parkinsonia aculeata (Apr. 2018). All of the legume drawings in the Sonoran Desert Florilegium that were prepared for the upcoming book Legumes of Arizona: An Illustrated Flora and Reference can be seen here. — Cindy Hartwell

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