Parkinsonia is a genus of about 12 species of flowering plants in the pea family, Fabaceae, placed in the order Fabales. The genus is native to semi-desert regions of Africa and the Americas, where species grow as large shrubs or small trees typically reaching 5–12 metres in height. Plants are characterised by their distinctively green bark, which carries out photosynthesis throughout the dry season when the sparse, bipinnate leaves — each carrying numerous tiny leaflets — are absent or reduced. The thorny, open crowns give the trees a wispy silhouette, and the yellow or white flowers have five nearly symmetrical petals.
Most American species are collectively known as palo verde, from the Spanish for "green tree," a name bestowed because of the chlorophyll-rich green stems and branches that sustain the plant between rains. The palo verde is the official state tree of Arizona. The genus was formerly divided between Parkinsonia and the closely related Cercidium, but molecular studies led to the merger of Cercidium into Parkinsonia. Among the best-known species are Parkinsonia aculeata (Mexican palo verde), Parkinsonia florida (blue palo verde), Parkinsonia microphylla (foothill palo verde), and Parkinsonia africana, native to southern Africa.
Etymology
The genus name Parkinsonia honors John Parkinson (1567–1650), an English apothecary and botanist. Parkinson is best known for his herbal works, including Paradisi in Sole Paradisus Terrestris (1629).
Distribution
Parkinsonia species are native to arid and semi-arid regions spanning Africa and the Americas. American species range from the Sonoran and Chihuahuan deserts of the southwestern United States and Mexico south to Argentina, including the Galápagos Islands; African species are found in Botswana, Namibia, South Africa, and East African countries including Kenya, Ethiopia, Somalia, and Djibouti.
Ecology
In the southwestern United States and western Mexico, Parkinsonia flowers are pollinated primarily by Centris pallida, a solitary ground-nesting bee known as the digger or pallid bee. This bee collects nectar and pollen from Parkinsonia to provision its brood cells, with the pollen imparting a strong orange colour to the bee bread stored for larvae.
Taxonomy Notes
Parkinsonia was formerly split between two genera: Parkinsonia sensu stricto and Cercidium. Phylogenetic analyses showed the two groups are not distinct, and Cercidium was subsumed into Parkinsonia. Several species therefore carry synonyms prefixed with Cercidium (e.g. Parkinsonia florida syn. Cercidium floridum, Parkinsonia microphylla syn. Cercidium microphyllum).
