Parkinsonia aculeata. JERUSALEM THORN. Tropical America


A deciduous desert tree, with characteristic pea-family pinnate leaves modified for dry conditions. The leafl ets, normally up to a foot long, are much reduced in size (about 1/8inch) and very numerous (40 to 80) if present at all (the plant often gets along with the interestingly shaped leaf rachis only). Yellow ½ -inch flowers come in racemes standing several inches high. The seed pods are several inches long, constricted between the seeds. No examples are known on campus, but in Palo Alto three specimens are in front of 586 College Avenue and another is at Palo Verde Elementary School, 3450 Louis Road. Walk in the main entrance and see the tree on the right in the courtyard just beyond the principal’s office.

Illus. right: George B. Sudworth. Forest Trees of the Pacific Slope. USDA, 1907. Click for full image.

Illustrations (links open new windows): habit |


The Pea family is among the largest flowering plant families with about 18,000 species in 630 genera worldwide. Caesalpina is the type genus for the Fabaceae subfamily Caesalpinioideae, following Cronquist (1981). This subfamily is intermediate in flower morphology between the other two higher-level classifications Mimosoideae and Faboideae (or Papilionoideae). The latter and largest group includes plants with papilionaceous, "butterfly-like", corollas (with standard, wings, and keel)—as the common garden pea.

All subfamilies are well represented on campus for closer study. Caesalpinioideae and Mimosoideae include mainly tropical trees with pinnately or bipinnately compound, alternate leaves. Mimosoideae flowers are regular (radially symmetrical), the corolla with equal petals often fused into a tube. Campus representatives include Acacia, Albizia, and Gymnocladus. Caesalpinioideae flowers are usually more or less zygomorphic (divisible into equal halves in one plane only). As with Caesalpina spp., the petals are distinct, the uppermost often smaller than the laterals. Other campus members of the subfamily include Bauhinia, Cassia, Cercis, Gleditsia, and Parkinsonia. It would be instructive to work out the higher classification of the many other campus peas (genera listed in the family index) from field observation throughout the year.

— further reading: Wendy Zomlefer, Guide to Flowering Plant Families, Univ. of North Carolina Press, 1994.

Name derivation, genus | species after John Parlinson (1567-1650) london apothecary and author of gardening books | prickly

Related material: Canopy Trees for Palo Alto Tree Library

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