This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Trees or shrubs often with spines in the axils, with 2-pinnate leaves, the leaflets few or numerous, and small spicate or capitate perfect flowers. Calyx campanulate, with 5 short teeth. Petals 5, valvate, distinct, or connate below. Stamens 10, distinct; filaments long. Ovary often stalked, many-ovuled; styles slender or filiform; stigma very small. Pod linear, straight or curved, compressed, leathery, indehiscent, the mesocarp spongy or dry. Seeds flattened. [Ancient name for some very different plant.]
About 15 species, natives of warm and tropical regions. Besides the following, 2 or 3 others occur in the southwestern United States. Type species: Prosopis spicigera L.
P. glandulosa Torr. Ann. Lyc. N. Y. 2: 192. pl. 2. 1828.
A glabrous or minutely pubescent shrub, the axils usually with a pair of sharp spines. Leaves petioled, with 2 spreading short-stalked pinnae, each of numerous sessile leaflets which are linear or linear-oblong, entire, acute or obtuse, mostly mucronulate, firm, veiny, 1/2'-2' long, 1"-2" wide; spikes or spikelike racemes axillary, often numerous, peduncled, very densely many-flowered, 2'-5' long, nearly 1/2' thick; pedicels 1/2"-1" long; calyx campanulate; petals 2-4 times as long as the calyx; ovary villous; pods linear, stipitate, 4'-8' long, 4"-6" wide, constricted between the seeds.
Kansas to Texas, Arizona, California and Mexico. Apparently distinct from the tropical P. juliflora (Sw.) DC. April-June.