The North American Mesquites Prosopis Sect. Algarobia (Leguminosae)

Abstract

Bentham (1875) included nearly all the plants of Prosopis Sect. Algarobia DC. emend. Burk. (Leguminosae) in one polymorphic species, P. juliflora (Sw.) DC., distributed from Chile and Argentina to the United States. His treatment has been followed by most North American authors, e.g. Sargent (1902). Most of these authors have recognized in the United States the varieties glandulosa (Torr.) Coekerell and velutina (Woot.) Sarg. The discovery of an older name, P. chilensis (Mol.) Stuntz, based on Chilean plants, was followed by nomenclatural transfers of these varieties. In the same section, Burkart (1940) can recognize a number of species, among others P. chilensis sens. str., P. jutliflora sens. str., P. articutlata S. Wats., P. glandulosa Torr. and P. velutina Woot. That members of this section have indistinct morphic boundaries is probably due, says Burkart, to hybridization. Burkart cited, and presumably saw, rather few North American collections. Benson (1941) agrees that P. chilensis is not conspecific with the North American mesquites, but can recognize only one species in the United States, P. juliflora, with three varieties, glandulosa, veluttina, and torreyana. The oftenremarked taxonomic difficultv of this section needs no further emphasis than to point out these divergences in treatment. Field and herbarium studies of mesquites have been undertaken to determine whether considerable discontinuities exist between populations of mesquites, and whether combinations of certain characters show geographic localization. Some recent work of this nature has been done by Graham (1960) in northeastern Mexico. Graham demonstrates the existence of a narrow contact zone or zone of morphic inter(gradation between the lonI-leaved P. glanidulosa and a kind of "short-leaf" mesquite (included in P. laevigata in this paper). The characters which Graham finds to be siQmifieant are quanititative foliage characters. Benson (1941) distincuished the mesquites of the United States mainly on the basis of quantitative foliage characters. Preliminary studies on all the members of this section likewise indicate that foliage characters are geographically segregated and are of prime taxonomic value. Characters of fully mature fruits are also geomraphically segregated, and well correlated with those of foliage, but such fruits are so infrequently present in the field or on herbarium specimens that their taxonomic value is secondary. Quantitative inflorescence characters, such as absolute length of raceme, or raceme-length relative to leaf-length, may prove to be distinctive, but their evaluation is difficult. Quantitative characters of the flowers are very difficult to evaluate because of the proterogynous development (Burkart 1940) ; measurements have to be taken at comparable stages of developmnent, since the genitalia differ so greatlv in relation. to each other at different stages. The geography of some of the quantitative foliage characters is presented on the nmap, Fig. 2. The data were taken from herbariuim sheets. They do not