Morphological Variation in Mesquite (Prosopis, Leguminosae) in the Lowlands of Northeastern Mexico

Abstract

A detailed study of morphological variation in mesquite (Prosopis) from 17 sites in northeastern Mexico shows a definite correlation of leaf length with secondary-leaflet size, number of pairs of primary leaflets per leaf, and number of spines per node. These morphological characters are also correlated to some extent with geographic location, with some exceptions; intermediates occur which indicate the occurrence of hybridization between the long-leaf mesquite, P. glandulosa Torr., and the short-leaf form, for which Neltuma palmeri Britt. & Rose is the only avail- able name. The contact zone between the two kinds remains fairly static at present because of environmental pressures and/or partial isolation, the mechanisms for which are at present unknown. Mesquites (Prosopis Sect. Algarobia DC. emend. Burk.) of the southwestern United States and Mexico are polymorphic and variable (Standley, 1922; Burkhart, 1940; Benson, 1941). During the course of work in northeastern Mexico, opportunity was provided for a de- tailed study of some of the morphological variation. Preliminary reconnaissance over a wide area in the Gulf lowlands of northeastern Mexico showed that, in general, the mesquite growing in southern Tamaulipas, southern Nuevo Leon, extreme eastern San Luis Potosi, and extreme northern Veracruz had, among other dif- ferences, shorter leaves and smaller secondary leaflets than those growing in northern Tamaulipas and northern Nuevo Leon, and that the contact zone between the short-leaf and long-leaf mesquites is in the vicinity of the Rio San Fernando. The map (figure 1) shows this contact zone.