Ziziphus obtusifolia (Hook. ex Torr. & A.Gray) A.Gray
A somewhat uncommon low-elevation shrub in New Mexico. For some reason I never see if flowering. Vegetatively, it is easily confused with Lycium berlandieri, but has greyish-green thorns with distinctly hardened, reddish tips and stipules (which persist on young stems after the leaves fall), rather than brownish thorns without reddish tips nor stipules.
A branch on the north side of Tortugas Mountain, Doña Ana County, New Mexico, 15 Sep 2012.
Young stem with thorns, north side of Tortugas Mountain, 15 Sep 2012.
A young thorn with senescing leaves, north side of Tortugas Mountain, 15 Sep 2012.
An older thorn, north side of Tortugas Mountain, 15 Sep 2012.