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Prosopis alpataco Phil. and P. argentina Burk. are shrubs growing under different ecological conditions within the Monte Desert, Argentina. Both species have similar wood structure: semi-ring-porosity, vessels with a bimodal diameter distribution, short vessel elements, paratracheal confluent axial parenchyma, libriform fibres and homocellular rays. Prosopis alpataco has wider vessels and a greater proportion of solitary vessels than P. argentina which has narrower vessels, mainly in clusters, and a larger number of vessels per mm(2). The wood structure of P. argentina suggests a safer, but less efficient, water-conducting system than that of P. alpataco. This could be related to P. argentina’s tolerance to high water stress, as this species occurs mostly in sand dunes. Instead P. alpataco is a phreatophyte, getting a regular water supply.