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[[underlined]]Acacia constricta[[/underlined]]- Common from Benson to Tucson.  Not found at Casa Grande or west of there.

[[underlined]]Mimosa roemeriana?[[/underlined]] - This catsclaw was found at Lordsburg & Duncan New Mex., and at Benson, Tucson, Casa Grande & west to the Mohawk Mts., Arizona, but no farther.

[[underlined]]Fouquiera spinosa[[/underlined]]- Common at Lordsburg N.M., at Benson, Ariz, and westward to Tucson, Casa Grande - all the way to Yuma & beyond on mesas & stoney slopes.

[[underlined]]Chilopsis saligva [[/underlined]]- Common along the Rillita creek near Tucson, but not found west of there.

Other plants that range west to Tucson & no farther are:-

Opuntia englemani
"[[ditto for Opuntia]]s (mainly spineless)
"[[ditto for Opuntia]](brown spined)
"[[ditto for Opuntia]] leptricaulis
Agave [[space]] fig
Yucca palmeri?
"[[ditto for Yucca]] radiosa 
Koeberlinia spinosa
Ephedra trifurcata?

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There seems to be a marked break in range of species just west of Tucson but it needs careful working out with camp outfit.  It is evidently the change from Lower Sonoran to arid subtropical - or the "lower division" of lower sonoran.

The marginal distribution of Giant cactus and palms (Washingtonia) around the sides of valleys would suggest that a hotter belt than is found in the bottom of the valleys.  A series of experiments would show whether this is the case and whether the giant cactus belt is frostless.