Opuntia macrorhiza Engelmann. Tuberous-rooted Prickly-pear, Plains Prickly-pear. Phen: May-Jun; Aug-Dec (-Feb). Hab: Sandy or silty prairies, glades, limestone rock outcrops. Dist: Mostly of the southwestern United States and northern Mexico, w. LA, AR, MO into the Midwestern states.

Origin/Endemic status: Native

Taxonomy Comments: What is currently recognized as Opuntia macrorhiza s.l. is a group of closely related taxa (species complex), that is currently under revision (Majure, unpubl. data). Material that occurs in w. MS is of the O. grandiflora form (see Britton & Rose 1920), which is mostly spineless or may have 1-3 spines per areole. Those plants form large colonies and grow slightly ascending, produce inner tepals that are yellow with red bases adaxially and elongated fruit maturing dark purple to red (see Majure and Ervin 2008; treated as O. aff. allairei). This species often has tuberous roots. A species complex with diploid and tetraploid components (2n=22, 44).

Synonymy: = C, FNA4, Il, K3, K4, Mex, NcTx; = Opuntia macrorhiza Engelm. var. macrorhiza – GrPl, Tx; = Opuntia tortispina – F, misapplied; ? Opuntia allairei Griffiths; > Opuntia compressa (Salisb.) J.F.Macbr. var. macrorhiza (Engelm.) L.D.Benson – Weniger (1984); > Opuntia grandiflora Engelm. – S13; > Opuntia leptocarpa MacKenson – S13, Weniger (1984); > Opuntia macrorhiza Engelm. – F, S13; > Opuntia roseana Mackensen – S13; > Opuntia sphaerocarpa Engelm. – Weniger (1984); > Opuntia tortispina – S13

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Heliophily: 9