CACTACEAE: Cactus Family
KEY TO OPUNTIA SPECIES
1. Spines absent ......................................................................................................................... 2
2. Plants erect; lower joints enlarging, becoming nearly terete, fusing increasingly, and forming a trunk; the larger terminal joints 30-60 cm .......... O. ficus-indica
2. Plants sprawling to erect; lower joints remaining flat or nearly so, not forming a trunk, the larger terminal joints 10-25(40) cm ............................ O. stricta
1. Spines present ........................................................................................................................ 3
3. Plants creeping, often mat-forming; joints mostly 1.5-3 cm across, easily detached and dispersed, clinging by strongly barbed spines ............................................................ O. pusilla
3. Plants creeping to erect; joints mostly 4-40 cm across, adhering firmly together, spines not strongly barbed ...................................................................... 4
4. Plants erect; the lower joints enlarging, fusing more prominently, becoming nearly terete, and forming a conspicuous trunk; the larger terminal joints mostly 30-60 cm .................. O. ficus-indica
4. Plants trailing or decumbent to erect; if erect, any trunk rarely over 30 cm; the larger terminal joints mostly 10-30 cm ....................... 5
5. Spines somewhat needlelike, very narrowly elliptic to nearly linear in cross section........................ 6
6. Spines weakly barbed, nearly linear in cross-section; apex of inner sepals acute to acuminate ................................... O. stricta
6. Spines not barbed, very narrowly elliptic in cross-section; apex of inner sepals truncate or notched ................................ O. engelmanii var lindheimeri
5. Spines needlelike, elliptic to nearly circular in cross-section ....................... 7
7. Spines 0-1(2) per areole, spreading, 0.5-1 mm across base ................................................... O. humifusa
7. Spines 0-6 per areole, most deflexed, 0.25-0.5 mm across base......................................... O. macrorhiza