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Key to Opuntia

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1 Plants forming low, spreading shrubs.
  2 Tepals yellow with red or maroon bases adaxially.
    3 Erect-spreading shrubs, chains of cladodes radiating from center of plant, the flat surface generally held perpendicular to the ground surface; cladodes remaining turgid throughout the year; stigma usually green; fruit barrel-shaped (widest near the middle); [of c. TN]
    3 Spreading shrubs, plants clump forming or with chains of cladodes spreading from the central axis of the plant, the flat surface generally parallel to the ground surface; cladodes becoming cross-wrinkled during fall and winter months, fruit clavate (widest towards the tip); [collectively widespread in our area].
      4 Central spines mostly 0-2 (when 2, both spines generally in the same plane, i.e., both reflexed or both erect); spines monomorphic (central spines only produced); [widespread in eastern US, mostly west of the Appalachian Mts. and east of the MS River]
      4 Central spines mostly 0-3 per areole (when 2-3, the lower spines generally reflexed and the upper spine porrect on upper cladodes); spines monomorphic (central spines only) or dimorphic (with smaller hairlike radial spines and larger central spines); [mostly west of the Mississippi River, in our area as a rare disjunct in w. MS]
  2 Tepals entirely yellow or greenish yellow.
        5 Cladodes spineless; cladodes never easily disarticulating; areoles typically 4-5 per diagonal row at the widest point of the cladode; [central Appalachian Mts. and n. Atlantic Coast, disjunct in nc. MS]
        5 Cladodes generally with 1 or more spines per areole on at least some of the uppermost areoles; cladodes easily disarticulating or not; areoles typically 1-4 per diagonal row at the widest point of the cladode; [widespread in Atlantic and Gulf Coastal Plain and Atlantic Piedmont].
          6 Terminal cladodes easily disarticulating, 0.8-11.1 cm × 0.6-3.4 cm, often cylindrical or only moderately flattened; spines strongly retrorsely barbed; terminal cladodes with 1-2 areoles per diagonal row at the widest point of the cladode
          6 Terminal cladodes disarticulating or not, 3.1-17.7 cm × 2-9 cm, strongly flattened; spines retrorsely barbed (to the touch) or not; cladodes with 3-4 areoles per diagonal row at the widest point of the cladode.
             7 Seeds with funicular envelope smooth, only moderate, if any, protrusion of the cotyledons and hypocotyl, cladodes typically scalloped-margined, elliptical or rotund in outline, spines delicate, 0.7-0.9 mm in diameter; [restricted to the outer Gulf Coastal and Atlantic Coastal Plain]
             7 Seeds with funicular envelope bumpy, cotyledons and hypocotyl noticeably protruding, cladodes typically smooth-margined, obovate or rotund in outline, spines stout, 0.95-1.3 mm in diameter; [of Atlantic Southern Piedmont, FL panhandle along the coast west to coastal MS and e. LA, otherwise mostly inner Gulf Coastal and Atlantic Coastal Plain]
1 Plants forming erect or ascending shrubs or small trees.
               8 Tepals red, pink, yellow with red midribs abaxially, or yellow with red or reddish maroon bases adaxially; stamens either thigmonastic (i.e., moving towards the stigma in response to manipulation of filaments) or not.
                 9 Cladodes generally spineless; tepals red or pink, erect; stamens and style exserted past the corolla; filaments red or pink, the stamens not thigmonastic; [exotic, rarely persistent or naturalizing in FL]
                 9 Cladodes generally spiny; tepals yellow and variously tinged red or maroon adaxially or abaxially; stamens and style included within the corolla; filaments yellow or yellow green, the stamens thigmonastic.
                   10 Erect shrubs or small trees; spines chalky-white; cladodes dark, glossy green; tepals yellow with red abaxial midrib; stigmas white to cream; fruit clavate (widest towards the tip), often proliferous; [exotic, known from FL only in our area]
                   10 Spreading or erect shrubs; spines pale white or white with maroon or brown bands; cladodes light, glaucous green, dull; tepals yellow basally tinged red or maroon adaxially; stigmas usually greenish; fruit barrel-shaped (widest near the midpoint), not proliferous; [of nc. TN]
               8 Tepals yellow or greenish yellow; stamens thigmonastic.
                     11 Spines white (at least on second year’s growth), generally round at the base and thus round in cross section, strongly retrorsely barbed or not; glochids stramineous-colored; mature fruit red, greenish yellow or pink; [restricted to FL]
                     11 Spines white to yellow or reddish-brown (on second year’s growth), often flattened at the base and thus lacunar or elliptical in cross section; glochids bright yellow; mature fruit usually dark purple or dark pink; [of coastal areas].
                       12 Glochid pattern generally of an adaxial crescent in the areole, i.e, glochids forming a dense fascicle in the upper portion of the areole, not widely separated throughout, exserted or not from the areole; spines yellow, sometimes with dark lateral banding; cladodes typically tuberculate with scalloped margins; [native, collectively widespread in coastal areas and also occasionally cultivated].
                          13 Cladodes usually few-spined, spines mostly 0-3 per areole, generally straight, produced erect from the areole or only slightly spreading, usually rounded, slightly twisted, or only slightly flattened at base, yellow
                          13 Cladodes spiny throughout, the spines 0-13 per areole, curved and spreading from the areole margins, or stiffly porrect, or mostly concealed within glochids
                            14 Spines reddish-brown in color (on second year’s growth), prominently twisted and with conspicuous dark, red-brown bands on the lower 80% of spine; unbanded spine tips translucent to opaque and whitish-yellow
                            14 Spines white to yellow (on second year’s growth), not prominently twisted or banded for most of the spine length, the marginal spines usually strongly flattened at the base, then with 1-4 central spines, which are dark yellow and commonly with dark brown or black bands towards to base and middle of the spine.
                              15 Spines 4-13 per areole, short and mostly concealed by glochids on mature stems so that the plant appears unarmed from a distance; tepals salmon in color
                              15 Spines 0-11 per areole, conspicuous on mature stems, curved and spreading from the areole; tepals bright yellow
                       12 Glochid pattern generally of a pin-cushion type, with the glochids widely separated and exerted from the areole; spines white with dark bases or yellow; cladodes not strongly tuberculate, the margins smooth not scalloped; [cultivated widely, and seemingly introduced in remote areas on barrier islands of SC, NC, and se. VA].
                                16 Spines yellow, with or without dark bases.
                                  17 Cladodes elongate, ovate or narrowly ovate (lanceolate) in outline, appearing indeterminate as a result of the lengthening and narrowing of the cladode apex
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