Escobaria vivipara
 
Pincushion Cactus

Tubercle Top

Oro Lake Regional Park
26-August-2011

Note the stem is covered with rounded projections called tubercles.  Spines arise from an area at the apex of each tubercle called the areole.  There are 3-4 (uncommonly more) central spines pointing more or less at right angles to the surface, and 12 to 40 radial spines pointing more or less parallel to the surface.

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Mamillaria/Coryphantha: Note that Budd's Flora places this species in genus Mamillaria, while Flora of Alberta places it in genus Coryphantha.  The following are the answers to the key questions in these flora that lead to Mamillaria or Coryphantha. 
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globose or cushion-like plants; NOT [plants branched and jointed, the stems more or less flattened]

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flowers red or purple; NOT [flowers yellow or orange]

 

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Cactaceae: Answers to key questions in Budd's Flora  leading to this family. 
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succulent, spiny plants with leaves absent or scale-like, inconspicuous; NOT [plants with leaves]

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flowers with two floral rings, and with each petal distinct from the others