Oxalis stricta
 
Yellow Wood-Sorrel

Stamen Tube Cut Open

Regina
07-July-2017

The stamen tube has been split to reveal the pistil inside.  The pistil is composed of five united carpels.  The pubescent styles contrast with the glabrous filaments.

I am including S. europaea within a larger S. stricta.  The key to the families in Budd's Flora requires "stamens usually separate or partly so, not in a column".  This is not true.  The filaments in S. stricta are united in a column around the pistil.  I have deleted that item from the family key answers below.

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Stricta: Answers to key questions in Budd's Flora leading to this species. 
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stem NOT rooting at nodes; NOT [stem often rooting at nodes]

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stem erect or decumbent in age; NOT [stem creeping]

 

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Oxalidaceae: Answers to key questions in Budd's Flora leading to this family. 
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herbs; NOT [shrubs or trees]

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plants terrestrial or semiaquatic; NOT [plants aquatic; leaves submerged or floating]

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plants NOT with colored milky juice

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plants with more than one normal leaf

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some or all leaves alternate; NOT [leaves opposite, whorled or basal]

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leaves with stipules; NOT [leaves without stipules, or having glands]

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leaves of 3 leaflets; NOT [leaves palmately divided]

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flowers with two floral rings

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flowers regular in shape; NOT [flowers irregular in shape]

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calyx regular; NOT [calyx irregular, some sepals smaller than others]

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each petal distinct from the others

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stamens 5 or 10; NOT [stamens usually numerous]

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ovary of 5 united carpels; NOT [ovary of one or more carpels, either separate or enclosed by a fleshy receptacle]

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fruit not beaked; NOT [fruit with a long beak]