The blue Violets that flower in April are very much alike. See here the Common Dog Violet (Viola riviniana).
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It look rather rectangular. It has to be admitted, violets do show a temperament.
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This one grows in woods on neutre or slightly acidic soils in little rather loose bunches. If there is a leaf rosette of a kind, it is not at ground level but halfway up a stalk.
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Common Dog Violet has a big white spur and pointed sepals...
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... and a striped lower lip.
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Here it grows between leaves of a Sweet Chestnut that, like the Violet, does not like calcareous soils.