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Photograph (A) and schematic model (B) of heteromorphic SI in primrose. (A) The pin (long pistil and short stamen length) form is shown on the left, and thrum (short pistil and long stamen length) is on the right. (B) Selfed pollen and pollen derived from the same flower form are incompatible. Crossed pollen and pollen derived from the different flower form are compatible.
