Nieuwhof (1969) attributed this self-incompatibility to the fact that pollen on the stigma of the same plant germinates poorly, and he agreed with Knuth (1908*, pp. 74 - 128) that this incompatibility is strongest in freshly opened flowers.
(Nieuwhof (1969; attributed; this self-incompatibility; to the fact) (he; agreed; with Knuth) (this incompatibility; is; strongest in freshly opened flowers)
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attributed agreed is
this self-incompatibility to the fact
with Knuth
strongest in freshly opened flowers