In some cases the seed heads of crop circle plants do not contain any seeds. This mainly occurs in formations formed in young crop, i.e. formations that appear in April - June, when the crop is not yet ripe.
Left: normal controle seed heads with seeds
Right: crop circle seed heads without seeds
In 2004 I found this wheat plant in a formation near Wijchen (the Netherlands). The upper half of the seed head had shrunk and the seeds had disappeared:
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