It was Dobby who gave Harry the Gillyweed, after he heard a staged conversation between the fake ‘Mad-eye’ and Professor McGonagall.
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Dobby to the rescue
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The fourth film did not include Dobby, so Neville gave the gillyweed to Harry in the film. This was a neat change on the part of the film team because in the books Imposter Moody intended Harry to find out about gillyweed from Neville in the first place. Only when Imposter Moody found out Harry was too proud and independent (in his opinion) to ask anyone but Ron and Hermione for help, did he concoct the plan for Dobby to overhear him and Professor McGonagall talking in the staffroom.--EJ