Biologists have discovered why reputedly harmful radicals are important to plant fertilization.

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Scar in the carpel of the Arabidopsis thaliana plant: The pollen grains (green on top of the scar) germinate and form pollen tubes (green strings). Fertilization takes place once the tubes reach the ovules. Without reactive oxygen species, the process is disrupted (right) .Source: Roman Lassig 

Supplemental movies visualizing the growth of pollen tubes

Source: Roman Lassig