One type of phenolic that plants find very useful in fruit is the vicinal (or ortho-) diphenol. This is simply a phenol with two -OH groups in adjacent positions on the ring. This molecule has the magical ability, in reaction with oxygen, to attach itself to another phenol and then to re-create its original reactive diphenol structure. This odd trait allows it to react over and over, daisy-chaining to create long polymers. The browning we see in freshly cut apples and bananas is polymerized diphenols, sealing the fruit in case of injury.
