Old Man Willow cast a spell on the hobbits (Frodo, Sam, Merry and Pippin), causing them to feel sleepy. Merry and Pippin go to lean against the trunk of the willow and fall asleep, while Frodo sits on a root to dangle his feet in the water, before he also falls asleep. The willow then traps Merry and Pippin in cracks of its trunk and tips Frodo into the stream, but the latter is saved by Sam, who, suspicious, manages to remain awake. After Frodo and Sam talk about possibly burning the tree so that it is frightened enough to release the others, Merry yells from inside to put the fire out at the risk of the tree squeezing them to death. They are saved by the timely arrival of Tom Bombadil who 'sings' to the ancient tree to release Merry and Pippin. The tree then ejects the two hobbits.
According to Tom Bombadil, of the corrupted trees of the Old Forest, "none were more dangerous than the Great Willow; his heart was rotten, but his strength was green; and he was cunning, and a master of winds, and his song and thought ran through the woods on both sides of the river. His grey thirsty spirit drew power out of the earth and spread like fine root-threads in the ground, and invisible twig-fingers in the air, till it had under its dominion nearly all the trees of the Forest from the Hedge to the Downs."
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*update* Thanks to Zinnware for coord update.