The first appearance of the Brain Tree in Essex folk-lore is in the Cronicle of Chelmesford. Under the entry for 1352, following a brief description of the recurrence of bubonic plague in this year, a meat-fisted scribbler briefly displaces the usual neat monkish hand and scrawls “Bee ware ye BRANETRE”. Recent attempts to link this with the catastrophic outbreak of bran-induced flatulence that rendered Chelmsford uninhabitable for large periods in the late 14th century have been refuted, and Dr. Plectrode’s suggestion that this is a first-hand witness of the Brain Tree remains the most plausible.
According to legend, the Brain Tree is a large willow usually associated with the River Pant (previously Shitpant), although some stories also link it with the Blackwater. It lulls weary travellers to sleep in its shade, then wraps their heads in its long trailing branches, and eats their brains. The victims are then discovered by passers-by, incapable of rational thought or coherent speech, and are integrated seamlessly into Essex society. In the sixteenth century, belief in the Brain Tree was so strong that mobs swept the Essex countryside looking for willow trees to burn them down, with the result that Essex now has fewer oak trees than any other region of comparable size in England. The last recorded mention of the Brain Tree in Essex oral tradition comes from the Reminiscences of Derek Poke (1884), in which he mentions that a tree branch knocked his uncle’s brains out, but that his uncle survived to father four children and become a vicar after a pig’s brain proved to be an adequate substitute.
from The Book of Essex Monsters by Prof. Dreg Twedloxx & Assorted Authors (1947)
