The Romantic poet John Keats once defined a state of mind wherein a person "is capable of being in uncertainties, mysteries, doubts, without any irritable reaching after fact and reason." He called it negative capability. It stands in direct contrast to the quality known as negative pressure, a scientific conundrum that recently caused much irritable grasping after fact and reason among Discover readers.
The trouble started last September, when Discover published my Works in Progress column about how water gets to the tops of trees. As water evaporates from a tree's leaves, I reported, it tugs on water remaining in the xylem, a network of inert pipes that reaches from roots to shoots. Water molecules in the xylem, I innocently explained, are linked by bonds that form among their positively and negatively charged poles. As water evaporates up top, it stretches the bonds between interlinked molecules in the column below, generating considerable tension. "The pressure inside the xylem tubes of the tallest trees could be as low as -20 atmospheres," I wrote.
