we think of all the water on Earth as an Olympic-sized swimming pool, the amount that’s available to plants within the soil would fill less than one soda bottle. Trees require so much water—more than a gallon is needed to build a handful of leaves—that it is tempting to envision the roots as actively sucking the soil. But the reality is quite different: the roots of a tree are absolutely passive. Water flows passively into the roots during the day and passively out of them at night, faithful as the tides of the ocean drawn by the moon. Root tissue functions like a sponge: when placed dry upon
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