How it works

A river that runs uphill.

When sunlight warms a leaf, water evaporates inside it and slips out through tiny pores. Each molecule that leaves tugs on the next, because water clings to itself, and the tug runs down an unbroken thread through twigs, limbs, trunk and roots to the finest root tips. The whole column rises. Two scientists in Dublin, Henry Dixon and John Joly, proposed this in 1894, and more than a century of measurement has borne them out.

Almost all of a kapok’s roots lie in the top meter of soil, running out past the edge of the crown, with a few sinkers and a taproot going deeper. In the drawing the building counts twenty floors of three meters. Four kapoks grow in the gardens; the first were planted in 2015. The same river runs through each of them, every sunny day.