Via Atlas Obscura (highly addictive).The Ficus elastica produces a series of secondary roots from higher up its trunk and can comfortably perch atop huge boulders along the riverbanks, or even in the middle of the rivers themselves.
The War-Khasis, a tribe in Meghalaya, long ago noticed this tree and saw in its powerful roots an opportunity to easily cross the area's many rivers. Now, whenever and wherever the need arises, they simply grow their bridges. In order to make a rubber tree's roots grow in the right direction - say, over a river - the Khasis use betel nut trunks, sliced down the middle and hollowed out, to create root-guidance systems:
The thin, tender roots of the rubber tree, prevented from fanning out by the betel nut trunks, grow straight out. When they reach the other side of the river, they're allowed to take root in the soil. Given enough time, a sturdy, living bridge is produced.