Y

This shape occurs in nature EVERYWHERE. In us, on many levels, in plants, in animals, in minerals, visible and microscopic.

Legs sprouting from a torso, fingers from a palm, branches from a tree trunk, twigs from a branch, antlers from a deer's skull, blood vessels, veins in a leaf, a nervous system, a root system, atoms organising into a chrystalline arrangement...is it a lung, is it a tree, is it a rock? 

Y ...a will forcing material to reach further.

Research:

Bent into shape: The rules of tree form