Certain roots, such as spruce, make excellent cordage. You don't even
have to twist them together -- just use them as is, or split them
into halves or quarters.
This photo shows a spruce root that has just been
extracted from the ground.
The roots are found close to the ground surface (up to
about 6" down), and usually extend outwards from the tree in a
fairly straight line.
You can often find roots several feet in length.
Traditionally they have been used to "sew"
together birch bark, as in birch bark canoes, wigwams, baskets, etc.