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Vrksasana - Tree Pose

The sole of the foot presses into the calf or thigh of the opposite leg, as the calf or thigh muscles press reciprocally into the foot. This creates contraction of all the muscles in the inner legs, which form the trunk of the tree. 

Avoid pressing on the knee, but you may wrap your heel slightly behind the knee, with toes coming to the inside front of the shin.

The top of the pelvic bone (Illiac crest) stays parallel with the ground, as normally in standing, so the spine rises straight up. Raise the arms to describe whatever angle of branches you wish. The hands too form leaves of your choice.

My hands form anjali mudra (angel hands) among a stand of dying trees. Alaska, 2013.

 Tree branches, by Abigail Allen Art, Gladwyne, PA.