To all of you that are interested in the symbolic meanings of ikebana - please check this out. Sogetsu teacher Ping Wei has an interesting comment on the principles of heaven, earth and human on his new blog.
"Heaven" is the sky, the freedom. In order to fully express "heaven" in our ikebana, we need to be free. ... "Earth" is the ground, the disciplines and the rules. Without disciplines and rules, things will fall apart. ... We "human" are in the middle. Our job is to bring "heaven" and "earth" in perfect harmony. It's not an easy task, but that's what we keep studying for.
(Quote from Ping Wei, Ikebana in Desert on Blogspot)