shape, line, and form
More than simply putting flowers in a container, ikebana is a disciplined art form in which nature and humanity are brought together. Contrary to the idea of floral arrangement as a collection of particolored or multicolored arrangement of blooms, ikebana often emphasizes other areas of the plant, such as its stems and leaves, and draws emphasis toward shape, line, form.
-From Wikipedia: Ikebana article
Here are a couple arrangements that caught my eye at the Japanese festival that was held in downtown Phoenix.