I think that plants and trees around the house are as important of a design element as any. They obviously add a necessary organic element to a modern element but they also create visual interest. A tall tree in the corner of a room with a romantic swoop is a terrific source to infuse personality into your space.
And don't forget that plants have a real impact on air quality. There are a ton of fairly common plants that do a decent job at cleaning the air. But don't take my word for it, NASA has it all sorted out for you in this article.