Printing out Plants

Several years ago (or more, few of my older journals are properly dated)  I became inspired by developments I had seen in the field of 3-D printing technologies, especially those that were being developed to produce replacement organs.  The idea that you could work from a canvas of living tissue as opposed simple metals and plastics was wondrous.  From that overwhelming image came a question for me, why not print out plants or coral reefs.  With advancements in biotechnology we might have the ability to print new ecosystems.  I began to imagine cities where the walls of buildings were living trees, not just dead concrete and metal.  I have since learned that grafting of more naturally grown plant structures is by leaps and bounds more cost effective, that being said I think it would be really really awesome if we could expand urban green zones into the z axis.
Visual Follow Up (Feb 7 2013)

Apologies to whoever on Imgur I stole this from,
Reddit didn't provide any copyright info.

Imagine structures like this, but with tree branches extending out, providing additional shade and productive biome.