I found an interesting article which discusses the idea that scientists may not have been able to locate plants on other planets from our own because these plants may be black in color! You can read about this hypothesis by clicking on the following link:
http://www.ecanadanow.com/space/2011/04/25/black-plant-life-could-be-on-other-planets/ This link
will take you to an article entitled, " Black Plant Life could be on other planets".
Here is a link to a Scientific America magazine podcast about this subject matter:
A text version of this podcast can be read by clicking on the following Scientific America website link: