What's the difference between appropriation and plagiarism?

In an article for Design Observer designer and author William Drenttel writes about how ideas come from many sources in graphic design: they recur, regenerate, take new forms, and mutate into alternative forms. In the world of design and photography, there seems to be an implicit understanding that any original work can and will evolve into the work of others, eventually working its way into our broader visual culture.Drenttel goes on to talk about how the charge of plagiarism is not a simple one. He says, “Designers should take note: the idea of borrowing ideas is getting more complex everyday. Inherent in the modern definition of originality, though, is that ideas are extended, language expanded, and syntax redefined. Take a psychologist’s ideas and experiences, as explained through the eyes of a journalist, and turn them into a play, a work of fiction—this is a work of complex, ‘appropriation,’ I believe the design world benefits greatly from such an understanding of complexity.”How does a designer know where the boundaries are when finding the line between appropriation and plagiarism?Source:http://www.designobserver.com/observatory/entry.html?entry=2837