Many recipes specifically call for either the white or the green part of the scallion. I have tasted both and can discern no difference. Is there a reason, aside from aesthetics, that a recipe would call for one and not the other?
Well, aesthetics isn't nothing; sometimes you just don't want the green. And I'm not sure that there's no difference in taste - I find the green tastes a bit more vegetal, the white more sharply oniony.
That said, when the scallion is to be sauteed slowly, I think recipes are justified in specifying "white portion only" because it will melt into a sweet, undefined mass. The green portion, with its more fibrous texture, will always retain its scallion-hood.
Erica Marcus writes for Newsday.