in Thoreau’s Journal:

Sometimes I would rather get a transient glimpse or side view of a thing—than stand fronting to it.  As those poly-podies— The object I caught a glimpse of as I went by haunts my thoughts a long time—is infinitely suggestive—& I do not care to front it & scrutinize it—for I know that the thing that really concerns me is not there, but in my relation to that. That is a mere reflecting surface. 

It is not the polypody in my pitcher or herbarium, or which I may possibly persuade to grow on a bank in my yard or which is described in botanies that interests me—but the one that I pass by in my walks a little distance off—when in the right mood. Its influence is sporadic—wafted thro’ the air to me— Do you imagine its fruit to stick to the back of the leaf all winter? At this season polypody is in the air—  It is worth the while to walk in swamps now, to bathe your eyes with greenness— The terminal shield fern is the handsomest & glossiest green.