If you find yourself empty-handed with your pants down, then nature has many potential substitutes for the fluffy stuff. But most Americans' diets are too rich to perform the clean pinch, especially if they stopped at McDonald's on the way into the woods. More often than not, you are forced to improvise and find a suitable substitute for the soft, fluffy stuff. When in search of leaves and grasses, choose the soft specimens and not the slick ones. Beware of sharp, cutting edges.
Large leaves work better than small ones.
Of course, know your poisonous species. Lichen is clearly the queen of nature's toilet papers, and don't underestimate the usefulness of a smooth rock, a pinecone, or a stick. Many campers in the past have had to use quick thinking and sound judgement to satisfy their emergent needs. These improvisations undoubtedly require split-second decision making, weighing the relative benefits of all the choices. Unfortunately, some choices made by campers have not been the most environmentally sound.