All of the plant tinders I have found I have discovered through experimentation before I read 18th century accounts. Some of these are indeed mentioned in period accounts, some under different names such as this one below, others I have found are not mentioned at all.
Alexander Mackenzie; among the Indians, 1789.
Touchwood is another name for punk wood. Here is another:
Patrick Campbell, 1792.
Spunge is also punk wood, and this name gives you some indication of what this tinder feels like. With the knowledge of what this tinder looks like and feels like it enables you to search out other plant materials of similar look and feel, and experiment to see if it will catch a spark, or work as an extender. An extender is a plant material that will not necassarily catch and hold a spark like true tinder will, but it will take from a smouldering ember and can add to the heat when blown upon in the tinderbox.
Treating tinder with saltpetre/potasium nitrate is all very well, but it is not a lot of good for extended use in the woods. So whatever material you find, first see if it will catch and hold a spark as is, and if it does not work, then try charring it a little in the fire.
The information in these posts is taken from the revised edition of Primitive Fire Lighting, Flint & Steel, which is now a part of "The New World Woodsman. His clothing, equipment & accoutrements. 1700-1760". By Keith H. Burgess. This new book is in its final stages of editing, and its publication date will be posted on this blog.