The instructions here are to break up the plants and add them to the
pulp - no cooking needed. In this case, the plants would show up as
When you cook the plants - the fibers become more a part of the sheet
of paper - the fibers "weave" themselves into the cotton or abaca
fibers (or whatever pulp you are using). You can also make paper
entirely out of plant fibers if you cook them in soda ash (lye).
Maybe check your library for this book:
The Art of Papermaking with Plants (Paperback) - by Marie-Jeanne Lorente.
I will forward your question to Elizabeth - she is not on Getty list.