Adam, the fibers are in the leaves. yucca and agave plants have very long broad leaves. The best way I have found to process them is to pound them between two smooth stones, and then scraping the outer layer of the leaves away. The pounding will separate the fibers down to very small hairlike fibers, when they are dry, take two bundles of them and start twisting! When the bundle starts thinning out, just weave in more fibers. The resulting string will be very strong.
Primitive Tim, What part of the us do you live in? I live in northeast texas, and we have yucca , but I have never found agave???
I have thouhgt about doing a tension test, but I think it's too risky haha