The latex of the opium poppy is the most famous of these. I doubt that anybody has tried to chemically characterize the latex of Olona. This could be an good research project. The cell walls of Olona fibers are thick and are composed of pure cellulose without much lignin. Consequently, they are very flexible.
Flax fibers have similar characteristics.
Flax fibers are stronger when they are wet. They were used to make fire hoses because of this. It would be interesting to see if the same was true for Olona. if it is so, this might also explain why they were so useful for naval ropes.