Kthysh was a type of vine native to the planet Kashyyyk. Specially treated kthysh vines could be used to make strings for that could be used to manually load bowcaster. Many warriors carried treated kthsyh vines in their utility pouches for emergencies.[1]
Chewbacca carried some kthysh vines in his bandolier pouch.
Sources
- The Essential Guide to Weapons and Technology
- The Official Star Wars Fact File 16 BOW 1-2: Chewbacca's Bowcaster
- The New Essential Guide to Weapons and Technology
"May The Facts Be With You: Part Two: #51-100" — Star Wars Insider 95 (reprinted in Special Edition 2010 and The Original Trilogy)