Same thing that holds a wooden arrow together when you take the bark off, saw it into pieces, and sand them. Plus a coat of varnish! I've seen such arrows used in SE Asia.
Split 'em square; run em through one of those dowel rounding rigs like I've seen here, then fine sand and varnish. Those "long thin strands" will run pretty much the length of the arrow. Heck - it's worth a try. I want lightweight bamboo/cane arrows, and everything I've seen is 'way too heavy. The "fly rod" shafts are light, but 'way too much work for everyday arrows...
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Well, I just went out and tried it with a much thinner (1/8") piece of culm. Split a "shaft". I don't have a rounding jig yet, so I used a scraper to head the 4-sided split in the direction of 8 sides. Then applied 120 grit sandpaper. Smooth as the proverbial baby's bottom.
Try it Mullet. You might like it! Or send a few thick-wall splits this way and I'll see what I can do.