They're not, of course, but they are common to yuccas.
One of the main differences between yuccas and agaves is these white fibers. About half of all yucca species have these threadlike hairs.
Sadly, I tossed the pot it came out of before writing down which variety this is -- perhaps my friends at Vivero will help me with a reminder?!
In any case, I loved the idea of this stark, spiny, rugged-looking yucca paired with the delicate, flowing clump of black foot daisies.
This is the kind of contrast I find so beautiful in the drought-tolerant native and adapted plants that are home to Central Texas. Do you have some pairings like this in your garden?