"The down of a thistle"

From Clement Clark Moore’s poem “A Visit from St. Nicholas”, I’m wondering what the expression “down of a thistle” means, and how it applies to the departing sleigh. I’d never really thought about it until someone asked & I didn’t know…how someone here does. Thx:-)

I never would have thought I’d find a Reading Comprehension Guide for 'Twas The Night Before Christmas which states:

down of a thistle = the light material on a certain type of weed that floats away in the air

also, the second picture on Wikipedia’s entry for thistle may help you make sense of it.

The image: Thistle seeds are much like dandelion seeds and are easily launched into the air with the slightest puff of breath, i.e. effortlessly.