An Excerpt from Jim Conrad's
from the January 12, 2007 Newsletter issued from Sierra Gorda Biosphere Reserve Headquarters in Jalpan, Querétaro, MÉXICO
FLOURY CLOAK FERN
On both sides of the reservoir several miles of one-lane gravel roads have been cut into the mountainsides. Besides displaying fine examples of the region's much folded and faulted limestone geology, the roadcuts display a goodly number of plants adapted to rock faces, especially ferns. One particular fern species is common and very distinctive -- and I've had a dickens of a time trying to identify it because of the lack of fieldguides here -- catches your eye nowadays because its frond undersurface is chalky white. When the frond is dry its blade margins curl up showing the whiteness, as seen above.
The fronds in that picture are pretty curled up, but since the picture was made it's been dry and now those fronds look like little more than popcorn puffs on black stems.
I'm about 90% sure it's the Floury Cloak Fern, CHEILANTHES FARINOSA. It's called "floury" not only because of the whiteness but because the whiteness rubs onto your fingers like flour.