These small ferns on this oak tree are resurrection ferns (Pleopeltis polypodioides). The resurrection fern gets its name because it can survive long periods of drought by curling up its fronds and appearing desiccated, grey-brown and dead. However, when just a little water is present, the fern will uncurl and reopen, appearing to "resurrect" and restoring itself to a vivid green color within about 24 hours. It has been estimated that these plants could last 100 years without water and still revive after a single exposure.
When the fronds dessicate they curl with their bottom sides upwards. In this way, they can rehydrate the quickest when rain comes, as most of the water is absorbed on the underside of the leaf blades. Experiments have shown that they can lose almost all their free water—up to 97%--and remain alive, though more typically they only lose around 76% in dry spells. For comparison, most other plants would die after losing only 8-12%. This fern can lose almost all the water not hydrating the cells in its leaves and survive. When drying, the plant synthesizes dehydrins which allow the cell walls to fold in a way which can be reversed later (Info from Wikipedia and Florida Plant Atlas).
This plant has even been the subject of an experiment in which it was sent to space to test it's dormancy ability in microgravity. The researchers were investigating the plant's characteristics for use as a potential food source for long-duration space missions.