In Texas, their preferred roosting sites are the frond "skirts" of both wild and ornamental palm trees, such as Sabal mexicana and Washingtonia robusta. These are collections of dead fronds against the trunk and provide a favored dark habitat for the bats.
The passage describes where bats prefer to roost in Texas, specifically on the edges of palm tree fronds that are located against the trunk. This provides a dark and secluded environment for them.