Granite outcrop specialists

By Amy Weiss

May 15 2019

Plants are amazing and can adapt to live in the most unlikely places—like shallow, ephemeral pools that form in depressions on granite outcrops. Three aquatic plant species that are uniquely adapted to life in these granite pools are federally listed in the United States as being threatened or endangered:

  • black-spored quillwort, Isoetes melanospora
  • snorkelwort, Gratiola amphiantha (listed under its previous name Amphianthus pusillus)
  • mat-forming quillwort, Isoetes tegetiformans

All are native to the Piedmont region of the southeastern U.S. and grow nowhere else. The vernal pools where they live fill with water during heavy rains, retaining the water for several weeks before drying out with summer droughts. These three species already have a limited area where they can thrive; plus their populations are threatened by quarrying and dumping, and by recreational impacts such as littering, trampling, and vandalism. A few pools have been fenced off in areas with heavy visitation, which has helped alleviate some of the foot traffic and littering.  

Isoetes deserves a special mention, as they are lycophytes; a group that, like ferns, has vascular tissue and produce spores, but has a separate evolutionary history from true ferns. These tiny Isoetes are all that remain of a lineage of plants that used to include lepidodendrids, huge tree-like plants that for almost 40 million years dominated the swamps of the late Carboniferous before becoming extinct in at the end of the Permian.

Explore further:

There are photos of these pools and outcrops on Wikimedia Commons.


Moran, R. C. 2004. A natural history of ferns. Portland, OR: Timber Press.

Nash, E. et al. 2008. Three granite outcrop plants: Black-spored quillwort (Isoetes melanospora), mat-forming quillwort (Isoetes tegetiformans), little amphianthus (Amphianthus pusillus). Five-year review: Summary and evaluation. Athens, GA: U.S. Fish and Wildlife Service.