Approximately one-third of the Earth's land surface is classified as arid or semi-arid. The classic 'hot' deserts of the world (e.g., Sahara) occur between 20 and 30 latitudes N and S, but arid conditions are found across the globe. Most arid areas experience high daily temperatures as well as low and unpredictable precipitation (<200 mm per year). Water lost by evapotranspiration often exceeds that gained by precipitation, resulting in water deficits. Factors contributing to aridity include rain shadows, distance from water, proximity to cold oceanic currents, and localized soil conditions. Even in wet tropical habitats, some plants (e.g., many epiphytes) experience periodic drought conditions because of microclimatic factors. Aridity presents special challenges to plants. As a group, pteridophytes are rare in extreme desert environments, but many grow in persistently and/or seasonally dry habitats. A very few of these are drought-avoiding ephemerals that complete their life cycle during wet periods. The majority, however, are drought-tolerating perennials. Typical xeromorphic adaptations include dissected or coriaceous leaves, dense indumentum, and/or cuticular deposits. In addition, many ferns and fern allies endure drought periods through physiological dormancy (e.g., the poikilohydrous resurrection plant, Selaginella lepidophylla). It is important to note that selection pressures act on gametophytes as well as sporophytes, but little is known about gametophyte adaptations to drought. Taxonomic diversity of xerophytic pteridophytes is concentrated in relatively few families: e.g., Pteridaceae and Selaginellaceae. However, xerically-adapted species occur in most lineages. Of note are several epiphytes in the Polypodiaceae as well as some typically aquatic species whose habitats are intermittently dry (e.g., Isoetaceae and Marsileaceae). Hotspots of pteridophyte diversity can be summarized for arid regions with reasonably complete floristic data.

Key words: arid climates, deserts, drought adaptations, pteridophytes