Lip ferns
The genus Cheilanthes comprises some 150 species, primarily distributed in the Western Hemisphere with a few species found in Eurasia, Africa and Australasia. They are xerophytic ferns with numerous adaptations to the deserts and dry, rocky regions in which they thrive. Some species are restricted to igneous rocks, e.g., granite, or sedimentary and metamorphic rocks, e.g., limestone or dolomite, whilst others are more cosmopolitan in the rocks upon which they will grow.
One of their most obvious features is that they curl up, become brown and go dormant during dry periods. When rains arrive, they turn green. They are so-called 'resurrection plants', technically they are poikilohydric, capable of tolerating low water content without sustaining physiological damage to their cells.
As with many ferns, the spores of lip ferns are produced on the lower leaf surface and when the plants dry these are exposed to the sun. The leaves have many adaptations to life under dry conditions. The leaves are small, and the leaf segments are often rounded, which has the effect of reducing the area over which water can be lost. Other adaptations to dry environments include a leaf margin with a thickened ridge, which adds rigidity to the leaf when it is dry and protects the spore-producing sori. The thickening gives the leaf segments a superficial appearance of lips, hence giving the ferns their common and generic names.
Lip fern leaves are covered with soft hairs, which in some species form a dense, felt-like layer. This is usually thicker on the lower than the upper surface. The hairs shade the leaf surface from high-light intensities, and trap humid air, preventing water loss. In some Cheilanthes species, there are specialised flat, translucent scales on the leaves which reflect light under high-light intensities, and shade the leaves.
Other adaptations for life with limited, seasonal water availability include leaves protected by thick layers of wax. The cells in the leaf stalks and stem may be impregnated with hydrophobic suberins and filled with water-retaining gums and tannins.
The roots of some Cheilanthes species have close associations with fungi. The mycorrhizae formed are important for nutrient uptake and, perhaps, also coping with water stress.
Fern spores germinate to produce thin, green structures (prothalli), which in most ferns are highly prone to water loss. In the case of lip ferns, the prothalli are capable of recovering from desiccation. Prothalli will eventually produce a young fern.
Further reading
Harten JB and Eickmeier WG 1987. Comparative desiccation tolerance of three desert pteridophytes: response to long-term desiccation. American Midland Naturalist 118: 337-347.
Hevly RH 1963. Adaptations of cheilanthoid ferns to desert environments. Journal of the Arizona Academy of Science 2: 164-175.
Palmieri M and Swatzell LJ 2004. Mycorrhizal fungi associated with the fern Cheilanthes lanosa. Northeastern Naturalist 11: 57-66.
Stephen Harris