Ecologically, the ferns are most commonly plants of shaded damp forests of both temperate and tropical zones. Some fern species grow equally well on soil and upon rocks; others are confined strictly to rocky habitats, where they occur in fissures and crevices of cliff faces, boulders, and taluses. Acidic rocks such as granites, sandstones, and quartzites are associated with characteristic fern species different from those of alkaline rocks such as calcites and dolomites. A few species appear to be confined to serpentine and related rocks. In the tropics as many as two-thirds of the ferns of an area may grow as epiphytes on the shaded lower trunks and branches or in the crowns of trees. A few so-called epiphytic ferns are actually climbers that originate upon the ground and grow up tree trunks. In these the lower leaves (bathyphylls) are usually vegetative and are often different in form from those at the higher levels (acrophylls), which are entirely or partly fertile in that they bear sporangia over their surfaces.