Water deficiency is the key limiting factor for pteridophytes in general, but particularly so in seasonally dry tropical areas (Kornaš 1985). The effect of moisture deficiency on pteridophytes may be particularly severe. Lack of moisture may inhibit intergametophytic mating. Also, juvenile ferns may suffer from drought, since the vulnerability of plant juvenile stages to drought tends always to be higher when diaspores are smaller (Ganade and Westoby 1999). Diellia species grow in relatively dry sites, and success of mating, as well as persistence of established individuals may therefore be low because of the lack of moisture in dry years. The regeneration of Diellia populations evidently follows the so-called “window of opportunity” (Jelinski and Cheliak 1992) model, where recruitment is a scarce event and is only possible in particularly favourable years.