Factors Influencing the Narrow Restriction of Pilularia americana in Texas

Abstract

Pilularia americana A. Br., was investigated in the laboratory in an effort to determine the factors influencing its restriction to semipermanent ponds in the granitic areas of Burnet and Llano Counties, Texas. Soporophytes transplanted to various soils and under various levels of submergence showed a broad range of tolerance but were strongly inhibited in development in calcareous soils. The sporophytes did not develop sporocarps in rapidly drying soils and were killed by complete desiccation. The spores germinated under a range of photo- period-temperature conditions, but spore germination was inhibited by calcareous soils and by certain other edaphic conditions. Sporocarps which developed in the laboratory under a range of conditions yielded viable spores. The niche of Pilularia americana restricts it in Texas to semipermanent pools on acidic, granitic soils. This specialized niche may play a major role in the restricted distribution of P. americana throughout its range. Pilularia americana, one of the rarest ferns in North America, was reported in Texas from Burnet County by LaMotte in 1940; it was also collected in Llano County during the present study. The plant was investigated in the laboratory to determine factors contributing to this apparent narrow restriction. The sporophytes were transplanted to different soils and placed under different levels of submergence. The spores were germinated in different media and under different photo- period-temperature conditions. The results of these investigations sug- gest that Pilularia has a soil-moisture relation that is restrictive to its distribution.