1. Spores of Pteris vittata L. germinated and grown in the light on a complex medium produce the usual cordate gametophytes. Callus formation occurred when very young gametophytes on the same medium were transferred to, and maintained in, the dark. 2. Three types of proliferation can be distinguished. (a) Type 1 callus is extremely friable, and subcultured fragments show no differentiation for a long time. When these friable tissues are lightly agitated in a liquid medium, small tissue pieces, a suspension of small groups of cells, or single cells are easily obtained. In either light or darkness the single cells or cell aggregates can grow on the complex medium. On a defined medium, they will not grow in darkness but, occasionally, will form normal richly chlorophyllous gametophytes in the light. (b) Type 2 calli tend to form sporophytes in the dark and gametophytes when placed in the light or transferred to a defined medium. (c) Calli of type 3 form only sporophytes and that relatively rarely.