AbstractTetraphis geniculata, of North American origin, grown in a north-facing glasshouse in Britain, showed marked seasonality in the duration of protonemal growth before the appearance of thalloid protonemal appendages and shoot buds, in the number of thalloid appendages formed, in the variation in the number of leaves borne on shoots and in the production of gemma-cups. The plastochron scale was used as a measure of earliness in the production of gemma-cups. Shoots formed early in the year, following a prolonged protonemal phase, had a low average leaf number but later in the year, when the protonemal phase was reduced, the average leaf-number was increased. In this period of the year leaf-number varied with temperature; the higher the temperature, the higher the average leaf-number. When the average maximum monthly temperature in midsummer was in the range 23°–28.5°C there was complete suppression of the development of gemma-cups and therefore a prolonged period of vegetative growth.