Abstract
The sporophyte of Laminaria hyperborea produces a new frond at daylengths of up to 12 h (white light). This short-day response is completely inhibited by a 1-h white light-break given in the middle of a 16-h dark period. Temperatures of -1·5-, 2-, and 8°C were found to be favourable for new frond formation whereas 15°C was largely inhibitory.