Abstract: The formation of sporangial sori on laminarian blades depends both on environmental and endogenous factors. In nature, most laminarian species form sorus during the slow growth period of the blade. There is recent experimental evidence for the existence of sporulation inhibitors produced by the actively growing basal blade meristem, moving in distal direction and preventing sorus formation. Sorus may readily develop during the active growth phase on isolated blade discs or distally of transverse cuts in entire blades, probably due to isolation from the inhibitor stream. In nature, growth activity of the blade decreases due to an endogenous circannual rhythm, possibly decreasing the sporulation inhibitor concentration down to a critical level enabling sorus formation. Light and temperature demands of sporogenesis for Laminaria digitata were investigated using isolated blade discs. Optimal sporogenesis took place in photon fluence rates of 20-200 µmol m-2 s-1; while no induction was achieved in continuous darkness. Daylength had a modulating effect with a significant promotion of sorus size in long days. Fastest sporogenesis was achieved between 10-17 °C while optimal sporogenesis with biggest sori took place in low temperatures (1-10°C). Possible effects on kelp reproduction due to global temperature change will be discussed.