The pathway from the intercalary frond meristem to distal frond portions was blocked in experimental sporophytes of Laminaria digitata in January by cutting 2 or 3 holes (15-25 mm diameter) 5 cm from the base of the frond. This procedure resulted in the formation of sori within 5-10 weeks adjacent to the distal edge of the holes. Frond portions that were cut at least 20 cm away from the stipe-frond transition readily formed sori within 5-7 weeks after isolation from the rest of the thallus. These findings suggest that the basal, actively dividing and expanding part of the laminarian frond is the source of inhibitors of sporangium formation which move in a distal direction and keep the young frond free of sori during the season of rapid growth, i.e. during the first part of the year. The natural occurrence of sori during the season of slow growth, i.e. the second part of the year in the Northern hemisphere, may be due to reduced synthesis and export of inhibitors of sporangium formation from the more or less resting basal meristem.