Evidence for inhibitors of sporangium formation in Laminaria digitata (Phaeophyceae) during the season of rapid growth (Note)

The pathway from the intercalary frond meristem to distal frond portions wa s blocked in experimental sporophytes of Laminaria digitata (Huds.) Lamour. 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 adjacent to the distal edge of the holes within 5-10 weeks. Frond portions that were cu t at least 20 cm away from the stipe-frond transition readily formed sori w ithin 5-7 weeks after isolation from the rest of the thallus. These finding s suggest that the basal actively dividing and expanding part of the lamina rian frond is the source of inhibitors of sporangium formation that 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 occu rrence of sori during the season of slow growth (i.e. the second part of th e rear in the Northern Hemisphere) may be due to reduced synthesis and expo rt of inhibitors of sporangium formation from the more or less resting basa l meristem.