Sorus development on median and marginal parts of the sporophyte of Laminaria japonicaAreschoug (Phaeophyceae)

Sorus formation on median and marginal parts of Laminaria japonciaAreschoug was investigated by cultivating sporophytes from December 1996 to September 1997 in Southern Hokkaido, Japan. Simultaneously, discs (ca. 3 cm in diameter) that were cut off monthly from median and marginal parts of the cultivated sporophytes were incubated in our laboratory. All the cultivated sporophytes formed sori on the median parts from January to August, while all the median and marginal discs formed sori at 10 under 46 μmol photon m-2s-1(12L:12D) in enriched medium. There was no difference in sorus area between median and marginal discs. These results indicated that the marginal parts are able to produce sori, although the greater part of them are actually washed away before forming sori in the sea. The marginal part is probably in a state of `resource limitation' on the reproduction because the resource outflowed from this part to the basal part for the growth of sporophytes.