To investigate the possible causes of the contrasting geographical distributions of Saccorhiza polyschides (Lightf.) Batt. and S. dermatodea (Pyl.) J. Ag., sporophytes of both species were cultured from zoospores under a variety of conditions. The spores of both species were able to survive in the dark for at least 6 months. Thus although, unlike S. dermatodea, S. polyschides is not found within the Arctic Circle, it too can survive the equivalent of the Arctic winter night without light.
Water temperatures from 3° C to 25° C were found to influence every stage in the development of both plants from spore settlement and germination, to the production and rate of growth of the sporophytes and it was particularly critical to the fertility of the female gametophyte and to the survival of the sporophyte. The female gametophytes of S. dermatodea became fertile only between 3°C and 10°C, producing numerous sporophytes at 3°C and 5°C but rather few at 10°C. In contrast, those of S. polyschides produced an abundance of sporophytes from 5°C to 17°C, relatively few at 20 and 23°C and none at either 3°C or 25°C. Some grossly abnormal sporophytes were produced at the temperatures close to the tolerance limits of the species. Gametophytes cultured at temperatures unfavourable for sporophyte production survived, but rapidly lost their ability to produce sporophytes.
The close correlation between these tolerances and the sea temperatures prevalent at the geographical limits of each species strongly suggests that the absence of S. polyschides from within the Arctic Circle is related to its inability to reproduce sexually or to survive at low water temperatures and that the southward extension of both S. dermatodea and S. polyschides and the unusual distribution of the latter species in the Mediterranean are probably the result of their failure to tolerate the higher sea temperatures during the growing period in summer.
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