Abstract
1. Over a wide range of salinity (10 to 200 per cent sea water) cells of Porphyra perforata accumulate K and partially exclude Na. Apparent osmotic volume is nearly constant between 50 and 100 per cent sea water. This imperfect volume control is thought to be due to differential shrinking and swelling of structural polysaccharides and not to active water secretion. 2. Survival of Porphyra in diluted and concentrated sea water is discussed with respect to ion transport. Rain is potentially a more serious threat to survival, due in part to breakdown of ion transport, than is concentration of sea water. Calcium seems especially important in maintaining membrane selectivity toward K and Na.
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