Considerable obscurity surrounds the mechanism of calcification in all biological material. The coralline algae, which lay down quantities of calcium salts in their tissues, are no exception. These forms are common in the littoral and sublittoral in exposed situations in northern waters and make up a large part of many of the reefs in tropical seas. This paper describes observations and experiments relating to growth and calcification in two northern corallines, Clathromorphum circumscriptum Foslie and Corallina officinalis Linn. Calcification is related to the production of an alkaline reaction by the cells when exposed to sunlight. The source of the base concerned will be considered in a following paper.