The writer became interested in the growth of algae in culture (4, 5, 6, 7, 8, 11, 20, 21, 23, 24, 25) while engaged in attempts to isolate a few of the common species. In particular the relation of these plants to the element calcium was considered. Molisch (15) was the first to report that certain algae can grow normally when calcium is excluded from the media in which they are cultured, while others soon die without this element. Benecke (1) also grew a few species in calcium-free culture solutions, and confirmed the conclusion of Molisch that calcium is not essential for the growth of some algae. In spite of many inoculations with fresh material of Chlamydom?nos ting ens A. Braun, however, Frank (9) did not succeed in growing this alga in calcium-deficient cultures. Richter (22) found that two diatoms, Nitzschia palea (Kuetz.) W. Sm. and Nav?cula min?scula Grun. also could not grow without calcium. Likewise, Maertens (12) proved that this element was indispensable for the bluegreens, Oscillatoria brevis Kuetz. (Oscillarla brevis Kuetz.), 0. tennis Ag.> and Nostoc sp., and that strontium could not be completely substituted for it in cultures of the last two algae (the first was not investigated). Waren (28) reported that, in general, cell divisions were not possible in the desmid Micrasterias rotata (Grev.) Ralfs when there was a deficiency of calcium. That a relationship might exist between the calcium requirements of a plant and its systematic position was suggested by Pringsheim (21). Algae which could grow without calcium were classed with the lower plants such as the fungi, while those which could not grow without it were grouped with the higher plants. He investigated a large number of algae, some of them producing no growth without calcium and others as Chlorella sp. 1 and 2 developing with or without this element. These latter species continued to develop after several transfers from one calcium-deficient solution to another, but notwithstanding the precautions which were taken his results were not always clear. Among other investigators who have cultivated different strains of Chlorella in solutions with a deficiency of calcium are Hopkins and Wann (10), Trelease and Selsam (26), Pratt (16, 17), and Pratt and Fong (18, 19). Their works will be referred to later.