We may summarize our communication in the next four points; (1) The water absorption in potato reserve tissue was found to be a real growth phenomenon, which is accompanied by an active synthesis of cellulose and furfurogens. As a consequence, the activation of water intake by auxins may be regarded as a normal growth activation of the same type as found in the common growing zones of plants. (2) Auxins cause a shift in cell wall composition, since they bring about a more rapid building up of pectic substances, in opposition to cellulose. (3) We suggest that this shift is the immediate cause of the increased extensibility of the cell walls, the pectic substances being more hydrated and less crystallizable than cellulose. (4) Hindrance of water intake by 4 % mannitol is accompanied by reduction of starch loss, of furfurogen and probably of cellulose synthesis. These reductions however are not proportional to the large decrease of water intake. It is suggested that mannitol is not only an osmotic growth retarder but also interferes directly with glucose metabolism. The authors are much endebted to Dr J. B. Thomas and to Dr L. Anker, who critically read the manuscript.