Concerning the Open Subsets of a Plane Continuum.

exchange of radioactive isotopes in the culture medium for non-radioactive isotopes in the roots. The fraction of potassium present in the root capable of rapid exchange for isotopes in the culture medium is calculated. This fraction is believed to be associated with the colloidal phases of the protoplasm and cell wall, and for this reason may have a special significance for the study of certain aspects of the ionic interrelations of the root and culture medium. Acknowledgments.-The authors are indebted to Professors D. R. Hoagland and H. Jenny for their interest and valuable suggestions, and to Professor E. 0. Lawrence, Director of the Radiation Laboratory, for supplying radioactive potassium. 1 Assistance in the preparation of the experimental material was furnished by the personnel of Works Progress Administration Official Project No. 465-03-3-587. ' Jenny, H., and Overstreet, R., Proc. Nat. Acad. Sci., 24, 384 (1938).. 3Compare Brooks, S. C., Protoplasma, 8, 389 (1929). 4Hoagland, D. R., and Broyer, T. C., Plant Physiol., 11, 471 (1936). 6Jenny, H., Overstreet, R., and Ayers, A. D., Soil Sci., 48, 9 (1939). * The star is used to designate the radioactive isotope of potassium.