Purpose of Investigation.-This paper is one of a series in which various problems involving the investigation of the osmotic pressure or osmotic concentration of the fluids of plant tissues are treated. Specifically it presents an extensive series of determinations of the freezing-point lowering of the extracted leaf sap of plants from the Blue Mountains of Jamaica, discusses the differences in these values in their relation to local differences in the environmental complex, and briefly compares the series as a whole with others now available. In another place (Harris, Lawrence and Gortner, I9I6) we have put forward in detail the arguments for the carrying out of such studies as a regular part of systematic and thoroughgoing phytogeographical investigation. It seems unnecessary, therefore, to repeat these arguments here. After completing a series of determinations of the osmotic concentration of the tissue fluids of a number of species of plants from the southwestern deserts, in the vicinity of the Desert Laboratory during the winter and spring months of I9I4, and comparing them (Harris, Lawrence and Gortner, I9I5) with a series made in the more mesophytic habitats in the neighborhood of the Station for Experimental Evolution on Long Island, the next most desirable step seemed to be the investigation of the sap properties of the plants of an extremely hygrophytic region. Since such field studies could be most conveniently carried out during the winter months, at a time when we could be absent from