WITH TWO FIGURES)Regional variety tests have shown that the tensile strength of cotton fibers of any giveni variety is influenced by environmental conditions. In general, the fiber seems to have greater strenigth wheni plant growth is checked by deficient water and less strengicth when water is not a limitinog factor. This observation has led to the assumption that the strength of cotton fibers might be influenced appreciably by the turgor of the developing fiber cells. If this assumption were correct, a knowledge of cell turgor, at critical stages in the growth of the fiber, should serve as ani inidex of the tensile strength of the fiber. Since tensile strength is a property-of the greatest importance, efforts were made to determine the diurnial and seasonal variations in turgor in the developing seeds of upland eottoin grown in Raleigh, N. C.Cell turgor cannot be imeasured directly btut it can be estiimiated indirectly in two ways: (a) byr measuring volume changtes in the cells, and (b) by determining the differenice between the osmotic pressures and the diffusioni pressure deficits of the growing cells (11). Both methods have been employed by the writers. The data obtained by the use of the first method have beenl reported in a previous paper (1). This paper is a report of the results obtained in the application of the seconid method.The osmiiotic quantities of plant cells and their relation to the intercellular movements of water have beein diseussed by URSPRUNG (15) and by MEYER (11). The terminology used by MIEYER is adopted in this paper.The movement of water between plant cells, and between cells and their environment has commonlv been regarded as an osm1otic phenomnenioni. The osmotic quantity designated as the diffuision pressure deficit is generally regarded by physiologists as having a mutch greater significance in determining the direction and rate of water movements into and out of plant cells than the osmotic pressure of the cells (12). This is due to the fact that the direction and rate of water miiovement between cells is believed to be controlled bv the relative diffusion pressure of the water in the different cells. Movement of water will occur from regionis of higher to regions of lower difftusion pressure. The diffusionl pressure of water is influenced both by the osmiiotic pressure and by the turg-or of the cells. The presence of solutes decreases the diffusion pressure of water. The amount of this reduction of diffusion pressure is equivalent to