In explaining osmotic movement of water, it is customary in plant physiology to regard water movement between cells as taking place along gradients of diffusion pressure deficit or suction force. Efforts to describe water movement quantitatively are based on the assumption that the rate of water movement between two cells should be proportional to the difference between the diffusion pressure deficits (or suction forces) of the two cells. The purpose of this paper is to consider some aspects of the theory of osmotic water movement which arise out of this elementary rate problem, and indicate some fundamental difficulties in the concepts currently in use. First we shall examine critically the meaning of the concepts "suction force" and "diffusion pressure deficit". Then their applicability as parameters of osmotic water movement will be considered. Finally, the value of rate measurements in indicating the mechanism of osmosis will be reviewed, and a hypothesis concerning the mechanism of osmotic flow will be presented.