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ing in all directions where the substance is most concentrated. Water 

 may be diffusing out of a cell, while the solutes in the water are diffusing 

 into the cell or vice versa. The factors that influence diffusion most are 

 temperature in relation to the rate of molecular motion, the concentration 

 of the diffusing substance, and the nature of the medium into which 

 it is diffusing. Diffusion under certain special conditions may be dis- 

 tinguished as imbibition and osmosis. Imbibition results in the swelling 

 of colloidal membranes and cell walls. Osmosis results in the enlarge- 

 ment of vacuoles. In the following chapter we shall note how diffusion 

 that is called osmosis underlies certain plant behavior familiar to all of 

 us. In later chapters we shall see how the energy of diffusion is the force 

 underlying some of the mass movement of materials within a plant. 



