Leaf transpiration of eight genotypes of Pima cotton was measured in the field of the Maricopa Agricultural Center in August 1994. Photomicrographs of leaf cross-sections and of the leaf surfaces were scanned and analyzed with the image analysis software. The data were used to parameterize the new WALL model, developed in this study to analyze the leaf transpiration with a special emphasis to liquid water movement inside the leaf. The transpiration stream was assumed to go from vein endings in two directions, towards the upper and lower leaf surfaces. These fluxes were presented as two parallel currents driven by the water vapour concentration difference between the atmosphere and the open surface of the vein endings and on the mesophyll and epidermis (inner parts) cells’ surfaces as a flow in thin films of water. Simulations were run to estimate quantitatively the contribution of the cuticular transpiration to the total amount of leaf transpiration stream, to evaluate the role of the mesophyll cell walls’ surfaces in the water transfer inside the leaf, and to calculate the dependence of transpiration and its components on temperature. Simulation results showed (1) a major role of the cuticular transpiration as a leaf cooling mechanism and (2) that the cell wall properties can affect water film characteristics that also affect the transpiration course.