Diffusion through multiperforate septa was investigated, using two series of septa and three types of diffusion with the object of attempting to formulate an interference equation;A simple derivation of small-pore diffusion is presented which indicates that spheroidal uniform pressure surfaces yield diffusion rates showing proportionality with pore diameter;The experiments and empirical interference equation of Huber were analyzed and found to be unreliable because he failed to measure interference as an isolated variable;A theoretical interference equation is derived which shows that interference between pores of constant size, uniformly spaced in a septum, should be inversely proportional to the distance between pores: I=kQ1D2 ;The theoretical interference equation is confirmed by data presented. Graphs of evaporation per pore against 1000D2 show a linear relationship over a considerable range of spacings;An extended analysis of the interference equation indicates that small pores should show relatively greater interference than large pores when spaced an equal number of diameters apart; interference being inversely proportional to pore diameter;The inverse relation of pore diameter to interference is supported by a comparison of interference between the 0.2 mm- and 0.3 mm-pored septa. Interference in the 0.2 mm series at 10-diameter spacing was 1.67 times that observed in the 0.3 m series, and at 5-diameter spacing it was 1.38 times that observed in the 0.3 nm series. The theoretical ratio is 1.5 to 1;A survey of stomatal characteristics of thirteen plants (data of Eckerson) showed that, although wide variations in stomatal size, density, and spacing are evident, yet relative spacing (spacing in terms of effective diameters) showed much less variation. The relative stomatal spacings of all the plants surveyed seem to favor diffusion rates closely approaching those of a free surface;The application of interference data to leaf diffusion problems indicates that the lack of correlation between stomatal characteristics (size, density, and aperture) and observed diffusion rates may be partly explained by the magnitude of interference effects and by the fading of interference during stomatal closure.