Finite parallel wavelength is shown to have a significant effect on the linear growth of instabilities of the Rayleigh-Taylor or E¿B gradient drift types. The flute approximation, which explicitly neglects electric fields parallel to the ambient magnetic field, is asymptotically correct for long perpendicular wavelengths, if the parallel component of the electric field is primarily inductive in character. However, when the parallel electric field is primarily electrostatic, there is a profound reduction in the growth rate for long perpendicular wavelengths. This reduction in growth rate arises because parallel electron currents short-circuit the perpendicular electric field driving the instability. Parallel electric fields are more likely to be electrostatic in lower-density plasmas than in higher-density plasmas. Consequently, the extrapolation of structuring from low-density plasmas, such as those found in the spread-F environment, to higher-density ionospheric plasma may not be reasonable.