Theory is presented to provide insight into the observation that attenuation through vegetation is proportional to vegetation water content. In this analysis, the canopy is modeled as a sparse layer of randomly oriented particles (leaves, stalks, etc.) over a flat, homogeneous ground plane (soil) and an expression is obtained for the "optical depth". The formulas developed by Ulaby and El Rayes are used to relate this expression to the water content of the canopy. In the low frequency extreme (Rayleigh scatterers), the attenuation varies almost linearly with water content and inversely with wavelength. In contrast, in the high frequency limit, the attenuation is independent of both water content and frequency, in between, geometry dependent "resonances" occur even at the low frequency end of the microwave spectrum (e.g. L-band) making the dependence of attenuation on frequency and water content specific to canopy architecture.