Deficiencies of P or N strongly decrease root hydraulic conductance (volume flux of water per unit pressure per unit length of root system) in young cotton (Gossypium hirsutum L.) plants. One hypothesis about this effect is that nutrient deficiency might decrease numbers or diameters of xylem vessels, thereby increasing the 'axial' component of root resistance. Studies reported here address this hypothesis. In plants on complete nutrients, the cross-sectional pattern of xylem in lateral roots was virtually 100% tetrarch (four protoxylem poles). In P-or N-deficient roots, about 80% of lateral roots were again tetrarch, but the remaining 20% were divided between triarch and pentarch patterns. Triarcli roots were narrower than tetrarch roots, with fewer vessels and a smaller- central metaxylem vessel, but this difference was apparently compensated by the presence of pentarch roots on the same plants. Nutrition did not alter mean diameters of populations of lateral roots. Because of the -mall number of aberrant roots, and because of the "compensation" in anatomical patterns, vasculature of lateral roots could not account for the large nutritional effects on root hydraulic conductance.