Flax plants were grown in vertical cylinders of soil contained in polyethylene tubing. Root growth was estimated from the number of roots crossing vertical and horizontal lines on the tube surface. After the plants were 29 days old, some of the soil cylinders received no more water, and root growth and soil water content in each 10 cm layer of soil were determined at 2-day intervals. When a graph of root growth against soil water potential is plotted, points from each 10 cm layer fall approximately on the same concave curve. A marked reduction in root growth first occurred at — 7 bars total water potential. At — 15 bars, root growth was 20% or less of the rate before drying, but some root growth occurred in soil drier than — 20 bars. The results suggest that root growth in each layer was not influenced by water potential elsewhere in the plant.