a mathematical model, based on probabilistic assumptions concerning root growth directions, was applied to observations from minirhizotrons, in order to characterize the dominant root system orientation. the model uses the average number of roots that intersect the wall of several tubes buried in the soil, and not the numbers of root on individual observation tubes. by determining the ratio of the numer of roots that intersect the top of an angled minirhizotron to the number that intersect the bottom, the model predicts whether the root system is more horizontally or more vertically oriented than random. application of the model to measurements of root intersections with minirhizotrons indicated that the cotton root system was more horizontally oriented than random this orientation was confirmed by examining the cotton root system on a vertical trench wall. the modeled and observed ratios had the same absolute value when the probability was considered random that a given root was oriented upward or downward. (edited author abstract) 11 refs