Development of the root system of peas grown on filter paper between vertical glass sheets was studied during the period 6 to 20 days after germination. Elongation of each whole root system proceeded at alternate high and low rates. At the end of each period of low growth rate, a higher order of roots emerged. Secondary roots tended to occur in clumps. Secondary roots with the greatest growth potential occurred midway within that region of the primary root from which secondary roots emerged, and also, but less markedly, at the positions of clumps. The timing and duration of elongation of roots with high growth potential differed from that of roots with lower growth.Growth potential of secondary root appears to be partly determined at the time of root initiation. Such a process would precondition a seedling to a particular environment.