Studies on transport mechanism in plants, using soybean seedlings and C¹⁴-labeled tracers, show that: 2,4-D* movement is largely restricted to phloem; monuron* movement takes place in xylem and cell walls; amino triazole* moves in phloem, xylem and cell walls; and maleic hydrazide* moves like amino triazole* and may leak from phloem to xylem and thus, circulate in the plant. Applied to epicotyl, 2,4-D* moves into and along the phloem, monuron* into and along the xylem, and amino triazole* and maleic hydrazide* into and along both phloem and xylem. These distribution patterns show selectivity of solute translocation, and support the concept of mass flow as the mechanism responsible for rapid, long-distance transport of foods in the plant.