Summary.Peas, barley and linseed were grown in water‐cuture. Roots were divided into two portions and contained in separate vessels. The effect of herbicides applied to one‐third of the roots was compared with their effect when applied to the whole root system.Propham, chlorpropham, atratone and 2,3,6‐TBA killed the plant whether application was to one‐third or all of the roots. Roots in direct contact with propham, chlorpropham and 2,3,6‐TBA were deformed or killed, while roots in uncontaminated solution remained healthy. The effect of atratone and high concentrations of chlorpropham on a portion of the roots was transmitted to roots not in the toxic solution.Certain concentrations of MCPA and DNOC applied to the whole root system killed the plant while three times the concentration to one‐third of the roots caused local damage but had little effect on the plant as a whole. Sufficient herbicide may be taken up by one‐ third of the roots from high concentrations to kill the whole plant.TGA gave variable results with respect to the health of the aerial portion of the plant but had no obvious direct effect on root growth.Propham, chlorpropham, atratone and 2,3,6‐TBA reduced transpiration rates whether applied to part or whole root systems. MCPA and DNOC prevented water uptake by roots with which they Were in direct contact.Etudes sur les réactions de certaines plantes à des herbicides appliqués aux racines I. Effet d'une ap
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