Summary.Senescent, expanded or expanding leaves of lettuce plants were treated individually with MCPA. The subsequent development of auxin‐induced symptoms throughout the plants was recorded. The number of leaves affected and the severity of the symptoms increased with application to the less mature leaves. In addition more leaves were affected when the application was made to larger plants than to smaller ones. These results were associated with absorption/translocation differences in the leaves of different ages, and in their spatial relationship to tissues at particularly susceptible stages of development. In further experiments, applications of similar weights of MCPA to lettuce plants in droplet sizes of approximately 100 μ and 500 μ were compared. There were consistent trends for the small droplets to produce greater effects.Quetques effets de la taille des gouttelettes et de leur position sur la reaction de la laitue à des doses sub‐létales
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