Summary. The growth of wild oat (Avena fatua L.) was accompanied by an increase in the amount of spray retained per plant and per unit weight of plant, probably because the projected area (plan view) increased as the plant became larger. Flax (Linum usitatissimum L.) retained a fairly constant volume of spray per plant from the 8‐leaf to the 35–40‐leaf stages, so that the spray retained per unit plant weight decreased considerably as the plants grew. This was related to the decrease in projected area as a proportion of the whole. There was also a difference in retentivity between the cotyledons and true leaves; the cotyledons retained ten to twenty times more spray per unit weight. These results suggest that maximum selectivity would be achieved by spraying at an advanced stage of growth. Wild oat at the 3‐leaf stage retained more than half the aqueous spray in the two leaf axils, while flax retained most on the cotyledons. Distribution was shown to be a probable reason for selectivity. Addition of a surfactant greatly increased retention by both species, but although the effect of a spray on wild oat might thereby be increased, under certain conditions injury to flax might be increased and the margin of selectivity reduced. Croissance et retention des bouillies herbicides par la folle‐avoine et le lin en rapport avec la sélectivité

Outdoor pot experiments, carried out between 1967 and 1971, mainly with Polygonum lapathifolium L. and Stellaria media (L.) Vill., showed that herbicidal potency of ioxynil and bromoxynil salts and esters was affected by climatic conditions, particularly solar radiation and relative humidity. Multiple regression equations were frequently obtained which accounted for 80% or more of the observed variation in effective dose (ED90). By contrast, experiments in growth cabinets suggested that air temperature was the most important climatic variable; the reasons for this disagreement are discussed. It is concluded that climatic factors affect the potency of ioxynil and bromoxynil via their influence on leaf uptake; ester formulations (emulsions) were much less affected than aqueous salt solutions.