Experiments have been pursued on the biological activity of products derived from the in vitro breakdown of polyenes including Vitamin A aldehydes (Retinenes) and carotenoids. The results are consistent with the working hypothesis that these polyenes, when chemically denaturated following irradiation with visible light and other treatments, possess inhibitory properties on seed germination and enhance abscission. The dormin/abscisin-like derivatives are themselves, to some extent, photolabile. The significance of these researches for the morphogenetical manifestations is briefly considered.