Leaching effects, biological activity of leachates and extracts from unripe endosperms of Triticum. — Unripe endosperms ofTriticum durumcv.Cappelliinhibit germination and depress growth of after-ripe embryos, as revealed by embryo transplantation technique. Leaching of unripe endosperms increases their capacity to depress germination and growth of after-ripe embryos. Bioassay of leachates showed that water soluble substances are removed from the endosperm. These are able to stimulate growth of lettuce hypocothyl. Their removal seems to allow leached endosperms to exibit a depressing activity, unopposed by leached stimulating substances, stronger than unleached endosperms. Fractionation on Sephadex G 10 column of aqueous extract from unripe leached endosperms was carried out to display substances responsible of depressing activity. Bioassay of 12 eluted fractions revealed the presence of two groups of active fractions, which differ for molecular weight (m.w.). These are able to stimulate — fractions with high m.w. — or to depress — fractions with low m.w. — growth of lettuce hypocothyl. The depressing activity of unripe endosperms could be due to low m.w. fractions, of which activity exceeds the stimulating effects of fractions with high m.w. The germination and growth behaviour of unripe caryopses ofTriticummay be explained as a consequence of the relationships between depressing and stimulating growth substances. Their unstable balance controls the behaviour of seed during ripening.