Volatile excretions from rotting weeds and after-harvest residues sharply depressed germination of seeds of cultivated plants, partly due to excretion of NH3. The volatile vapours from macerated leaves of weeds had a strong depressive effect on photosynthesis in maize and beet, a smaller depressive effect on this process in clover, sunflower and beans and the least effect on potato leaves. Perennial weeds act more intensely than annuals.