Scientific research concerning growth inhibitors, which has been pursued for several decades already, dealt mainly with the effect of these substances on the germination process. WIESNER (1894) demonstrated the presence of a growth inhibitor in the slime of the mistletoe (Viscum album) which prevented the germination of a great variety of seeds. OPPENHEIMER (1922) supplemented the analysis by placing seeds on the pulp of ripe tomatoes and he observed a strong inhibitive effect as a result of this treatment. In addition, however, he found that the inhibiting substance is thermolabile and insoluble in ether or alcohol. REINHARD (1933) corroborated Oppenheimer’s results for the most part. According to this author, however, the inhibiting agent in tomato juice is thermostabile, and it is not destroyed by boiling, neiher by neutralisation or by diluting the juice 50 times. In other fleshy fruits such as apples, pears and quinces KÖCKEMANN (1934) detected inhibiting substances capable of preventing the germination of Lepidium seeds. These substances were reported to be sensitive to peroxide and to alkali, thermostabile and soluble in water and in ether, but insoluble in petroleum ether. On the other hand, the inhibiting agent extracted by LEHMANN (1937) from the exocarp if buckwheat is thermolabile. In Helianthus annuus and Avena sativa, finally, RUGE (1939) demonstrated the presence of an inhibitor that reduces the speed of germination to a considerable extent. FRÖSCHEL’S investigations on Trifolium and Beta will be dealt with in 4. This survey is not quite exhaustive, but clearly demonstrates that the inhibiting agent should not be regarded as a definite, well-defined chemical substance which is always the same in every individual case, but as a group of substances with analogous activities but most probably with widely divergent physical and chemical properties. Following KÖCKEMANN (1934) we can classify the inhibiting substances into two groups, as follows : 1. inhibiting substances in the testa or in the seed, and 2. inhibiting substances in the mesocarp of pulpy fruits.