Whereas scientific research on inhibiting substances has mainly occupied itself with the effect of these substances on the germination process, I was able to demonstrate the presence of a root-inhibiting agent during my studies on root formation in Fuchsia hybrida and Pelargonium zonale (Stolk, 1952). In connection with this previous investigation I tried to find out whether a similar substance is present as well in bulbous plants and, by doing so, as a suitable species for my experiments the Liliaceous Allium Cepa was selected. My principal object was to corroborate and, if possible, extend the results obtained with Fuchsia and Pelargonium. A confirmation appeared highly desirable because the experiments with Fuchsia and Pelargonium could not possibly be very accurate on account of the direct method of measuring and that is why I used in my experiments with Allium a root-auxanometer which will be described in the following section. Substances retarding germination were demonstrated in the slime of Viscum album by WIESNER (1894), in the fruit pulp of ripe tomatoes by OPPENHEIMER (1922), in tomato juice by REINHARD (1933), in other pulpy fruits such as apples, pears, quinces and tomatoes by KÖCKEMANN (1934), in the exocarp of buckwheat by LEHMANN (1937), in the fruits of Helianthus annuus and Avena sativa by RUGE (1939) and in Beta by FRÖSCHEL (1939, 1940). That not one and the same inhibitor is involved, is evident from the fact that the above-mentioned authors found differences between the inhibiting substances they studied in their behaviour towards high temperatures. The inhibiting substances found by OPPENHEIMER and LEHMANN, for instance, are thermolabile. those discovered by REINHARD, KÖCKEMANN and FRÖSCHEL thermostabile. Also in their chemical behaviour certain differences can be demonstrated. Whereas the substance studied by KÖCKEMANN is soluble in ether, this is not the case with the inhibiting substance found by OPPENHEIMER in tomatoes.