In a previous paper (6) the writer assumed that the methods of seed stimulation, advocated by Popoff (9) are of little value for agriculture and horticulture [cf. also Bredeman (1)]. Many publications witness the great interest on the subject of seed stimulation. The occurrence of stimulation itself and the effect of so-called stimulants upon yield is still in controversy. However, the search for principles involved has1 been meager. These investigations aim to contribute to the knowledge of one of these principles, which is not the least important in the writer's opinion. The well established fact of semipermeability of seed coverings' was overlooked by Popoff and his associates. In this paper the term seed coverings is used to include all structures outside the embryo and endosperm. The significance of the semipermeability of the seed coats was stated by Shull (11, p. 191) : "The testa has physical and chemical characters, which may enable it to modify greatly any factors entering into germination behavior and the effects of these characters must be known before any sound conclusions can be drawn. Semipermeability is now shown to be common enough that its existence should be proved or disproved before proceeding to use stimuli acting through membranes/7 This was written ten years before Popoff started to publish his papers on the topic of seed stimulation. Popoff (9) claimed to explain the real cause of stimulation by reagents. Unfortunately, he and his1 associates when presenting successful results did not give the exact composition of the stimulants1 used and therefore their work is not susceptible of verification, but it is certain that they have paid no attention to the permeability of the seed coverings.