1. The pycnospores of the sooty mould fungus, Capnodium fuliginodes REHM, germinate well in the solutions of honeydews secreted by aphides and scale insects as well as in some plant decoctions. However, they germinate neither in redistilled water nor in both the solutions of glucose and sucrose, and scarcely in RICHARDS' nutrient solution. The pyncnospores of the fungus seem to be unable to germinate unless being reinforced by certain substances derived from some living organisms. It is inferred that the honeydew solutions as well as the plant decoctions contain a certain substance or substances wich stimulate the spore germination of the sooty mould fungus.
2. The pyncnopores of the fungus germinate well in the solutions of honeydews secreted by the aphides and scale insects, including Tichoregma bambusifoliae TAK., Aphis sacchari ZEHNT., Thoracaphis fici TAK., Pulvinaria psidii MASK., Pulvinaria polygonata COCK., and Tachardina theae GREEN et MANN. The growth of the germ tubes is also favoured in these solutions. The specific effect of the honeydew on the spore germination of the fungus does not appear distinctly according to the different kinds of aphides and scale insects.
3. The solutions of honeydews secreted by the aphides and scale insects were treated with charcoal, siliceous earth or Japanese acid clay, respectively, and the germination of the pycnospores was examined in these treated solutions. They germinated never in the honeydew solution treated with charcoal, but did well in the solution treated with Japanese acid clay or siliceous earth as well as in the untreated solution. The results of the germination tests stated above showed that the effective substance or substances stimulating the spore germination are perfectly adsorbed by charcoal, but not by Japanese acid clay and siliceous earth.
4. The solution of the honeydew secreted by Trichoregma bambusifoliae TAK. was kept for 8 or 16 hours continuously at 100°C in a Koch's steam sterilizer, and the germination of spores and the growth of germ tubes of the fungus were examined in the treated solution, and the results showed that both are rather better in the solution treated with heat than in the untreated one. Therfore, the effective substance or substances contained in the honeydew solution seem to be thermostable. The spore germination of the fungus is also favoured on normal agar media which contain enough biotin referred to as a growth promoting substance. It is suggested by the results obtained in the experiments that the effective substance or substances in the honeydews secreted by aphides and scale insects may be biotin or some allied substances.