1. High-vitality and high-frequency components of the undergrowth of red-pine forests, in which the cover degree of Pinus densiflora was higher than 4.5, were tabulated for the wide ranges of Japan (mainly according to Yoshioka⁶⁾) and South Korea (near Seoul).
2. Leaf extract and root extract of P. densiflora inhibited heavily the seed germination of weeds, e. g. Amaranthus and Achyranthes, which did not naturally grow in the red-pine forest. No inhibition was observed in Pinus seed.
3. Soils taken from the red-pine forests of various localities were detected inhibiting the growth of plants. Amaranthus, Achyranthes, Phytolacca, Chenopodium, Setaria and Galinsoga, which rarely appeared in the red-pine forest with very low vitalities. The growth of high-vitality components, Pinus, Miscanthus, Atractylis and Paederia, was the same or slightly better on the red-pine-forest soils than on ordinary nursery soils.
4. The growth-inhibitive effect of Pinus extracts was demonstrated in low-vitality plants, Amaranthus, Achyranthes and Setaria, by planting on an artificial soil supplied with mineral nutrients. On the contrary, the high-vitality plants, Pinus and. Miscanthus, did not show any depression of their growth.
5. Negative geotropism was observed in the roots of Amaranthus seeds which were sprouted on filter paper wetted respectively by Pinus shoot extract, Quercus seed extract and tannic acid solution.
6. The chemical nature of the germination and growth inhibitors secreted by P. densiflora was preliminarily investigated. In the Pinus extract as well as in the red-pine-forest-soil extract, the fluorescence of p-coumaric acid-like substance was detected by paper-chromatography.
7. It was suggested that the floristic composition of a plant community may sometimes be decided by chemical influences on other species, allelopathy.